1. Structure, topography and functon of the lver and pancreas

2. Structure, topography and functon of large ntestne

3. Structure and topography of pertoneum, relations with organs

Lesson No 14

Theme 1. Structure, topography and functon of the lver and pancreas

The liver, the largest gland in the body, also performs important exocrine and metabolic functions:

          The secretion of bile.

          The protective role by detoxifying substances.

          The storehouse for various substances.

          Metabolising the products of digestion.

          The synthesis of proteins.

          The metabolism of carbohydrates and the regulation of blood glucose.

          The metabolism of fats and the regulation of blood lipids.

          The conjugation of substances.

          The transformation of substances.

          The production of carbohydrates from proteins.

          The haemopoietic function - especially during foetal life the liver is a centre for haemopoiesis and new-born.

          The production of thrombolitic agents.

          The synthesis of procoagulants.

Topography of the liver.

Holotopy: Liver occupies right hypochondriac region, proper epigastric region and small part of left hypochondriac region. Skeletotopy: The upper edge of the liver projects in right 10th intercostal space (middle axillar line). Than it lifts to level of 4th rib (middle clavicular line) and passes across the sternum a bit upper from xiphoid process, terminates in left 5th intercostal space (between middle clavicular line and parasternal lines). The lower edge of the liver passes along the costal arch from right 10th intercostal space (middle axillar line). Than it crosses cartilage of right 9th rib and runs in epigastrium 1,5 cm lower from xiphoid process to cartilage of left 8th rib and meets the upper margin.

: : : : : : image1086

 

Inferior surface of the liver

 

We distinguish the convex diaphragmatic surface of the liver and lower visceral surface. Visceral surface adjoins to the organs, which form on surface of the liver suitable tracks: renal, adrenal, gastric, duodenal, oesophageal and colic impressions. Diaphragmatic surface carries cardiac impression.

Liver is almost entirely covered with peritoneum except posteriorly positioned area nuda. The superior surface is attached to the diaphragm and anterior abdominal wall by a fold of peritoneum, the falciform ligament, in the free margin of which is a rounded cord, the ligamentum teres (obliterated umbilical vein). The liver is connected to the lower surface of the diaphragm by the coronal ligament and the right and left triangular ligaments. The falciform ligament conventionally separates greater right lobe of liver and lesser left lobe of liver.

The porta hepatis, the entrance into the liver forms a cross-connection between the sagittal grooves which together are shaped like an H. Visceral surface carries furrows: right sagittal sulcus and left sagittal sulcus, which communicate by transversal sulcus (is called porta hepatis). Left sagittal sulcus anteriorly contain fissure of teres ligament, where umbilical vein in foetus passes. It obliterates in adult and forms teres liver ligament. Posterior portion of left sagittal sulcus is formed by fissura of venous ligament (obliterated venous duct of Arantii). Right sagittal sulcus anteriorly contains fossa of gall bladder, and behind - sulcus of inferior vena cava. Vena portae, proper hepatic artery and nerves enter through the porta hepatis into liver, common hepatic duct and lymphatic vessels leave the parenchyma in this place. Sagittal and transversal sulcuses limit the quadrate lobe, positioned ventrally and caudate lobe, disposed dorsally. Caudate lobe carries papillary and caudate processes.

The liver is held together by a tense connective tissue capsule Glisson 's capsule. At the porta it separates the lobules of liver. The lobules form the chief mass of the hepatic substance. Branches of portal vein, hepatic artery and biliary duct form a hepatic triad are situated in stratums between liver lobules.

Unlike all other organs a liver obtains arterial blood from proper hepatic artery and venous - from portal vein. Entering into liver porta, a portal vein and hepatic artery disintegrate into right and left lobar, segmental and lobular veins and arteries, which pass along interlobular bile duct. Capillaries from these vessels joining together form sinusoid capillaries that receive mixed blood and empty into a central vein, which occupies the centre of the lobule. Central vein drains into hepatic veins, which leave the liver to end in the inferior vena cava. This system is called as wonderful venous liver net.

Hepatic cells hepatocytes excrete the bile, which get into bile canaliculi. Last pass to periphery emtpy into interlobular ductuli that form right hepatic duct and left hepatic duct (from right and left hepatic lobes). Common hepatic duct, which originated in porta, passes in hepatoduodenal ligament, meets the cystic duct and forms ductus choledochus. It flows together with pancreatic duct and forms common hepalopancreatic ampulla, which opens on major duodenal papilla. The ampulla may itself be closed by its own sphincter muscle, the sphincter ampullae (Oddi).

The liver, the largest gland in the body, has both external and internal secretions, which are formed in the hepatic cells. Its external secretion, the bile, is collected after passing through the bile capillaries by the bile ducts, which join like the twigs and branches of a tree to form two large ducts that unite to form the hepatic duct. The bile is either carried to the gall-bladder by the cystic duct or poured directly into the duodenum by the common bile duct where it aids in digestion. The internal secretions are concerned with the metabolism of both nitrogenous and carbohydrate materials absorbed from the intestine and carried to the liver by the portal vein. The carbohydrates are stored in the hepatic cells in the form of glycogen which is secreted in the form of sugar directly into the blood stream. Some of the cells lining the blood capillaries of the liver are concerned in the destruction of red blood corpuscles. It is situated in the upper and right parts of the abdominal cavity, occupying almost the whole of the right hypochondrium, the greater part of the epigastrium, and not uncommonly extending into the left hypochondrium as far as the mammillary line. In the male it weighs from 1.4 to 1.6 kilogm., in the female from 1.2 to 1.4 kilogm. It is relatively much larger in the fetus than in the adult, constituting, in the former, about one-eighteenth, and in the latter about one thirty-sixth of the entire body weight. Its greatest transverse measurement is from 20 to 22.5 cm. Vertically, near its lateral or right surface, it measures about 15 to 17.5 cm., while its greatest antero-posterior diameter is on a level with the upper end of the right kidney, and is from 10 to 12.5 cm.

Opposite the vertebral column its measurement from before backward is reduced to about 7.5 cm. Its consistence is that of a soft solid; it is friable, easily lacerated and highly vascular; its color is a dark reddish brown, and its specific gravity is 1.05.

  To obtain a correct idea of its shape it must be hardened in situ, and it will then be seen to present the appearance of a wedge, the base of which is directed to the right and the thin edge toward the left. Symington describes its shape as that of a right-angled triangular prism with the right angle rounded off.

 

Surfaces.The liver possesses three surfaces, viz., superior, inferior and posterior. A sharp, well-defined margin divides the inferior from the superior in front; the other margins are rounded. The superior surface is attached to the diaphragm and anterior abdominal wall by a triangular or falciform fold of peritoneum, the falciform ligament, in the free margin of which is a rounded cord, the ligamentum teres (obliterated umbilical vein). The line of attachment of the falciform ligament divides the liver into two parts, termed the right and left lobes, the right being much the larger. The inferior and posterior surfaces are divided into four lobes by five fossæ, which are arranged in the form of the letter H. The left limb of the H marks on these surfaces the division of the liver into right and left lobes; it is known as the left sagittal fossa, and consists of two parts, viz., the fossa for the umbilical vein in front and the fossa for the ductus venosus behind. The right limb of the H is formed in front by the fossa for the gall-bladder, and behind by the fossa for the inferior vena cava; these two fossæ are separated from one another by a band of liver substance, termed the caudate process. The bar connecting the two limbs of the H is the porta (transverse fissure); in front of it is the quadrate lobe, behind it the caudate lobe.

  The superior surface (facies superior) comprises a part of both lobes, and, as a whole, is convex, and fits under the vault of the diaphragm which in front separates it on the right from the sixth to the tenth ribs and their cartilages, and on the left from the seventh and eighth costal cartilages. Its middle part lies behind the xiphoid process, and, in the angle between the diverging rib cartilage of opposite sides, is in contact with the abdominal wall. Behind this the diaphragm separates the liver from the lower part of the lungs and pleuræ, the heart and pericardium and the right costal arches from the seventh to the eleventh inclusive. It is completely covered by peritoneum except along the line of attachment of the falciform ligament.

 

liver. (From model by His.)

: : : : : : image1085

 

The superior surface of the

  The inferior surface (facies inferior; visceral surface)is uneven, concave, directed downward, backward, and to the left, and is in relation with the stomach and duodenum, the right colic flexure, and the right kidney and suprarenal gland. The surface is almost completely invested by peritoneum; the only parts devoid of this covering are where the gall-bladder is attached to the liver, and at the porta hepatis where the two layers of the lesser omentum are separated from each other by the bloodvessels and ducts of the liver. The inferior surface of the left lobe presents behind and to the left the gastric impression, moulded over the antero-superior surface of the stomach, and to the right of this a rounded eminence, the tuber omentale, which fits into the concavity of the lesser curvature of the stomach and lies in front of the anterior layer of the lesser omentum. The under surface of the right lobe is divided into two unequal portions by the fossa for the gall-bladder; the portion to the left, the smaller of the two, is the quadrate lobe, and is in relation with the pyloric end of the stomach, the superior portion of the duodenum, and the transverse colon. The portion of the under surface of the right lobe to the right of the fossa for the gall-bladder presents two impressions, one situated behind the other, and separated by a ridge. The anterior of these two impressions, the colic impression, is shallow and is produced by the right colic flexure; the posterior, the renal impression, is deeper and is occupied by the upper part of the right kidney and lower part of the right suprarenal gland. Medial to the renal impression is a third and slightly marked impression, lying between it and the neck of the gall-bladder. This is caused by the descending portion of the duodenum, and is known as the duodenal impression. Just in front of the inferior vena cava is a narrow strip of liver tissue, the caudate process, which connects the right inferior angle of the caudate lobe to the under surface of the right lobe. It forms the upper boundary of the epiploic foramen of the peritoneum.

: : : : : : image1087

 

Posterior and inferior surfaces of the liver.

 

  We distinguish the convex diaphragmatic surface of the liver and lower visceral surface. Visceral surface adjoins to the organs, which form on surface of the liver suitable tracks: renal, adrenal, gastric, duodenal, oesophageal and colic impressions. Diaphragmatic surface carries cardiac impression.

Liver is almost entirely covered with peritoneum except posteriorly positioned area nuda. The superior surface is attached to the diaphragm and anterior abdominal wall by a fold of peritoneum, the falciform ligament, in the free margin of which is a rounded cord, the ligamentum teres (obliterated umbilical vein). The liver is connected to the lower surface of the diaphragm by the coronal ligament and the right and left triangular ligaments. The falciform ligament conventionally separates greater right lobe of liver and lesser left lobe of liver.

The porta hepatis, the entrance into the liver forms a cross-connection between the sagittal grooves which together are shaped like an H. Visceral surface carries furrows: right sagittal sulcus and left sagittal sulcus, which communicate by transversal sulcus (is called porta hepatis). Left sagittal sulcus anteriorly contain fissure of teres ligament, where umbilical vein in foetus passes. It obliterates in adult and forms teres liver ligament. Posterior portion of left sagittal sulcus is formed by fissura of venous ligament (obliterated venous duct of Arantii). Right sagittal sulcus anteriorly contains fossa of gall bladder, and behind - sulcus of inferior vena cava. Vena portae, proper hepatic artery and nerves enter through the porta hepatis into liver, common hepatic duct and lymphatic vessels leave the parenchyma in this place. Sagittal and transversal sulcuses limit the quadrate lobe, positioned ventrally and caudate lobe, disposed dorsally. Caudate lobe carries papillary and caudate processes.

The liver is held together by a tense connective tissue capsule Glisson 's capsule. At the porta it separates the lobules of liver. The lobules form the chief mass of the hepatic substance. Branches of portal vein, hepatic artery and biliary duct form a hepatic triad are situated in stratums between liver lobules.

Unlike all other organs a liver obtains arterial blood from proper hepatic artery and venous - from portal vein. Entering into liver porta, a portal vein and hepatic artery disintegrate into right and left lobar, segmental and lobular veins and arteries, which pass along interlobular bile duct. Capillaries from these vessels joining together form sinusoid capillaries that receive mixed blood and empty into a central vein, which occupies the centre of the lobule. Central vein drains into hepatic veins, which leave the liver to end in the inferior vena cava. This system is called as wonderful venous liver net.

Hepatic cells hepatocytes excrete the bile, which get into bile canaliculi. Last pass to periphery emtpy into interlobular ductuli that form right hepatic duct and left hepatic duct (from right and left hepatic lobes). Common hepatic duct, which originated in porta, passes in hepatoduodenal ligament, meets the cystic duct and forms ductus choledochus. It flows together with pancreatic duct and forms common hepalopancreatic ampulla, which opens on major duodenal papilla. The ampulla may itself be closed by its own sphincter muscle, the sphincter ampullae (Oddi).

 

 

The posterior surface (facies posterior) is rounded and broad behind the right lobe, but narrow on the left. Over a large part of its extent it is not covered by peritoneum; this uncovered portion is about 7.5 cm. broad at its widest part, and is in direct contact with the diaphragm. It is marked off from the upper surface by the line of reflection of the upper layer of the coronary ligament, and from the under surface by the line of reflection of the lower layer of the coronary ligament. The central part of the posterior surface presents a deep concavity which is moulded on the vertebral column and crura of the diaphragm. To the right of this the inferior vena cava is lodged in its fossa between the uncovered area and the caudate lobe. Close to the right of this fossa and immediately above the renal impression is a small triangular depressed area, the suprarenal impression, the greater part of which is devoid of peritoneum; it lodges the right suprarenal gland. To the left of the inferior vena cava is the caudate lobe, which lies between the fossa for the vena cava and the fossa for the ductus venosus. Its lower end projects and forms part of the posterior boundary of the porta; on the right, it is connected with the under surface of the right lobe of the liver by theee caudate process, and on the left it presents an elevation, the papillary process. Its posterior surface rests upon the diaphragm, being separated from it merely by the upper part of the omental bursa. To the left of the fossa for the ductus venosus is a groove in which lies the antrum cardiacum of the esophagus.

  The anterior border (margo anterior) is thin and sharp, and marked opposite the attachment of the falciform ligament by a deep notch, the umbilical notch, and opposite the cartilage of the ninth rib by a second notch for the fundus of the gall-bladder. In adult males this border generally corresponds with the lower margin of the thorax in the right mammillary line; but in women and children it usually projects below the ribs.

  The left extremity of the liver is thin and flattened from above downward.

 

Fossæ.The left sagittal fossa (fossa sagittalis sinistra; longitudinal fissure) is a deep groove, which extends from the notch on the anterior margin of the liver to the upper border of the posterior surface of the organ; it separates the right and left lobes. The porta joins it, at right angles, and divides it into two parts. The anterior part, or fossa for the umbilical vein, lodges the umbilical vein in the fetus, and its remains (the ligamentum teres) in the adult; it lies between the quadrate lobe and the left lobe of the liver, and is often partially bridged over by a prolongation of the hepatic substance, the pons hepatis. The posterior part, or fossa for the ductus venosus, lies between the left lobe and the caudate lobe; it lodges in the fetus, the ductus venosus, and in the adult a slender fibrous cord, the ligamentum venosum, the obliterated remains of that vessel.

  The porta or transverse fissure (porta hepatis) is a short but deep fissure, about 5 cm. long, extending transversely across the under surface of the left portion of the right lobe, nearer its posterior surface than its anterior border. It joins nearly at right angles with the left sagittal fossa, and separates the quadrate lobe in front from the caudate lobe and process behind. It transmits the portal vein, the hepatic artery and nerves, and the hepatic duct and lymphatics. The hepatic duct lies in front and to the right, the hepatic artery to the left, and the portal vein behind and between the duct and artery.

  The fossa for the gall-bladder (fossa vesicæ felleæ) is a shallow, oblong fossa, placed on the under surface of the right lobe, parallel with the left sagittal fossa. It extends from the anterior free margin of the liver, which is notched by it, to the right extremity of the porta.

  The fossa for the inferior vena cava (fossa venæ cavæ) is a short deep depression, occasionally a complete canal in consequence of the substance of the liver surrounding the vena cava. It extends obliquely upward on the posterior surface between the caudate lobe and the bare area of the liver, and is separated from the porta by the caudate process. On slitting open the inferior vena cava the orifices of the hepatic veins will be seen opening into this vessel at its upper part, after perforating the floor of this fossa.

 

Lobes.The right lobe (lobus hepatis dexter) is much larger than the left; the proportion between them being as six to one. It occupies the right hypochondrium, and is separated from the left lobe on its upper surface by the falciform ligament; on its under and posterior surfaces by the left sagittal fossa; and in front by the umbilical notch. It is of a somewhat quadrilateral form, its under and posterior surfaces being marked by three fossæ: the porta and the fossæ for the gall-bladder and inferior vena cava, which separate its left part into two smaller lobes; the quadrate and caudate lobes. The impressions on the right lobe have already been described.

  The quadrate lobe (lobus quadratus) is situated on the under surface of the right lobe, bounded in front by the anterior margin of the liver; behind by the porta; on the right, by the fossa for the gall-bladder; and on the left, by the fossa for the umbilical vein. It is oblong in shape, its antero-posterior diameter being greater than its transverse.

  The caudate lobe (lobus caudatus; Spigelian lobe) is situated upon the posterior surface of the right lobe of the liver, opposite the tenth and eleventh thoracic vertebræ. It is bounded, below, by the porta; on the right, by the fossa for the inferior vena cava; and, on the left, by the fossa for the ductus venosus. It looks backward, being nearly vertical in position; it is longer from above downward than from side to side, and is somewhat concave in the transverse direction. The caudate process is a small elevation of the hepatic substance extending obliquely lateralward, from the lower extremity of the caudate lobe to the under surface of the right lobe. It is situated behind the porta, and separates the fossa for the gall-bladder from the commencement of the fossa for the inferior vena cava.

  The left lobe (lobus hepatis sinister) is smaller and more flattened than the right. It is situated in the epigastric and left hypochondriac regions. Its upper surface is slightly convex and is moulded on to the diaphragm; its under surface presents the gastric impression and omental tuberosity, already referred to page 1189.

 

Ligaments.The liver is connected to the under surface of the diaphragm and to the anterior wall of the abdomen by five ligaments; four of thesethe falciform, the coronary, and the two lateralare peritoneal folds; the fifth, the round ligament, is a fibrous cord, the obliterated umbilical vein. The liver is also attached to the lesser curvature of the stomach by the hepatogastric and to the duodenum by the hepatoduodenal ligament (see page 1157).

  The falciform ligament (ligamentum falciforme hepatis) is a broad and thin antero-posterior peritoneal fold, falciform in shape, its base being directed downward and backward, its apex upward and backward. It is situated in an antero-posterior plane, but lies obliquely so that one surface faces forward and is in contact with the peritoneum behind the right Rectus and the diaphragm, while the other is directed backward and is in contact with the left lobe of the liver. It is attached by its left margin to the under surface of the diaphragm, and the posterior surface of the sheath of the right Rectus as low down as the umbilicus; by its right margin it extends from the notch on the anterior margin of the liver, as far back as the posterior surface. It is composed of two layers of peritoneum closely united together. Its base or free edge contains between its layers the round ligament and the parumbilical veins.

  The coronary ligament (ligamentum coronarium hepatis) consists of an upper and a lower layer. The upper layer is formed by the reflection of the peritoneum from the upper margin of the bare area of the liver to the under surface of the diaphragm, and is continuous with the right layer of the falciform ligament. The lower layer is reflected from the lower margin of the bare area on to the right kidney and suprarenal gland, and is termed the hepatorenal ligament.

  The triangular ligaments (lateral ligaments) are two in number, right and left. The right triangular ligament (ligamentum triangulare dextrum) is situated at the right extremity of the bare area, and is a small fold which passes to the diaphragm, being formed by the apposition of the upper and lower layers of the coronary ligament. The left triangular ligament (ligamentum triangulare sinistrum) is a fold of some considerable size, which connects the posterior part of the upper surface of the left lobe to the diaphragm; its anterior layer is continuous with the left layer of the falciform ligament.

  The round ligament (ligamentum teres hepatis) is a fibrous cord resulting from the obliteration of the umbilical vein. It ascends from the umbilicus, in the free margin of the falciform ligament, to the umbilical notch of the liver, from which it may be traced in its proper fossa on the inferior surface of the liver to the porta, where it becomes continuous with the ligamentum venosum.

 

Fixation of the Liver.Several factors contribute to maintain the liver in place. The attachments of the liver to the diaphragm by the coronary and triangular ligaments and the intervening connective tissue of the uncovered area, together with the intimate connection of the inferior vena cava by the connective tissue and hepatic veins would hold up the posterior part of the liver. Some support is derived from the pressure of the abdominal viscera which completely fill the abdomen whose muscular walls are always in a state of tonic contraction. The superior surface of the liver is perfectly fitted to the under surface of the diaphragm so that atmospheric pressure alone would be enough to hold it against the diaphragm. The latter in turn is held up by the negative pressure in the thorax. The lax falciform ligament certainly gives no support though it probably limits lateral displacement.

 

: : : : : : image1088

 

Liver with the septum transversum. Human embryo 3 mm. long.

 

Biliary flow

The biliary tree

 

The term biliary tree is derived from the arboreal branches of the bile ducts. The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts. Within the liver, these ducts are called intrahepatic (within the liver) bile ducts, and once they exit the liver they are considered extrahepatic (outside the liver). The intrahepatic ducts eventually drain into the right and left hepatic ducts, which merge to form the common hepatic duct. The cystic duct from the gallbladder joins with the common hepatic duct to form the common bile duct.

 

Bile either drains directly into the duodenum via the common bile duct, or be temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the second part of the duodenum together at the ampulla of Vater.

Ligaments.The liver is connected to the under surface of the diaphragm and to the anterior wall of the abdomen by five ligaments; four of thesethe falciform, the coronary, and the two lateralare peritoneal folds; the fifth, the round ligament, is a fibrous cord, the obliterated umbilical vein. The liver is also attached to the lesser curvature of the stomach by the hepatogastric and to the duodenum by the hepatoduodenal ligament (see page 1157).

  The falciform ligament (ligamentum falciforme hepatis) is a broad and thin antero-posterior peritoneal fold, falciform in shape, its base being directed downward and backward, its apex upward and backward. It is situated in an antero-posterior plane, but lies obliquely so that one surface faces forward and is in contact with the peritoneum behind the right Rectus and the diaphragm, while the other is directed backward and is in contact with the left lobe of the liver. It is attached by its left margin to the under surface of the diaphragm, and the posterior surface of the sheath of the right Rectus as low down as the umbilicus; by its right margin it extends from the notch on the anterior margin of the liver, as far back as the posterior surface. It is composed of two layers of peritoneum closely united together. Its base or free edge contains between its layers the round ligament and the parumbilical veins.

  The coronary ligament (ligamentum coronarium hepatis) consists of an upper and a lower layer. The upper layer is formed by the reflection of the peritoneum from the upper margin of the bare area of the liver to the under surface of the diaphragm, and is continuous with the right layer of the falciform ligament. The lower layer is reflected from the lower margin of the bare area on to the right kidney and suprarenal gland, and is termed the hepatorenal ligament.

  The triangular ligaments (lateral ligaments) are two in number, right and left. The right triangular ligament (ligamentum triangulare dextrum) is situated at the right extremity of the bare area, and is a small fold which passes to the diaphragm, being formed by the apposition of the upper and lower layers of the coronary ligament. The left triangular ligament (ligamentum triangulare sinistrum) is a fold of some considerable size, which connects the posterior part of the upper surface of the left lobe to the diaphragm; its anterior layer is continuous with the left layer of the falciform ligament.

  The round ligament (ligamentum teres hepatis) is a fibrous cord resulting from the obliteration of the umbilical vein. It ascends from the umbilicus, in the free margin of the falciform ligament, to the umbilical notch of the liver, from which it may be traced in its proper fossa on the inferior surface of the liver to the porta, where it becomes continuous with the ligamentum venosum.

 

Fixation of the Liver.Several factors contribute to maintain the liver in place. The attachments of the liver to the diaphragm by the coronary and triangular ligaments and the intervening connective tissue of the uncovered area, together with the intimate connection of the inferior vena cava by the connective tissue and hepatic veins would hold up the posterior part of the liver. Some support is derived from the pressure of the abdominal viscera which completely fill the abdomen whose muscular walls are always in a state of tonic contraction. The superior surface of the liver is perfectly fitted to the under surface of the diaphragm so that atmospheric pressure alone would be enough to hold it against the diaphragm. The latter in turn is held up by the negative pressure in the thorax. The lax falciform ligament certainly gives no support though it probably limits lateral displacement.

 

 

Functional anatomy

Correspondence between anatomic lobes and Couinaud segmentsSegment*

The central area where the common bile duct, hepatic portal vein, and hepatic artery proper enter is the hilum or "porta hepatis". The duct, vein, and artery divide into left and right branches, and the portions of the liver supplied by these branches constitute the functional left and right lobes.

 

The functional lobes are separated by an imaginary plane (historically called Cantlie's line) joining the gallbladder fossa to the inferior vena cava. The plane separates the liver into the true right and left lobes. The middle hepatic vein also demarcates the true right and left lobes. The right lobe is further divided into an anterior and posterior segment by the right hepatic vein. The left lobe is divided into the medial and lateral segments by the left hepatic vein. The fissure for the ligamentum teres also separates the medial and lateral segments. The medial segment is also called the quadrate lobe. In the widely used Couinaud (or "French") system, the functional lobes are further divided into a total of eight subsegments based on a transverse plane through the bifurcation of the main portal vein. The caudate lobe is a separate structure which receives blood flow from both the right- and left-sided vascular branches.

 

Synthesis

Further information: Proteins produced and secreted by the liver

A large part of amino acid synthesis

The liver performs several roles in carbohydrate metabolism:

Gluconeogenesis (the synthesis of glucose from certain amino acids, lactate or glycerol)

Glycogenolysis (the breakdown of glycogen into glucose)

Glycogenesis (the formation of glycogen from glucose)(muscle tissues can also do this)

The liver is responsible for the mainstay of protein metabolism, synthesis as well as degradation.

The liver also performs several roles in lipid metabolism:

Cholesterol synthesis

Lipogenesis, the production of triglycerides (fats).

A bulk of the lipoproteins are synthesized in the liver.

The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and XI, as well as protein C, protein S and antithrombin.

In the first trimester fetus, the liver is the main site of red blood cell production. By the 32nd week of gestation, the bone marrow has almost completely taken over that task.

The liver produces and excretes bile (a yellowish liquid) required for emulsifying fats and help the absorption of vitamin K from the diet. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder.

The liver also produces insulin-like growth factor 1 (IGF-1), a polypeptide protein hormone that plays an important role in childhood growth and continues to have anabolic effects in adults.

The liver is a major site of thrombopoietin production. Thrombopoietin is a glycoprotein hormone that regulates the production of platelets by the bone marrow.

 

Breakdown

The breakdown of insulin and other hormones

The liver glucoronidates bilirubin, facilitating its excretion into bile.

The liver breaks down or modifies toxic substances (e.g., methylation) and most medicinal products in a process called drug metabolism. This sometimes results in toxication, when the metabolite is more toxic than its precursor. Preferably, the toxins are conjugated to avail excretion in bile or urine.

The liver converts ammonia to urea (urea cycle).

 

Other functions

The liver stores a multitude of substances, including glucose (in the form of glycogen), vitamin A (12 years' supply), vitamin D (14 months' supply)[citation needed], vitamin B12 (13 years' supply), vitamin K, iron, and copper.

The liver is responsible for immunological effectsthe reticuloendothelial system of the liver contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the portal system.

The liver produces albumin, the major osmolar component of blood serum.

The liver synthesizes angiotensinogen, a hormone that is responsible for raising the blood pressure when activated by renin, an enzyme that is

 

Development.The liver arises in the form of a diverticulum or hollow outgrowth from the ventral surface of that portion of the gut which afterward becomes the descending part of the duodenum. This diverticulum is lined by entoderm, and grows upward and forward into the septum transversum, a mass of mesoderm between the vitelline duct and the pericardial cavity, and there gives off two solid buds of cells which represent the right and the left lobes of the liver. The solid buds of cells grow into columns or cylinders, termed the hepatic cylinders, which branch and anastomose to form a close meshwork. This network invades the vitelline and umbilical veins, and breaks up these vessels into a series of capillary-like vessels termed sinusoids (Minot), which ramify in the meshes of the cellular network and ultimately form the venous capillaries of the liver. By the continued growth and ramification of the hepatic cylinders the mass of the liver is gradually formed. The original diverticulum from the duodenum forms the common bileduct, and from this the cystic duct and gall-bladder arise as a solid outgrowth which later acquires a lumen. The opening of the common duct is at first in the ventral wall of the duodenum; later, owing to the rotation of the gut, the opening is carried to the left and then dorsalward to the position it occupies in the adult.

  As the liver undergoes enlargement, both it and the ventral mesogastrium of the fore-gut are gradually differentiated from the septum transversum; and from the under surface of the latter the liver projects downward into the abdominal cavity. By the growth of the liver the ventral mesogastrium is divided into two parts, of which the anterior forms the falciform and coronary ligaments, and the posterior the lesser omentum. About the third month the liver almost fills the abdominal cavity, and its left lobe is nearly as large as its right. From this period the relative development of the liver is less active, more especially that of the left lobe, which actually undergoes some degeneration and becomes smaller than the right; but up to the end of fetal life the liver remains relatively larger than in the adult.

 

: : : : : : image1089

Longitudinal section of a hepatic vein.

 

 

: : : : : : image1090

 

Longitudinal section of a small portal vein and canal.

 

Vessels and Nerves.The vessels connected with the liver are: the hepatic artery, the portal vein, and the hepatic veins.

  The hepatic artery and portal vein, accompanied by numerous nerves, ascend to the porta, between the layers of the lesser omentum. The bile duct and the lymphatic vessels descend from the porta between the layers of the same omentum. The relative positions of the three structures are as follows: the bile duct lies to the right, the hepatic artery to the left, and the portal vein behind and between the other two. They are enveloped in a loose areolar tissue, the fibrous capsule of Glisson, which accompanies the vessels in their course through the portal canals in the interior of the organ.

  The hepatic veins convey the blood from the liver, and are described on page 680. They have very little cellular investment, and what there is binds their parietes closely to the walls of the canals through which they run; so that, on section of the organ, they remain widely open and are solitary, and may be easily distinguished from the branches of the portal vein, which are more or less collapsed, and always accompanied by an artery and duct.

  The lymphatic vessels of the liver are described on page 711.

  The nerves of the liver, derived from the left vagus and sympathetic, enter at the porta and accompany the vessels and ducts to the interlobular spaces. Here, according to Korolkow, the medullated fibers are distributed almost exclusively to the coats of the bloodvessels; while the non-medullated enter the lobules and ramify between the cells and even within them. 

 

: : : : : : image1091

 

Section of injected liver

  

Structure of the Liver.The substance of the liver is composed of lobules, held together by an extremely fine areolar tissue, in which ramify the portal vein, hepatic ducts, hepatic artery, hepatic veins, lymphatics, and nerves; the whole being invested by a serous and a fibrous coat.

  The serous coat (tunica serosa) is derived from the peritoneum, and invests the greater part of the surface of the organ. It is intimately adherent to the fibrous coat.

  The fibrous coat (capsula fibrosa [Glissoni]; areolar coat) lies beneath the serous investment, and covers the entire surface of the organ. It is difficult of demonstration, excepting where the serous coat is deficient. At the porta it is continuous with the fibrous capsule of Glisson, and on the surface of the organ with the areolar tissue separating the lobules.

  The lobules (lobuli hepatis) form the chief mass of the hepatic substance; they may be seen either on the surface of the organ, or by making a section through the gland, as small granular bodies, about the size of a millet-seed, measuring from 1 to 2.5 mm. in diameter. In the human subject their outlines are very irregular; but in some of the lower animals (for example, the pig) they are well-defined, and, when divided transversely, have polygonal outlines. The bases of the lobules are clustered around the smallest radicles (sublobular) of the hepatic veins, to which each is connected by means of a small branch which issues from the center of the lobule (intralobular). The remaining part of the surface of each lobule is imperfectly isolated from the surrounding lobules by a thin stratum of areolar tissue, in which is contained a plexus of vessels, the interlobular plexus, and ducts. In some animals, as the pig, the lobules are completely isolated from one another by the interlobular areolar tissue.

The liver stores a multitude of substances, including glucose (in the form of glycogen), vitamin A (12 years' supply), vitamin D (14 months' supply)[citation needed], vitamin B12 (13 years' supply), vitamin K, iron, and copper.

The liver is responsible for immunological effectsthe reticuloendothelial system of the liver contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the portal system.

The liver produces albumin, the major osmolar component of blood serum.

The liver synthesizes angiotensinogen, a hormone that is responsible for raising the blood pressure when activated by renin, an enzyme that is

 

Development.The liver arises in the form of a diverticulum or hollow outgrowth from the ventral surface of that portion of the gut which afterward becomes the descending part of the duodenum. This diverticulum is lined by entoderm, and grows upward and forward into the septum transversum, a mass of mesoderm between the vitelline duct and the pericardial cavity, and there gives off two solid buds of cells which represent the right and the left lobes of the liver. The solid buds of cells grow into columns or cylinders, termed the hepatic cylinders, which branch and anastomose to form a close meshwork. This network invades the vitelline and umbilical veins, and breaks up these vessels into a series of capillary-like vessels termed sinusoids (Minot), which ramify in the meshes of the cellular network and ultimately form the venous capillaries of the liver. By the continued growth and ramification of the hepatic cylinders the mass of the liver is gradually formed. The original diverticulum from the duodenum forms the common bileduct, and from this the cystic duct and gall-bladder arise as a solid outgrowth which later acquires a lumen. The opening of the common duct is at first in the ventral wall of the duodenum; later, owing to the rotation of the gut, the opening is carried to the left and then dorsalward to the position it occupies in the adult.

  As the liver undergoes enlargement, both it and the ventral mesogastrium of the fore-gut are gradually differentiated from the septum transversum; and from the under surface of the latter the liver projects downward into the abdominal cavity. By the growth of the liver the ventral mesogastrium is divided into two parts, of which the anterior forms the falciform and coronary ligaments, and the posterior the lesser omentum. About the third month the liver almost fills the abdominal cavity, and its left lobe is nearly as large as its right. From this period the relative development of the liver is less active, more especially that of the left lobe, which actually undergoes some degeneration and becomes smaller than the right; but up to the end of fetal life the liver remains relatively larger than in the adult.

 

: : : : : : image1089

Longitudinal section of a hepatic vein.

 

 

: : : : : : image1090

 

Longitudinal section of a small portal vein and canal.

 

Vessels and Nerves.The vessels connected with the liver are: the hepatic artery, the portal vein, and the hepatic veins.

  The hepatic artery and portal vein, accompanied by numerous nerves, ascend to the porta, between the layers of the lesser omentum. The bile duct and the lymphatic vessels descend from the porta between the layers of the same omentum. The relative positions of the three structures are as follows: the bile duct lies to the right, the hepatic artery to the left, and the portal vein behind and between the other two. They are enveloped in a loose areolar tissue, the fibrous capsule of Glisson, which accompanies the vessels in their course through the portal canals in the interior of the organ.

  The hepatic veins convey the blood from the liver, and are described on page 680. They have very little cellular investment, and what there is binds their parietes closely to the walls of the canals through which they run; so that, on section of the organ, they remain widely open and are solitary, and may be easily distinguished from the branches of the portal vein, which are more or less collapsed, and always accompanied by an artery and duct.

  The lymphatic vessels of the liver are described on page 711.

  The nerves of the liver, derived from the left vagus and sympathetic, enter at the porta and accompany the vessels and ducts to the interlobular spaces. Here, according to Korolkow, the medullated fibers are distributed almost exclusively to the coats of the bloodvessels; while the non-medullated enter the lobules and ramify between the cells and even within them. 

 

: : : : : : image1091

 

Section of injected liver

  

Structure of the Liver.The substance of the liver is composed of lobules, held together by an extremely fine areolar tissue, in which ramify the portal vein, hepatic ducts, hepatic artery, hepatic veins, lymphatics, and nerves; the whole being invested by a serous and a fibrous coat.

  The serous coat (tunica serosa) is derived from the peritoneum, and invests the greater part of the surface of the organ. It is intimately adherent to the fibrous coat.

  The fibrous coat (capsula fibrosa [Glissoni]; areolar coat) lies beneath the serous investment, and covers the entire surface of the organ. It is difficult of demonstration, excepting where the serous coat is deficient. At the porta it is continuous with the fibrous capsule of Glisson, and on the surface of the organ with the areolar tissue separating the lobules.

  The lobules (lobuli hepatis) form the chief mass of the hepatic substance; they may be seen either on the surface of the organ, or by making a section through the gland, as small granular bodies, about the size of a millet-seed, measuring from 1 to 2.5 mm. in diameter. In the human subject their outlines are very irregular; but in some of the lower animals (for example, the pig) they are well-defined, and, when divided transversely, have polygonal outlines. The bases of the lobules are clustered around the smallest radicles (sublobular) of the hepatic veins, to which each is connected by means of a small branch which issues from the center of the lobule (intralobular). The remaining part of the surface of each lobule is imperfectly isolated from the surrounding lobules by a thin stratum of areolar tissue, in which is contained a plexus of vessels, the interlobular plexus, and ducts. In some animals, as the pig, the lobules are completely isolated from one another by the interlobular areolar tissue

: : : : : : image1092

 

A single lobule of the liver

 

  If one of the sublobular veins be laid open, the bases of the lobules may be seen through the thin wall of the vein on which they rest, arranged in a form resembling a tesselated pavement, the center of each polygonal space presenting a minute aperture, the mouth of an intralobular vein .

  Microscopic AppearanceEach lobule consists of a mass of cells, hepatic cells, arranged in irregular radiating columns between which are the blood channels (sinusoids). These convey the blood from the circumference to the center of the lobule, and end in the intralobular vein, which runs through its center, to open at its base into one of the sublobular veins. Between the cells are also the minute bile capillaries. Therefore, in the lobule there are all the essentials of a secreting gland; that is to say: (1) cells, by which the secretion is formed; (2) bloodvessels, in close relation with the cells, containing the blood from which the secretion is derived; (3) ducts, by which the secretion, when formed, is carried away.

  1. The hepatic cells are polyhedral in form. They vary in size from 12 to 25μ in diameter. They contain one or sometimes two distinct nuclei. The nucleus exhibits an intranuclear network and one or two refractile nucleoli. The cells usually contain granules; some of which are protoplasmic, while others consist of glycogen, fat, or an iron compound. In the lower vertebrates, e.g., frog, the cells are arranged in tubes with the bile duct forming the lumen and bloodvessels externally. According to Delépine, evidences of this arrangement can be found in the human liver.

  2. The Bloodvessels.The blood in the capillary plexus around the liver cells is brought to the liver principally by the portal vein, but also to a certain extent by the hepatic artery.

  The hepatic artery, entering the liver at the porta with the portal vein and hepatic duct, ramifies with these vessels through the portal canals. It gives off vaginal branches, which ramify in the fibrous capsule of Glisson, and appear to be destined chiefly for the nutrition of the coats of the vessels and ducts. It also gives off capsular branches, which reach the surface of the organ, ending in its fibrous coat in stellate plexuses. Finally, it gives off interlobular branches, which form a plexus outside each lobule, to supply the walls of the interlobular veins and the accompanying bile ducts. From this plexus lobular branches enter the lobule and end in the net-work of sinusoids between the cells.

  The portal vein also enters at the porta, and runs through the portal canals, enclosed in Glissons capsule, dividing in its course into branches, which finally break up into a plexus, the interlobular plexus, in the interlobular spaces. These branches receive the vaginal and capsular veins, corresponding to the vaginal and capsular branches of the hepatic artery. Thus it will be seen that all the blood carried to the liver by the portal vein and hepatic artery finds its way into the interlobular plexus. From this plexus the blood is carried into the lobule by fine branches which converge from the circumference to the center of the lobule, and are connected by transverse branches. The walls of these small vessels are incomplete so that the blood is brought into direct relationship with the liver cells. The lining endothelium consists of irregularly branched, disconnected cells (stellate cells of Kupffer). Moreover, according to Herring and Simpson, minute channels penetrate the liver cells themselves, conveying the constituents of the blood into their substance. It will be seen that the blood capillaries of the liver lobule differ structurally from capillaries elsewhere. Developmentally they are formed by the growth of the columns of liver cells into large blood spaces or sinuses, and hence they have received the name of sinusoids. Arrived at the center of the lobule, the sinusoids empty themselves into one vein, of considerable size, which runs down the center of the lobule from apex to base, and is called the intralobular vein. At the base of the lobule this vein opens directly into the sublobular vein, with which the lobule is connected. The sublobular veins unite to form larger and larger trunks, and end at last in the hepatic veins, these converge to form three large trunks which open into the inferior vena cava while that vessel is situated in its fossa on the posterior surface of the liver.

  3. The bile ducts commence by little passages in the liver cells which communicate with canaliculi termed intercellular biliary passages (bile capillaries). These passages are merely little channels or spaces left between the contiguous surfaces of two cells, or in the angle where three or more liver cells meet, and they are always separated from the blood capillaries by at least half the width of a liver cell. The channels thus formed radiate to the circumference of the lobule, and open into the interlobular bile ducts which run in Glissons capsule, accompanying the portal vein and hepatic artery. These join with other ducts to form two main trunks, which leave the liver at the transverse fissure, and by their union form the hepatic duct.

  Structure of the Ducts.The walls of the biliary ducts consist of a connective-tissue coat, in which are muscle cells, arranged both circularly and longitudinally, and an epithelial layer, consisting of short columnar cells resting on a distinct basement membrane.

 

Excretory Apparatus of the Liver.The excretory apparatus of the liver consists of (1) the hepatic duct, formed by the junction of the two main ducts, which pass out of the liver at the porta; (2) the gall-bladder, which serves as a reservoir for the bile; (3) the cystic duct, or the duct of the gall-bladder; and (4) the common bile duct, formed by the junction of the hepatic and cystic ducts.

 

The Hepatic Duct (ductus hepaticus).Two main trunks of nearly equal size issue from the liver at the porta, one from the right, the other from the left lobe; these unite to form the hepatic duct, which passes downward and to the right for about 4 cm., between the layers of the lesser omentum, where it is joined at an acute angle by the cystic duct, and so forms the common bile duct. The hepatic duct is accompanied by the hepatic artery and portal vein.

  The Gall-bladder (vesica fellea) is a conical or pear-shaped musculomembranous sac, lodged in a fossa on the under surface of the right lobe of the liver, and extending from near the right extremity of the porta to the anterior border of the organ. It is from 7 to 10 cm. in length, 2.5 cm. in breadth at its widest part, and holds from 30 to 35 c.c. It is divided into a fundus, body, and neck. The fundus, or broad extremity, is directed downward, forward, and to the right, and projects beyond the anterior border of the liver; the body and neck are directed upward and backward to the left. The upper surface of the gall-bladder is attached to the liver by connective tissue and vessels. The under surface is covered by peritoneum, which is reflected on to it from the surface of the liver. Occasionally the whole of the organ is invested by the serous membrane, and is then connected to the liver by a kind of mesentery.

 

: : : : : : image1095

The gall-bladder and bile ducts laid open.

 

 Relations.The body is in relation, by its upper surface, with the liver; by its under surface, with the commencement of the transverse colon; and farther back usually with the upper end of the descending portion of the duodenum, but sometimes with the superior portion of the duodenum or pyloric end of the stomach. The fundus is completely invested by peritoneum; it is in relation, in front, with the abdominal parietes, immediately below the ninth costal cartilage; behind with the transverse colon. The neck is narrow, and curves upon itself like the letter S; at its point of connection with the cystic duct it presents a well-marked constriction.

 

StructureThe gall-bladder consists of three coats: serous, fibromuscular, and mucous.

  The external or serous coat (tunica serosa vesicæ felleæ) is derived from the peritoneum; it completely invests the fundus, but covers the body and neck only on their under surfaces.

  The fibromuscular coat (tunica muscularis vesicæ felleæ), a thin but strong layer forming the frame-work of the sac, consists of dense fibrous tissue, which interlaces in all directions, and is mixed with plain muscular fibers, disposed chiefly in a longitudinal direction, a few running transversely.

  The internal or mucous coat (tunica mucosa vesicæ felleæ) is loosely connected with the fibrous layer. It is generally of a yellowish-brown color, and is elevated into minute rugæ. Opposite the neck of the gall-bladder the mucous membrane projects inward in the form of oblique ridges or folds, forming a sort of spiral valve.

  The mucous membrane is continuous through the hepatic duct with the mucous membrane lining the ducts of the liver, and through the common bile duct with the mucous membrane of the duodenum. It is covered with columnar epithelium, and secretes mucin; in some animals it secretes a nucleoprotein instead of mucin.

  The Cystic Duct (ductus cysticus).The cystic duct about 4 cm. long, runs backward, downward, and to the left from the neck of the gall-bladder, and joins the hepatic duct to form the common bile duct. The mucous membrane lining its interior is thrown into a series of crescentic folds, from five to twelve in number, similar to those found in the neck of the gall-bladder. They project into the duct in regular succession, and are directed obliquely around the tube, presenting much the appearance of a continuous spiral valve. When the duct is distended, the spaces between the folds are dilated, so as to give to its exterior a twisted appearance.

  The Common Bile Duct (ductus choledochus).The common bile duct is formed by the junction of the cystic and hepatic ducts; it is about 7.5 cm. long, and of the diameter of a goose-quill.

  It descends along the right border of the lesser omentum behind the superior portion of the duodenum, in front of the portal vein, and to the right of the hepatic artery; it then runs in a groove near the right border of the posterior surface of the head of the pancreas; here it is situated in front of the inferior vena cava, and is occasionally completely imbedded in the pancreatic substance. At its termination it lies for a short distance along the right side of the terminal part of the pancreatic duct and passes with it obliquely between the mucous and muscular coats. The two ducts unite and open by a common orifice upon the summit of the duodenal papilla, situated at the medial side of the descending portion of the duodenum, a little below its middle and about 7 to 10 cm. from the pylorus . The short tube formed by the union of the two ducts is dilated into an ampulla, the ampulla of Vater.

 

Structure.The coats of the large biliary ducts are an external or fibrous, and an internal or mucous. The fibrous coat is composed of strong fibroareolar tissue, with a certain amount of muscular tissue, arranged, for the most part, in a circular manner around the duct. The mucous coat is continuous with the lining membrane of the hepatic ducts and gall-bladder, and also with that of the duodenum; and, like the mucous membrane of these structures, its epithelium is of the columnar variety. It is provided with numerous mucous glands, which are lobulated and open by minute orifices scattered irregularly in the larger ducts.

 

 

A CT scan in which the liver and portal vein are shown.

 

Anterior MIP image of anomalous hepatic veins

VIDEO

 

The Gallbladder

 

is a pear-shaped, thin-walled bag, which collects up to 30-50 ml bile. We distinguish fundus, body and neck of gallbladder, which continues into cystic duct. The gallbladder lies in a fossa in the liver to which it is attached by connective tissue and covered by peritoneum from below (mesoperitoneal position). The lumen of the neck of the gallbladder and of its connections with the cystic duct is incompletely subdivided by spiral fold of mucosa, known as the spiral fold (Heisler's valve).

 

: : : : : : image1097

The pancreas is the most important intestinal gland. The pancreas is shaped like a horizontal wedge with its thin end on the left. The head is the thickest part, fills into the duodenal loop to the right of the spine. The horizontal body continues into tail. The pancreatic duct runs right through the length of the gland. It receives short, vertical tributaries from the lobules and has owns sphincter muscle of pancreatic duct. The pancreatic duct ends together with the common bile duct on the major duodenal papilla. If present, the accessory pancreatic duct ends above the bile duct on the minor duodenal papilla.

: : : : : : image1098

Topography of the pancreas. Pancreas lies in upper abdominal region behind the peritoneum (retroperitoneal position) at the level of the from 1st to 3d lumbar vertebrae. Along the upper margin of the pancreas runs the splenic artery. The right kidney and adrenal gland adjoin to body of pancreas. Anterior surface of gland touches the stomach, posterior surface inferior vena cava and aorta. Tail adjoins to splenic hilus.

: : : : : : image1099

 

The pancreas and duodenum from behind

 

Endocrine part of pancreas is represented by islets of Langerhans. They produce insulin and glucagon that regulate metabolism of carbohydrates, regulative a sugar contents in organism. Attached to insufficient production of these hormonal disease sugar diabetes arises.

 

 

Theme 2. Structure, topography and functon of large ntestne

 

The horseshoe-shaped large intestine begins at the end of the ileum and ends at the anus. The large intestine lies inferior to the stomach and liver and almost completely frames the small intestine. The major functions of the large intestine include (1) the reabsorption of water and compaction of intestinal contents into feces, (2) the absorption of important vitamins liberated by bacterial action, and (3) the storing of fecal material prior to defecation.

The large intestine, or the large bowel , has an average length of about 1.5 meters (5 ft) and a width of 7.5 cm (3 in.). We can divide it into three parts: (1) the pouchlike cecum, the first portion of the large intestine; (2) the colon, the largest portion; and (3) the rectum, the last 15 cm (6 in.) of the large intestine and the end of the digestive tract.

 

The Cecum

Cecum and ileum

Ileo-cecal valve

Material arriving from the ileum first enters an expanded pouch called the cecum. The ileum attaches to the medial surface of the cecum and opens into the cecum at the ileocecal valve. The cecum collects and stores chyme and begins the process of compaction. The slender, hollow vermiform appendix (vermis, worm), or simply appendix, is attached to the posteromedial surface of the cecum. The appendix is generally about 9 cm (3.5 in.) long, but its size and shape are quite variable. A small mesentery called the mesoappendix connects the appendix to the ileum and cecum. The mucosa and submucosa of the appendix are dominated by lymphoid nodules, and the appendix's primary function is as an organ of the lymphatic system. Inflammation of the appendix is known as appendicitis.

 

The Colon

Front of abdomen, showing surface markings for liver, stomach, and large intestine.

 

The colon has a larger diameter and a thinner wall than the small intestine. Distinctive features of the colon include the following:

        The wall of the colon forms a series of pouches, or haustra (singular, haustrum). Cutting into the intestinal lumen reveals that the creases between the haustra affect the mucosal lining as well, producing a series of internal folds. Haustra permit the expansion and elongation of the colon rather like the bellows that allow an accordion to lengthen.

        Three separate longitudinal ribbons of smooth musclethe taenia coli , are visible on the outer surfaces of the colon just beneath the serosa. These bands correspond to the outer layer of the muscularis externa in other portions of the digestive tract. Muscle tone within these bands creates the haustra.

        The serosa of the colon contains numerous teardrop-shaped sacs of fat called epiploic appendages.

 

VIDEO

 

Regions of the Colon

We can subdivide the colon into four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon:

1.     The ascending colon begins at the superior border of the cecum and ascends along the right lateral and posterior wall of the peritoneal cavity to the inferior surface of the liver. At this point, the colon makes a sharp bend to the left at the right colic flexure, or hepatic flexure. This flexure marks the end of the ascending colon and the beginning of the transverse colon. The ascending colon is retroperitoneal, and only its lateral and anterior surfaces are covered by the peritoneum.

2.     The transverse colon curves anteriorly from the right colic flexure and crosses the abdomen from right to left. It is supported by the transverse mesentery and is separated from the anterior abdominal wall by the layers of the greater omentum. As the transverse colon reaches the left side of the body, it passes inferior to the greater curvature of the stomach. Near the spleen, the colon makes a 90 turn at the left colic flexure, or splenic flexure, and becomes the descending colon.

3.     The descending colon proceeds inferiorly along the left side until reaching the iliac fossa. The descending colon is retroperitoneal and firmly attached to the abdominal wall. At the iliac fossa, the descending colon curves at the sigmoid flexure and becomes the sigmoid colon.

4.     The sigmoid flexure is the start of the sigmoid colon (sigmeidos, the Greek letter S), an S-shaped segment that is only about 15 cm (6 in.) long. It lies posterior to the urinary bladder, suspended from the sigmoid mesocolon. The sigmoid colon empties into the rectum.

The large intestine receives blood from tributaries of the superior mesenteric and inferior mesenteric arteries, and venous blood is collected by the superior mesenteric and inferior mesenteric veins.

The Rectum

The rectum forms the last 15 cm (6 in.) of the digestive tract. The rectum is an expandable organ for the temporary storage of fecal material. Movement of fecal materials into the rectum triggers the urge to defecate.

The last portion of the rectum, the anorectal canal, contains small longitudinal folds, the rectal columns. The distal margins of the rectal columns are joined by transverse folds that mark the boundary between the columnar epithelium of the proximal rectum and a stratified squamous epithelium like that in the oral cavity. Very close to the anus, or anal orifice (the exit of the anorectal canal), the epidermis becomes keratinized and identical to the surface of the skin.

There is a network of veins in the lamina propria and submucosa of the anorectal canal. If venous pressures there rise too high due to straining during defecation, the veins may become distended, producing hemorrhoids. The circular muscle layer of the muscularis externa in this region forms the internal anal sphincter. The smooth muscle cells of the internal anal sphincter are not under voluntary control. The external anal sphincter guards the anus. This sphincter, which consists of a ring of skeletal muscle fibers, is under voluntary control.

 

Although the diameter of the colon is roughly three times that of the small intestine, its wall is much thinner. The major characteristics of the colon are the lack of villi, the abundance of goblet cells, and the presence of distinctive intestinal glands. The glands in the large intestine are deeper than those of the small intestine, and they are dominated by goblet cells. The mucosa of the large intestine does not produce enzymes; any digestion that occurs results from enzymes introduced in the small intestine or from bacterial action. The mucus is important in providing lubrication as the fecal material becomes less moist and more compact. Mucous secretion occurs as local stimuli, such as friction or exposure to harsh chemicals, trigger short reflexes involving local nerve plexuses. Large lymphoid nodules are scattered throughout the lamina propria and submucosa.

The muscularis externa of the large intestine is unusual because the longitudinal layer has been reduced to the muscular bands of the taenia coli. However, the mixing and propulsive contractions of the colon resemble those of the small intestine.

 

LARGE INTESTINE extends from the end of the ileum to the anus. It is about 1.5 meters long, being one-fifth of the whole extent of the intestinal canal. It owns the special signs, which distinguish it from small intestine: 1) teniae colli - thickened bands of longitudinal muscle. There are tenia libera, omental tenia and mescolic tenia. Large intestine has 2) epiploic appendices located along teniae. Externally positioned sacculations between the teniae - 3) intestinal haustrae that relate to spaces between semilunar folds on internal surface.

The Cecum is situated in right iliac fossa, projected on right inguinal region. Cecum covered by peritoneum fully (intraperitoneal position) and does not have own mesentery. In place of gathering of three teniae ostium of worm-shaped 8-cm in length blind tube vermiform appendix disposes, which has own mesentery. Transition of ileum into cecum is ileocolic junction. Superior labium and inferior labium of this orifice communicate by frenulum, they form ileocolic valve, which serves for closing of ileocolic ostium and prevents return the chyme into ileum.

The Ascending colon is situated in right lateral abdominal area, covered by peritoneum from three sides (mesoperitoneal position), does not have an own mesentery. Transition into transverse colon generates a right flexure of colon, which adjoins to right lobe of the liver that is why called as hepatic flexure of colon.

The Transverse colon passes in abdominal cavity from the right to the left, so mobile portion of bowel because has mesentery (intraperitoneal position). Transition into descending colon forms left colic flexura, which adjoins to spleen (splenic flexura).

The Descending colon positioned in left lateral abdominal region, covered by peritoneum from three sides (mesoperitoneal position), without mesentery.

S-shaped Sigmoid colon contained in left iliac fossa; covered by peritoneum from all sides (intraperitoneally) and has a mesentery.

 

VIDEO

 

The Rectum is continuous above with the sigmoid colon, while below it ends in the anal canal. It forms two flexurae in sagittal plane: superior sacral flexura, responses concave anterior surface of sacrum and inferior anrectal flexura or perineal flexura, that is situated on transition of rectum into anal canal in pelvic perineal diaphragm. Also rectum has lateral flexurae in frontal plane. Upper broadened part of rectum called as the rectal ampulla. External layer of rectum wall is connective tissue adventitia and serous peritoneum; middle layer is muscularis, which has the longitudinal and circular fibres; mucous membrane forms the transverse folds of rectum (plicae transversae recti - superior, middle and inferior).

The Anal Canal is terminal portion of the large intestine, begins at the level of the apex of the prostate, is directed downward and backward, and ends at the anus. External sphincter muscle of anus (musculus sphincter ani externus) lies under skin. It formed by striated (voluntary controlled) muscles of perineum. Internal sphincter of anus (m. sphincter ani internus) positioned deeper. It is formed by continuation of the circular musclular layer of the intestine and built by smooth muscular fibres (involuntary). Above the external and the internal sphincter muscles lies the puborectal muscle (part of levalor ani muscle) is important muscle of the sphincter. Part of the pubococcygeal muscle also takes part in anal closure. The muscles are under permanent tension except during the act of defecation.

Mucous membrane of the anal canal forms 8-10 permanent longitudinal folds columne anales, between which are anal sinuses, which end below in small valve-like folds, termed anal valves. They join together the lower ends of the rectal columns. Submucous stratum in this area contains developed venous plexus hemorrhoidal plexus.

The rectum is covered by peritoneum above on its anterior surface and sides; below, on its anterior aspect only; the anal canal is entirely devoid of any serous covering.

 

 

Theme 3. Structure and topography of pertoneum, relations with organs

The Peritoneum (Tunica Serosa)The peritoneum is the largest serous membrane in the body, and consists, in the male, of a closed sac, a part of which is applied against the abdominal parietes,

while the remainder is reflected over the contained viscera. In the female the peritoneum is not a closed sac, since the free ends of the uterine tubes open directly into the peritoneal cavity. The part which lines the parietes is named the parietal portion of the peritoneum; that which is reflected over the contained viscera constitutes the visceral portion of the peritoneum. The free surface of the membrane is smooth, covered by a layer of flattened mesothelium, and lubricated by a small quantity of serous fluid. Hence the viscera can glide freely against the wall of the cavity or upon one another with the least possible amount of friction. The attached surface is rough, being connected to the viscera and inner surface of the parietes by means of areolar tissue, termed the subserous areolar tissue. The parietal portion is loosely connected with the fascial lining of the abdomen and pelvis, but is more closely adherent to the under surface of the diaphragm, and also in the middle line of the abdomen.

: : : : : : image1035

Vertical disposition of the peritoneum. Main cavity, red; omental bursa, blue.

 

  The space between the parietal and visceral layers of the peritoneum is named the peritoneal cavity; but under normal conditions this cavity is merely a potential one, since the parietal and visceral layers are in contact. The peritoneal cavity gives off a large diverticulum, the omental bursa, which is situated behind the stomach and adjoining structures; the neck of communication between the cavity and the bursa is termed the epiploic foramen (foramen of Winslow). Formerly the main portion of the cavity was described as the greater, and the omental bursa as the lesser sac.

  The peritoneum differs from the other serous membranes of the body in presenting a much more complex arrangement, and one that can be clearly understood only by following the changes which take place in the digestive tube during its development.

  To trace the membrane from one viscus to another, and from the viscera to the parietes, it is necessary to follow its continuity in the vertical and horizontal directions, and it will be found simpler to describe the main portion of the cavity and the omental bursa separately.

The PERITONEUM is serous membrane that covers the walls of abdominal cavity and viscera in abdomen and pelvis. It may be subdivided into parietal peritoneum and visceral peritoneum. Transition of parietal peritoneum into visceral peritoneum realizes by derivatives: ligament, mesentery and omentum. If organ covered by peritoneum from all sides, such position is called intraperitoneal; if from three sides - mesoperitoneal position; if only one side - extraperitoneal or retroperitoneal.

Abdominal cavity is limited:

        above - by diaphragm

        anteriorly and laterally - by muscles, fasciae, skin

        behind - by lumbar and sacral portions of backbone and lumbar muscles

        from below by bones, ligaments and muscles of pelvis.

Abdominal cavity contains the organs of digestive and urogenital systems and spleen.

Peritoneal cavity is complex of fissure between abdominal organs and walls lined by parietal and visceral sheets that contain serous liquid. It can be subdivided into superior storey and inferior storey, also cavity of lesser pelvis.

The abdomen is the largest cavity in the body. It is of an oval shape, the extremities of the oval being directed upward and downward. The upper extremity is formed by the diaphragm which extends as a dome over the abdomen, so that the cavity extends high into the bony thorax, reaching on the right side, in the mammary line, to the upper border of the fifth rib; on the left side it falls below this level by about 2.5 cm. The lower extremity is formed by the structures which clothe the inner surface of the bony pelvis, principally the Levator ani and Coccygeus on either side. These muscles are sometimes termed the diaphragm of the pelvis. The cavity is wider above than below, and measures more in the vertical than in the transverse diameter. In order to facilitate description, it is artificially divided into two parts: an upper and larger part, the abdomen proper; and a lower and smaller part, the pelvis. These two cavities are not separated from each other, but the limit between them is marked by the superior aperture of the lesser pelvis.

  The abdomen proper differs from the other great cavities of the body in being bounded for the most part by muscles and fasciæ, so that it can vary in capacity and shape according to the condition of the viscera which it contains; but, in addition to this, the abdomen varies in form and extent with age and sex. In the adult male, with moderate distension of the viscera, it is oval in shape, but at the same time flattened from before backward. In the adult female, with a fully developed pelvis, it is ovoid with the narrower pole upward, and in young children it is also ovoid but with the narrower pole downward.

 

Boundaries.It is bounded in front and at the sides by the abdominal muscles and the Iliacus muscles; behind by the vertebral column and the Psoas and Quadratus lumborum muscles; above by the diaphragm; below by the plane of the superior aperture of the lesser pelvis. The muscles forming the boundaries of the cavity are lined upon their inner surfaces by a layer of fascia.

  The abdomen contains the greater part of the digestive tube; some of the accessory organs to digestion, viz., the liver and pancreas; the spleen, the kidneys, and the suprarenal glands. Most of these structures, as well as the wall of the cavity in which they are contained, are more or less covered by an extensive and complicated serous membrane, the peritoneum.

 

The Apertures in the Walls of the Abdomen.The apertures in the walls of the abdomen, for the transmission of structures to or from it, are, in front, the umbilical (in the fetus), for the transmission of the umbilical vessels, the allantois, and vitelline duct; above, the vena caval opening, for the transmission of the inferior vena cava, the aortic hiatus, for the passage of the aorta, azygos vein, and thoracic duct, and the esophageal hiatus, for the esophagus and vagi. Below, there are two apertures on either side: one for the passage of the femoral vessels and lumboinguinal nerve, and the other for the transmission of the spermatic cord in the male, and the round ligament of the uterus in the female.

 

: : : : : : image1034

 

Front view of the thoracic and abdominal viscera. a. Median plane. b b. Lateral planes. c c. Trans tubercular plane. d d. Subcostal plane. e e. Transpyloric plane.

  

Regions.For convenience of description of the viscera, as well as of reference to the morbid conditions of the contained parts, the abdomen is artificially divided into nine regions by imaginary planes, two horizontal and two sagittal, passing through the cavity, the edges of the planes being indicated by lines drawn on the surface of the body. Of the horizontal planes the upper or transpyloric is indicated by a line encircling the body at the level of a point midway between the jugular notch and the symphysis pubis, the lower by a line carried around the trunk at the level of a point midway between the transpyloric and the symphysis pubis. The latter is practically the intertubercular plane of Cunningham, who pointed out that its level corresponds with the prominent and easily defined tubercle on the iliac crest about 5 cm. behind the anterior superior iliac spine. By means of these imaginary planes the abdomen is divided into three zones, which are named from above downward the subcostal, umbilical, and hypogastric zones. Each of these is further subdivided into three regions by the two sagittal planes, which are indicated on the surface by lines drawn vertically through points half-way between the anterior superior iliac spines and the symphysis pubis.

  The middle region of the upper zone is called the epigastric; and the two lateral regions, the right and left hypochondriac. The central region of the middle zone is the umbilical; and the two lateral regions, the right and left lumbar. The middle region of the lower zone is the hypogastric or pubic region; and the lateral regions are the right and left iliac or inguinal.

  The pelvis is that portion of the abdominal cavity which lies below and behind a plane passing through the promontory of the sacrum, lineæ terminales of the hip bones, and the pubic crests. It is bounded behind by the sacrum, coccyx, Piriformes, and the sacrospinous and sacrotuberous ligaments; in front and laterally by the pubes and ischia and Obturatores interni; above it communicates with the abdomen proper; below it is closed by the Levatores ani and Coccygei and the urogenital diaphragm. The pelvis contains the urinary bladder, the sigmoid colon and rectum, a few coils of the small intestine, and some of the generative organs.

  When the anterior abdominal wall is removed, the viscera are partly exposed as follows: above and to the right side is the liver, situated chiefly under the shelter of the right ribs and their cartilages, but extending across the middle line and reaching for some distance below the level of the xiphoid process. To the left of the liver is the stomach, from the lower border of which an apron-like fold of peritoneum, the greater omentum, descends for a varying distance, and obscures, to a greater or lesser extent, the other viscera. Below it, however, some of the coils of the small intestine can generally be seen, while in the right and left iliac regions respectively the cecum and the iliac colon are partly exposed. The bladder occupies the anterior part of the pelvis, and, if distended, will project above the symphysis pubis; the rectum lies in the concavity of the sacrum, but is usually obscured by the coils of the small intestine. The sigmoid colon lies between the rectum and the bladder.

  When the stomach is followed from left to right it is seen to be continuous with the first part of the small intestine, or duodenum, the point of continuity being marked by a thickened ring which indicates the position of the pyloric valve. The duodenum passes toward the under surface of the liver, and then, curving downward, is lost to sight. If, however, the greater omentum be thrown upward over the chest, the inferior part of the duodenum will be observed passing across the vertebral column toward the left side, where it becomes continuous with the coils of the jejunum and ileum. These measure some 6 meters in length, and if followed downward the ileum will be seen to end in the right iliac fossa by opening into the cecum, the commencement of the large intestine. From the cecum the large intestine takes an arched course, passing at first upward on the right side, then across the middle line and downward on the left side, and forming respectively the ascending transverse, and descending parts of the colon. In the pelvis it assumes the form of a loop, the sigmoid colon, and ends in the rectum.

  The spleen lies behind the stomach in the left hypochondriac region, and may be in part exposed by pulling the stomach over toward the right side.

  The glistening appearance of the deep surface of the abdominal wall and of the surfaces of the exposed viscera is due to the fact that the former is lined, and the latter are more or less completely covered, by a serous membrane, the peritoneum.

 

The Digestive Apparatus (Apparatus Digestorius; Organs Of Digestion)

The apparatus for the digestion of the food consists of the digestive tube and of certain accessory organs.

The Digestive Tube (alimentary canal) is a musculomembranous tube, about 9 metres long, extending from the mouth to the anus, and lined throughout its entire extent by mucous membrane. It has received different names in the various parts of its course: at its commencement is the mouth, where provision is made for the mechanical division of the food (mastication), and for its admixture with a fluid secreted by the salivary glands (insalivation); beyond this are the organs of deglutition, the pharynx and the esophagus, which convey the food into the stomach, in which it is stored for a time and in which also the first stages of the digestive process take place; the stomach is followed by the small intestine, which is divided for purposes of description into three parts, the duodenum, the jejunum, and ileum. In the small intestine the process of digestion is completed and the resulting products are absorbed into the blood and lacteal vessels. Finally the small intestine ends in the large intestine, which is made up of cecum, colon, rectum, and anal canal, the last terminating on the surface of the body at the anus.

The accessory organs are the teeth, for purposes of mastication; the three pairs of salivary glandsthe parotid, submandibular, and sublingualthe secretion from which mixes with the food in the mouth and converts it into a bolus and acts chemically on one of its constituents; the liver and pancreas, two large glands in the abdomen, the secretions of which, in addition to that of numerous minute glands in the walls of the alimentary canal, assist in the process of digestion.

The Development of the Digestive Tube.The primitive digestive tube consists of two parts, viz.: (1) the fore-gut, within the cephalic flexure, and dorsal to the heart; and (2) the hind-gut, within the caudal flexure. Between these is the wide opening of the yolk-sac, which is gradually narrowed and reduced to a small foramen leading into the vitelline duct. At first the fore-gut and hind-gut end blindly. The anterior end of the fore-gut is separated from the stomodeum by the buccopharyngeal membrane the hind-gut ends in the cloaca, which is closed by the cloacal membrane.

The Mouth.The mouth is developed partly from the stomodeum, and partly from the floor of the anterior portion of the fore-gut. By the growth of the head end of the embryo, and the formation of the cephalic flexure, the pericardial area and the buccopharyngeal membrane come to lie on the ventral surface of the embryo. With the further expansion of the brain, and the forward bulging of the pericardium, the buccopharyngeal membrane is depressed between these two prominences. This depression constitutes the stomodeum. It is lined by ectoderm, and is separated from the anterior end of the fore-gut by the buccopharyngeal membrane. This membrane is devoid of mesoderm, being formed by the apposition of the stomodeal ectoderm with the fore-gut entoderm; at the end of the third week it disappears, and thus a communication is established between the mouth and the future pharynx. No trace of the membrane is found in the adult; and the communication just mentioned must not be confused with the permanent isthmus faucium. The lips, teeth, and gums are formed from the walls of the stomodeum, but the tongue is developed in the floor of the pharynx.

The visceral arches extend in a ventral direction between the stomodeum and the pericardium; and with the completion of the mandibular arch and the formation of the maxillary processes, the mouth assumes the appearance of a pentagonal orifice. The orifice is bounded in front by the fronto-nasal process, behind by the mandibular arch, and laterally by the maxillary processes. With the inward growth and fusion of the palatine processes, the stomodeum is divided into an upper nasal, and a lower buccal part. Along the free margins of the processes bounding the mouth cavity a shallow groove appears; this is termed the primary labial groove, and from the bottom of it a downgrowth of ectoderm takes place into the underlying mesoderm. The central cells of the ectodermal downgrowth degenerate and a secondary labial groove is formed; by the deepening of this, the lips and cheeks are separated from the alveolar processes of the maxillæ and mandible.

The Salivary Glands.The salivary glands arise as buds from the epithelial lining of the mouth; the parotid appears during the fourth week in the angle between the maxillary process and the mandibular arch; the submandibular appears in the sixth week, and the sublingual during the ninth week in the hollow between the tongue and the mandibular arch.

The Tongue is developed in the floor of the pharynx, and consists of an anterior or buccal and a posterior or pharyngeal part which are separated in the adult by the V-shaped sulcus terminalis. During the third week there appears, immediately behind the ventral ends of the two halves of the mandibular arch, a rounded swelling named the tuberculum impar, which was described by His as undergoing enlargement to form the buccal part of the tongue. More recent researches, however, show that this part of the tongue is mainly, if not entirely, developed from a pair of lateral swellings which rise from the inner surface of the mandibular arch and meet in the middle line. The tuberculum impar is said to form the central part of the tongue immediately in front of the foramen cecum, but Hammar insists that it is purely a transitory structure and forms no part of the adult tongue. From the ventral ends of the fourth arch there arises a second and larger elevation, in the center of which is a median groove or furrow. This elevation was named by His the furcula, and is at first separated from the tuberculum impar by a depression, but later by a ridge, the copula, formed by the forward growth and fusion of the ventral ends of the second and third arches. The posterior or pharyngeal part of the tongue is developed from the copula, which extends forward in the form of a V, so as to embrace between its two limbs the buccal part of the tongue. At the apex of the V a pit-like invagination occurs, to form the thyroid gland, and this depression is represented in the adult by the foramen cecum of the tongue. In the adult the union of the anterior and posterior parts of the tongue is marked by the V-shaped sulcus terminalis, the apex of which is at the foramen cecum, while the two limbs run lateralward and forward, parallel to, but a little behind, the vallate papillæ.

The Palatine Tonsils are developed from the dorsal angles of the second branchial pouches. The entoderm which lines these pouches grows in the form of a number of solid buds into the surrounding mesoderm. These buds become hollowed out by the degeneration and casting off of their central cells, and by this means the tonsillar crypts are formed. Lymphoid cells accumulate around the crypts, and become grouped to form the lymphoid follicles; the latter, however, are not well-defined until after birth.

The Further Development of the Digestive Tube.The upper part of the fore-gut becomes dilated to form the pharynx, in relation to which the branchial arches are developed (see page 65); the succeeding part remains tubular, and with the descent of the stomach is elongated to form the esophagus. About the fourth week a fusiform dilatation, the future stomach, makes its appearance, and beyond this the gut opens freely into the yolk-sac. The opening is at first wide, but is gradually narrowed into a tubular stalk, the yolk-stalk or vitelline duct. Between the stomach and the mouth of the yolk-sac the liver diverticulum appears. From the stomach to the rectum the alimentary canal is attached to the notochord by a band of mesoderm, from which the common mesentery of the gut is subsequently developed. The stomach has an additional attachment, viz., to the ventral abdominal wall as far as the umbilicus by the septum transversum. The cephalic portion of the septum takes part in the formation of the diaphragm, while the caudal portion into which the liver grows forms the ventral mesogastrium. The stomach undergoes a further dilatation, and its two curvatures can be recognized, the greater directed toward the vertebral column and the lesser toward the anterior wall of the abdomen, while its two surfaces look to the right and left respectively. Behind the stomach the gut undergoes great elongation, and forms a V-shaped loop which projects downward and forward; from the bend or angle of the loop the vitelline duct passes to the umbilicus. For a time a considerable part of the loop extends beyond the abdominal cavity into the umbilical cord, but by the end of the third month it is withdrawn within the cavity. With the lengthening of the tube, the mesoderm, which attaches it to the future vertebral column and carries the bloodvessels for the supply of the gut, is thinned and drawn out to form the posterior common mesentery. The portion of this mesentery attached to the greater curvature of the stomach is named the dorsal mesogastrium, and the part which suspends the colon is termed the mesocolon. About the sixth week a diverticulum of the gut appears just behind the opening of the vitelline duct, and indicates the future cecum and vermiform process. The part of the loop on the distal side of the cecal diverticulum increases in diameter and forms the future ascending and transverse portions of the large intestine. Until the fifth month the cecal diverticulum has a uniform caliber, but from this time onward its distal part remains rudimentary and forms the vermiform process, while its proximal part expands to form the cecum. Changes also take place in the shape and position of the stomach. Its dorsal part or greater curvature, to which the dorsal mesogastrium is attached, grows much more rapidly than its ventral part or lesser curvature to which the ventral mesogastrium is fixed. Further, the greater curvature is carried downward and to the left, so that the right surface of the stomach is now directed backward and the left surface forward, a change in position which explains why the left vagus nerve is found on the front, and the right vagus on the back of the stomach. The dorsal mesogastrium being attached to the greater curvature must necessarily follow its movements, and hence it becomes greatly elongated and drawn lateralward and ventralward from the vertebral column, and, as in the case of the stomach, the right surfaces of both the dorsal and ventral mesogastria are now directed backward, and the left forward. In this way a pouch, the bursa omentalis, is formed behind the stomach, and this increases in size as the digestive tube undergoes further development; the entrance to the pouch constitutes the future foramen epiploicum or foramen of Winslow. The duodenum is developed from that part of the tube which immediately succeeds the stomach; it undergoes little elongation, being more or less fixed in position by the liver and pancreas, which arise as diverticula from it. The duodenum is at first suspended by a mesentery, and projects forward in the form of a loop. The loop and its mesentery are subsequently displaced by the transverse colon, so that the right surface of the duodenal mesentery is directed backward, and, adhering to the parietal peritoneum, is lost. The remainder of the digestive tube becomes greatly elongated, and as a consequence the tube is coiled on itself, and this elongation demands a corresponding increase in the width of the intestinal attachment of the mesentery, which becomes folded.

At this stage the small and large intestines are attached to the vertebral column by a common mesentery, the coils of the small intestine falling to the right of the middle line, while the large intestine lies on the left side.

The gut is now rotated upon itself, so that the large intestine is carried over in front of the small intestine, and the cecum is placed immediately below the liver; about the sixth month the cecum descends into the right iliac fossa, and the large intestine forms an arch consisting of the ascending, transverse, and descending portions of the colonthe transverse portion crossing in front of the duodenum and lying just below the greater curvature of the stomach; within this arch the coils of the small intestine are disposed. Sometimes the downward progress of the cecum is arrested, so that in the adult it may be found lying immediately below the liver instead of in the right iliac region.

Further changes take place in the bursa omentalis and in the common mesentery, and give rise to the peritoneal relations seen in the adult. The bursa omentalis, which at first reaches only as far as the greater curvature of the stomach, grows downward to form the greater omentum, and this downward extension lies in front of the transverse colon and the coils of the small intestine. Above, before the pleuro-peritoneal opening is closed, the bursa omentalis sends up a diverticulum on either side of the esophagus; the left diverticulum soon disappears, but the right is constricted off and persists in most adults as a small sac lying within the thorax on the right side of the lower end of the esophagus. The anterior layer of the transverse mesocolon is at first distinct from the posterior layer of the greater omentum, but ultimately the two blend, and hence the greater omentum appears as if attached to the transverse colon. The mesenteries of the ascending and descending parts of the colon disappear in the majority of cases, while that of the small intestine assumes the oblique attachment characteristic of its adult condition.

The lesser omentum is formed, as indicated above, by a thinning of the mesoderm or ventral mesogastrium, which attaches the stomach and duodenum to the anterior abdominal wall. By the subsequent growth of the liver this leaf of mesoderm is divided into two parts, viz., the lesser omentum between the stomach and liver, and the falciform and coronary ligaments between the liver and the abdominal wall and diaphragm.

The Rectum and Anal Canal.The hind-gut is at first prolonged backward into the body-stalk as the tube of the allantois; but, with the growth and flexure of the tail-end of the embryo, the body-stalk, with its contained allantoic tube, is carried forward to the ventral aspect of the body, and consequently a bend is formed at the junction of the hind-gut and allantois. This bend becomes dilated into a pouch, which constitutes the entodermal cloaca; into its dorsal part the hind-gut opens, and from its ventral part the allantois passes forward. At a later stage the Wolffian and Müllerian ducts open into its ventral portion. The cloaca is, for a time, shut off from the anterior by a membrane, the cloacal membrane, formed by the apposition of the ectoderm and entoderm, and reaching, at first, as far forward as the future umbilicus. Behind the umbilicus, however, the mesoderm subsequently extends to form the lower part of the abdominal wall and symphysis pubis. By the growth of the surrounding tissues the cloacal membrane comes to lie at the bottom of a depression, which is lined by ectoderm and named the ectodermal cloaca.

The entodermal cloaca is divided into a dorsal and a ventral part by means of a partition, the urorectal septum, which grows downward from the ridge separating the allantoic from the cloacal opening of the intestine and ultimately fuses with the cloacal membrane and divides it into an anal and a urogenital part. The dorsal part of the cloaca forms the rectum, and the anterior part of the urogenital sinus and bladder. For a time a communication named the cloacal duct exists between the two parts of the cloaca below the urorectal septum; this duct occasionally persists as a passage between the rectum and urethra. The anal canal is formed by an invagination of the ectoderm behind the urorectal septum. This invagination is termed the proctodeum, and it meets with the entoderm of the hind-gut and forms with it the anal membrane. By the absorption of this membrane the anal canal becomes continuous with the rectum. A small part of the hind-gut projects backward beyond the anal membrane; it is named the post-anal gut, and usually becomes obliterated and disappears.

 

Note.Sometimes this condition persists throughout life, and it is then found that the duodenum does not cross from the right to the left side of the vertebral column, but lies entirely on the right side of the median plane, where it is continued into the jejunum; the arteries to the small intestine (aa. intestinales) also arise from the right instead of the left side of the superior mesenteric artery.

The Mouth (Cavum Oris; Oral Or Buccal Cavity)

The cavity of the mouth is placed at the commencement of the digestive tube it is a nearly oval-shaped cavity which consists of two parts: an outer, smaller portion, the vestibule, and an inner, larger part, the mouth cavity proper.

The Vestibule (vestibulum oris) is a slit-like space, bounded externally by the lips and cheeks; internally by the gums and teeth. It communicates with the surface of the body by the rima or orifice of the mouth. Above and below, it is limited by the reflection of the mucous membrane from the lips and cheeks to the gum covering the upper and lower alveolar arch respectively. It receives the secretion from the parotid salivary glands, and communicates, when the jaws are closed, with the mouth cavity proper by an aperture on either side behind the wisdom teeth, and by narrow clefts between opposing teeth.

The Mouth Cavity Proper (cavum oris proprium) is bounded laterally and in front by the alveolar arches with their contained teeth; behind, it communicates with the pharynx by a constricted aperture termed the isthmus faucium. It is roofed in by the hard and soft palates, while the greater part of the floor is formed by the tongue, the remainder by the reflection of the mucous membrane from the sides and under surface of the tongue to the gum lining the inner aspect of the mandible. It receives the secretion from the submandibular and sublingual salivary glands.

Structure.The mucous membrane lining the mouth is continuous with the integument at the free margin of the lips, and with the mucous lining of the pharynx behind; it is of a rosepink tinge during life, and very thick where it overlies the hard parts bounding the cavity. It is covered by stratified squamous epithelium.

The Lips (labia oris), the two fleshy folds which surround the rima or orifice of the mouth, are formed externally of integument and internally of mucous membrane, between which are found the Orbicularis oris muscle, the labial vessels, some nerves, areolar tissue, and fat, and numerous small labial glands. The inner surface of each lip is connected in the middle line to the corresponding gum by a fold of mucous membrane, the frenulumthe upper being the larger.

The Labial Glands (glandulœ labiales) are situated between the mucous membrane and the Orbicularis oris, around the orifice of the mouth. They are circular in form, and about the size of small peas; their ducts open by minute orifices upon the mucous membrane. In structure they resemble the salivary glands.

The Cheeks (buccæ) form the sides of the face, and are continuous in front with the lips. They are composed externally of integument; internally of mucous membrane; and between the two of a muscular stratum, besides a large quantity of fat, areolar tissue, vessels, nerves, and buccal glands.

Structure.The mucous membrane lining the cheek is reflected above and below upon the gums, and is continuous behind with the lining membrane of the soft palate. Opposite the second molar tooth of the maxilla is a papilla, on the summit of which is the aperture of the parotid duct. The principal muscle of the cheek is the Buccinator; but other muscles enter into its formation, viz., the Zygomaticus, Risorius, and Platysma.

The buccal glands are placed between the mucous membrane and Buccinator muscle: they are similar in structure to the labial glands, but smaller. About five, of a larger size than the rest, are placed between the Masseter and Buccinator muscles around the distal extremity of the parotid duct; their ducts open in the mouth opposite the last molar tooth. They are called molar glands.

The Gums (gingivœ) are composed of dense fibrous tissue, closely connected to the periosteum of the alveolar processes, and surrounding the necks of the teeth. They are covered by smooth and vascular mucous membrane, which is remarkable for its limited sensibility. Around the necks of the teeth this membrane presents numerous fine papillæ, and is reflected into the alveoli, where it is continuous with the periosteal membrane lining these cavities.

The Palate (palatum) forms the roof of the mouth; it consists of two portions, the hard palate in front, the soft palate behind.

The Hard Palate (palatum durum) is bounded in front and at the sides by the alveolar arches and gums; behind, it is continuous with the soft palate. It is covered by a dense structure, formed by the periosteum and mucous membrane of the mouth, which are intimately adherent. Along the middle line is a linear raphæ, which ends anteriorly in a small papilla corresponding with the incisive canal. On either side and in front of the raphé the mucous membrane is thick, pale in color, and corrugated; behind, it is thin, smooth, and of a deeper color; it is covered with stratified squamous epithelium, and furnished with numerous palatal glands, which lie between the mucous membrane and the surface of the bone.

The Soft Palate (palatum molle) is a movable fold, suspended from the posterior border of the hard palate, and forming an incomplete septum between the mouth and pharynx. It consists of a fold of mucous membrane enclosing muscular fibers, an aponeurosis, vessels, nerves, adenoid tissue, and mucous glands. When occupying its usual position, i. e., relaxed and pendent, its anterior surface is concave, continuous with the roof of the mouth, and marked by a median raphé. Its posterior surface is convex, and continuous with the mucous membrane covering the floor of the nasal cavities. Its upper border is attached to the posterior margin of the hard palate, and its sides are blended with the pharynx. Its lower border is free. Its lower portion, which hangs like a curtain between the mouth and pharynx is termed the palatine velum.

Hanging from the middle of its lower border is a small, conical, pendulous process, the palatine uvula; and arching lateralward and downward from the base of the uvula on either side are two curved folds of mucous membrane, containing muscular fibers, called the arches or pillars of the fauces.

The Teeth (dentes) Man is provided with two sets of teeth, which make their appearance at different periods of life. Those of the first set appear in childhood, and are called the deciduous or milk teeth. Those of the second set, which also appear at an early period, may continue until old age, and are named permanent.

The deciduous teeth are twenty in number: four incisors, two canines, and four molars, in each jaw.

The permanent teeth are thirty-two in number: four incisors, two canines, four premolars, and six molars, in each jaw.

The dental formulæ may be represented as follows:

Deciduous Teeth. Upper jaw Lower jaw

mol. 2 2

can. 1 1

in. 2 2

in. 2 2

can. 1 1

mol. 2 2

Total 20

 

Permanent Teeth. Upper jaw Lower jaw

mol. 3 3

pr.mol. 2 2

can. 1 1

in. 2 2

in. 2 2

can. 1 1

pr.mol. 2 2

mol. 3 3

Total 32

General Characteristics.Each tooth consists of three portions: the crown, projecting above the gum; the root, imbedded in the alveolus; and the neck, the constricted portion between the crown and root.

The roots of the teeth are firmly implanted in depressions within the alveoli; these depressions are lined with periosteum which invests the tooth as far as the neck. At the margins of the alveoli, the periosteum is continuous with the fibrous structure of the gums.

In consequence of the curve of the dental arch, terms such as anterior and posterior, as applied to the teeth, are misleading and confusing. Special terms are therefore used to indicate the different surfaces of a tooth: the surface directed toward the lips or cheek is known as the labial or buccal surface; that directed toward the tongue is described as the lingual surface; those surfaces which touch neighboring teeth are termed surfaces of contact. In the case of the incisor and canine teeth the surfaces of contact are medial and lateral; in the premolar and molar teeth they are anterior and posterior.

The superior dental arch is larger than the inferior, so that in the normal condition the teeth in the maxillæ slightly overlap those of the mandible both in front and at the sides. Since the upper central incisors are wider than the lower, the other teeth in the upper arch are thrown somewhat distally, and the two sets do not quite correspond to each other when the mouth is closed: thus the upper canine tooth rests partly on the lower canine and partly on the first premolar, and the cusps of the upper molar teeth lie behind the corresponding cusps of the lower molar teeth. The two series, however, end at nearly the same point behind; this is mainly because the molars in the upper arch are the smaller.

The Permanent Teeth (dentes permanentes) The Incisors (dentes incisivi; incisive or cutting teeth) are so named from their presenting a sharp cutting edge, adapted for biting the food. They are eight in number, and form the four front teeth in each dental arch.

The crown is directed vertically, and is chisel-shaped, being bevelled at the expense of its lingual surface, so as to present a sharp horizontal cutting edge, which, before being subjected to attrition, presents three small prominent points separated by two slight notches. It is convex, smooth, and highly polished on its labial surface; concave on its lingual surface, where, in the teeth of the upper arch, it is frequently marked by an inverted V-shaped eminence, situated near the gum. This is known as the basal ridge or cingulum. The neck is constricted. The root is long, single, conical, transversely flattened, thicker in front than behind, and slightly grooved on either side in the longitudinal direction.

The upper incisors are larger and stronger than the lower, and are directed obliquely downward and forward. The central ones are larger than the lateral, and their roots are more rounded.

The lower incisors are smaller than the upper: the central ones are smaller than the lateral, and are the smallest of all the incisors. They are placed vertically and are somewhat bevelled in front, where they have been worn down by contact with the overlapping edge of the upper teeth. The cingulum is absent.

The Canine Teeth (dentes canini) are four in number, two in the upper, and two in the lower arch, one being placed laterally to each lateral incisor. They are larger and stronger than the incisors, and their roots sink deeply into the bones, and cause well-marked prominences upon the surface.

The crown is large and conical, very convex on its labial surface, a little hollowed and uneven on its lingual surface, and tapering to a blunted point or cusp, which projects beyond the level of the other teeth. The root is single, but longer and thicker than that of the incisors, conical in form, compressed laterally, and marked by a slight groove on each side.

The upper canine teeth (popularly called eye teeth) are larger and longer than the lower, and usually present a distinct basal ridge.

The lower canine teeth (popularly called stomach teeth) are placed nearer the middle line than the upper, so that their summits correspond to the intervals between the upper canines and the lateral incisors.

The Premolars or Bicuspid teeth (dentes præmolares) are eight in number, four in each arch. They are situated lateral to and behind the canine teeth, and are smaller and shorter than they.

The crown is compressed antero-posteriorly, and surmounted by two pyramidal eminences or cusps, a labial and a lingual, separated by a groove; hence their name bicuspid. Of the two cusps the labial is the larger and more prominent. The neck is oval. The root is generally single, compressed, and presents in front and behind a deep groove, which indicates a tendency in the root to become double. The apex is generally bifid.

The upper premolars are larger, and present a greater tendency to the division of their roots than the lower; this is especially the case in the first upper premolar.

The Molar Teeth (dentes molares) are the largest of the permanent set, and their broad crowns are adapted for grinding and pounding the food. They are twelve in number; six in each arch, three being placed posterior to each of the second premolars.

The crown of each is nearly cubical in form, convex on its buccal and lingual surfaces, flattened on its surfaces of contact; it is surmounted by four or five tubercles, or cusps, separated from each other by a crucial depression; hence the molars are sometimes termed multicuspids. The neck is distinct, large, and rounded.

Upper Molars.As a rule the first is the largest, and the third the smallest of the upper molars. The crown of the first has usually four tubercles; that of the second, three or four; that of the third, three. Each upper molar has three roots, and of these two are buccal and nearly parallel to one another; the third is lingual and diverges from the others as it runs upward. The roots of the third molar (dens serotinus or wisdom-tooth) are more or less fused together.

Lower Molars.The lower molars are larger than the upper. On the crown of the first there are usually five tubercles; on those of the second and third, four or five. Each lower molar has two roots, an anterior, nearly vertical, and a posterior, directed obliquely backward; both roots are grooved longitudinally, indicating a tendency to division. The two roots of the third molar (dens serotinus or wisdom tooth) are more or less united.

The Deciduous Teeth (dentes decidui; temporary or milk teeth) The deciduous are smaller than, but, generally speaking, resemble in form, the teeth which bear the same names in the permanent set. The hinder of the two molars is the largest of all the deciduous teeth, and is succeeded by the second premolar. The first upper molar has only three cuspstwo labial, one lingual; the second upper molar has four cusps. The first lower molar has four cusps; the second lower molar has five. The roots of the deciduous molars are smaller and more divergent than those of the permanent molars, but in other respects bear a strong resemblance to them.

Structure of the Teeth.On making a vertical section of a tooth, a cavity will be found in the interior of the crown and the center of each root; it opens by a minute orifice at the extremity of the latter. This is called the pulp cavity, and contains the dental pulp, a loose connective tissue richly supplied with vessels and nerves, which enter the cavity through the small aperture at the point of each root. Some of the cells of the pulp are arranged as a layer on the wall of the pulp cavity; they are named the odontoblasts of Waldeyer, and during the development of the tooth, are columnar in shape, but later on, after the dentin is fully formed, they become flattened and resemble osteoblasts. Each has two fine processes, the outer one passing into a dental canaliculus, the inner being continuous with the processes of the connective-tissue cells of the pulp matrix.

The solid portion of the tooth consists of (1) the ivory or dentin, which forms the bulk of the tooth; (2) the enamel, which covers the exposed part of the crown; and (3) a thin layer of bone, the cement or crusta petrosa, which is disposed on the surface of the root.

The dentin (substantia eburnea; ivory) forms the principal mass of a tooth. It is a modification of osseous tissue, from which it differs, however, in structure. On microscopic examination it is seen to consist of a number of minute wavy and branching tubes, the dental canaliculi, imbedded in a dense homogeneous substance, the matrix.

The dental canaliculi (dentinal tubules) are placed parallel with one another, and open at their inner ends into the pulp cavity. In their course to the periphery they present two or three curves, and are twisted on themselves in a spiral direction. These canaliculi vary in direction: thus in a tooth of the mandible they are vertical in the upper portion of the crown, becoming oblique and then horizontal in the neck and upper part of the root, while toward the lower part of the root they are inclined downward. In their course they divide and subdivide dichotomously, and, especially in the root, give off minute branches, which join together in loops in the matrix, or end blindly. Near the periphery of the dentin, the finer ramifications of the canaliculi terminate imperceptibly by free ends. The dental canaliculi have definite walls, consisting of an elastic homogeneous membrane, the dentinal sheath of Neumann, which resists the action of acids; they contain slender cylindrical prolongations of the odontoblasts, first described by Tomes, and named Tomes fibers or dentinal fibers.

The matrix (intertubular dentin) is translucent, and contains the chief part of the earthy matter of the dentin. In it are a number of fine fibrils, which are continuous with the fibrils of the dental pulp. After the earthy matter has been removed by steeping a tooth in weak acid, the animal basis remaining may be torn into laminæ which run parallel with the pulp cavity, across the direction of the tubes. A section of dry dentin often displays a series of somewhat parallel linesthe incremental lines of Salter. These lines are composed of imperfectly calcified dentin arranged in layers. In consequence of the imperfection in the calcifying process, little irregular cavities are left, termed interglobular spaces. Normally a series of these spaces is found toward the outer surface of the dentin, where they form a layer which is sometimes known as the granular layer. They have received their name from the fact that they are surrounded by minute nodules or globules of dentin. Other curved lines may be seen parallel to the surface. These are the lines of Schreger, and are due to the optical effect of simultaneous curvature of the dentinal fibers.

Chemical Composition.According to Berzelius and von Bibra, dentin consists of 28 parts of animal and 72 parts of earthy matter. The animal matter is converted by boiling into gelatin. The earthy matter consists of phosphate of lime, carbonate of lime, a trace of fluoride of calcium, phosphate of magnesium, and other salts.

 

 

Prepared by

A.V. MIZ