1. milk dentition formula and permanent dentition formula, teething, replacement, occlusion.

2. pharynx, oesophagus, Stomach.

3. small intestine, large intestine

 

Lesson No 12

 

Theme 1. milk dentition formula and permanent dentition formula, teething, replacement, occlusion.

The teeth may be divided into deciduous (primary) teeth in chilhood age and permanent teeth in adult. Each tooth consists of three parts: crown, neck and root. The crown has 5 surfaces: lingual, vestibular (labial or buccal), contact (proximal and distal), occlusal. The neck is the part of the tooth between the crown and the root. The root is fixed in the alveolar socket by a fibrous periodontal ligament (gomphosis).

Tooth is composed of dentin that is covered by enamel over the crown - and cementum over the root. The pulp cavity contains connective tissue, blood vessels, and nerves. The last pass through the root canal and the apical foramen. The roots of the teeth fit into sockets called dental alveoli in the alveolar process of the mandible and maxillae. Each socket is lined with periodontal membrane.

Types of Teeth

Medial and lateral incisors - have a single root and chisel-shaped crown. Action: they cut off portions of food.

Canine - has a single root, conical crown. Action: holding and bite the food.

Premolar - has a single root, sometimes upper tooth has bifurcated root. Crown carries two tubercles. Action: crushing the food.

Molar – upper teeth have three roots, lower teeth have two roots. Crown carries 3-5 tubercles on occlusal surface. Action: grinding the food.

Formula of the deciduous (milk) teeth is 2102. Formula of the permanent teeth is 2123. It means that child before 6 years of age in each side of upper and lower jaw own 2 incisors, 1canine, no premolar and 2 molars. Permanent teeth include 2 incisors, 1canine, 2 premolars and 3 molars.

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Permanent teeth of upper dental arch, seen from below.

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Permanent teeth of right half of lower dental arch, seen from above

 

Age terms of eruption of deciduous teeth and permanent teeth:

Type of tooth

Deciduous

Permanent

Incisors

6 – 9 months

7 – 9 years

Canines

16 - 20 months

10 – 13 years

First Premolar

-

10 – 12 years

Second Premolar

-

11 –15 years

First Molar

12 - 15 months

6 –7 years

Second Molar

20 – 24 months

13 – 16 years

Third Molar ("wisdom tooth")

-

18 – 30 years

 

Order of cutting of milk teeth:

• Incisors;

• First molars;

• Canines;

• Second molars.

Order of cutting of milk teeth:

• Incisors;

• First molars;

• Canines;

• Second molars.

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Maxillæ at about one year.

 

Child should have 20 teeth till end of second year of age.

Order of eruption of permanent teeth:

first inferior molars;

• Medial incisors and first superior molars;

• Lateral incisors;

• First premolars;

• Canines;

• Second premolars;

• Second molars;

• Third molars (called "wisdom tooth", present not in all person).

 

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The complete temporary dentition (about three years), showing the relation of the developing permanent teeth.

 

There are 32 permanent teeth. Mutual arrangement of superior and inferior dental arches during closing the mouth called bite. There are the physiological and pathologic bites.

The gingivae are composed of fibrous tissue that is covered with mucous membrane. They are firmly attached to the margins of the alveolar processes (tooth sockets) of the jaws and to the necks of the teeth.

 

 

Maxillæ at about one year.

 

Child should have 20 teeth till end of second year of age.

Order of eruption of permanent teeth:

first inferior molars;

• Medial incisors and first superior molars;

• Lateral incisors;

• First premolars;

• Canines;

• Second premolars;

• Second molars;

• Third molars (called "wisdom tooth", present not in all person).

 

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The complete temporary dentition (about three years), showing the relation of the developing permanent teeth.

 

There are 32 permanent teeth. Mutual arrangement of superior and inferior dental arches during closing the mouth called bite. There are the physiological and pathologic bites.

The gingivae are composed of fibrous tissue that is covered with mucous membrane. They are firmly attached to the margins of the alveolar processes (tooth sockets) of the jaws and to the necks of the teeth.

 

Theme 2. pharynx, oesophagus, Stomach.

PHARYNX extends from skull base to level of C6 vertebra. The pharynx is located posterior to the nasal and oral cavities and the larynx and is the common route for air and food (conducts food to the oesophagus and air to the larynx and lungs). Its cavity subdivides into nasal part, oral part and laryngeal part.

Nasopharynx is respiratory part; it communicates by choanae with nasal cavity. Nasopharynx extends to the bodies of C2 vertebrae. Lateral walls contain the pharyngeal orifice of auditory tube, which communicate a pharynx with tympanic cavity. These foramens are limited behind and from above by torus tubarius. Between last and velum one can find pair agglomeration of lymphoid tissue – tubal tonsil. Pharyngeal (adenoid) tonsil disposed on border of pharyngeal fornix and posterior wall. Submucous layer of nasal part is absent; instead it there is fibrous membrane, which does not allow walls to close the cavity of nasopharynx.

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Oropharynx communicates with mouth cavity by fauceus, which is limited by velum palatine, root of tongue and palatine-pharyngeal arches. There are median glossoepiglottic fold and lateral (pair) glossoepiglottic folds. Posteriorly the oropharynx is related to the bodies of C2 to C4 vertebrae.

Laryngopharynx contains the entrance into larynx communicates with laryngeal cavity. It is related to the bodies of C4 to C6 vertebrae. Piriform recesses are situated on sides from entrance into larynx. Laryngopharynx continues with the oesophagus.

Pharyngeal wall consists of three layers: mucous membrane, muscular and connective tissue membrane (adventitia). Submucous stratum absent, instead of it is pharyngeо-basillar fascia, which is attached to the skull.

Peripharyngeal space round the pharynx and may be divided into retropharyngeal space (communicates with posterior mediastinum), lateropharyngeal space and parapharyngeal space.

Muscular membrane is built from longitudinal and circular striped fibres. Longitudinal muscles include:

Stylopharyngeus muscle elevates the pharynx and larynx and expands the sides of the pharynx thereby aiding in pulling the pharyngeal wall over a bolus of food during swallowing;

Palatopharyngeus muscle elevates pharynx and larynx during swallowing and speaking. It also stretches the palatopharyngeal arch;

Salpingopharyngeus muscle elevates the pharynx and larynx and opens the pharyngeal orifice of the auditory tube during swallowing.

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Circular muscles include:

• Superior constrictor muscle;

• Middle constrictor muscle;

• Inferior constrictor muscle.

Constrictor muscles contracting from up to down, push a bolus of food into oesophagus and act as a sphincter, preventing air from entering to the oesophagus.

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The OESOPHAGUS is about 25-30 cm of length tube, which extends from the phar­ynx to the stomach. Oesophagus has cervical part, thoracic part and abdominal part. Topography: thoracic portion positioned in superior and posterior mediastinum. Oesophagus is related to the bodies of C6 to Th 11 vertebrae, also to trachea in cervical part and behind aorta in mediastinum.

Oesophageal wall consists of mucous membrane, submucous stratum, muscular membrane and external connective tissue adventitia. Submucous stratum is well developed, that why mucous membrane forms the longitudinal folds. Submucous stratum contains the numerous of oesophageal glands. Muscular membrane consists of internal circular layer and external longitudinal layer. In superior third a muscular membrane is formed by striped muscles, in middle part gradually replaces by smooth muscles, and inferiorly has only the smooth muscles. Abdominal part of oesophagus is covered by peritoneum.

Oesophagus has 3 anatomic constrictions.

pharyngeоesophageal constriction is in place of transition from pharynx into oesophagus, on level of the C7 - 7th cervical vertebra;

• constriction of thoracic part is a place, where left principal bronchi, presses an oesophagus is on level of the 5th thoracic vertebrae;

phrenic constriction is a place, where an oesophagus passes through the lumbar part of the diaphragm on level of the 9th –10th  thoracic vertebrae.

Physiological constrictions (2):

aortic constriction is a place, where aorta bends and adjoins to oesophagus on level of the Th4 of thoracic vertebra;

abdominal (cardiac) constriction is in place of entry into cardiac portion of stomach - on level of the Th11 thoracic vertebra.

The esophagus or gullet is a muscular canal, about 23 to 25 cm. long, extending from the pharynx to the stomach. It begins in the neck at the lower border of the cricoid cartilage, opposite the sixth cervical vertebra, descends along the front of the vertebral column, through the superior and posterior mediastina, passes through the diaphragm, and, entering the abdomen, ends at the cardiac orifice of the stomach, opposite the eleventh thoracic vertebra. The general direction of the esophagus is vertical; but it presents two slight curves in its course. At its commencement it is placed in the middle line; but it inclines to the left side as far as the root of the neck, gradually passes to the middle line again at the level of the fifth thoracic vertebra, and finally deviates to the left as it passes forward to the esophageal hiatus in the diaphragm. The esophagus also presents antero-posterior flexures corresponding to the curvatures of the cervical and thoracic portions of the vertebral column. It is the narrowest part of the digestive tube, and is most contracted at its commencement, and at the point where it passes through the diaphragm.

 

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The position and relation of the esophagus in the cervical region and in the posterior mediastinum. Seen from behind.

         Relations.—The cervical portion of the esophagus is in relation, in front, with the trachea; and at the lower part of the neck, where it projects to the left side, with the thyroid gland; behind, it rests upon the vertebral column and Longus colli muscles; on either side it is in relation with the common carotid artery (especially the left, as it inclines to that side), and parts of the lobes of the thyroid gland; the recurrent nerves ascend between it and the trachea; to its left side is the thoracic duct.

  The thoracic portion of the esophagus is at first situated in the superior mediastinum between the trachea and the vertebral column, a little to the left of the median line. It then passes behind and to the right of the aortic arch, and descends in the posterior mediastinum along the right side of the descending aorta, then runs in front and a little to the left of the aorta, and enters the abdomen through the diaphragm at the level of the tenth thoracic vertebra. Just before it perforates the diaphragm it presents a distinct dilatation. It is in relation, in front, with the trachea, the left bronchus, the pericardium, and the diaphragm; behind, it rests upon the vertebral column, the Longus colli muscles, the right aortic intercostal arteries, the thoracic duct, and the hemiazygos veins; and below, near the diaphragm, upon the front of the aorta. On its left side, in the superior mediastinum, are the terminal part of the aortic arch, the left subclavian artery, the thoracic duct, and left pleura, while running upward in the angle between it and the trachea is the left recurrent nerve; below, it is in relation with the descending thoracic aorta. On its right side are the right pleura, and the azygos vein which it overlaps. Below the roots of the lungs the vagi descend in close contact with it, the right nerve passing down behind, and the left nerve in front of it; the two nerves uniting to form a plexus around the tube.

  In the lower part of the posterior mediastinum the thoracic duct lies to the right side of the esophagus; higher up, it is placed behind it, and, crossing about the level of the fourth thoracic vertebra, is continued upward on its left side.

  The abdominal portion of the esophagus lies in the esophageal groove on the posterior surface of the left lobe of the liver. It measures about 1.25 cm. in length, and only its front and left aspects are covered by peritoneum. It is somewhat conical with its base applied to the upper orifice of the stomach, and is known as the antrum cardiacum.

 

VIDEO

 

Structure. The esophagus has four coats: an external or fibrous, a muscular, a submucous or areolar, and an internal or mucous coat.

  The muscular coat (tunica muscularis) is composed of two planes of considerable thickness: an external of longitudinal and an internal of circular fibers.

  The longitudinal fibers are arranged, at the commencement of the tube, in three fasciculi: one in front, which is attached to the vertical ridge on the posterior surface of the lamina of the cricoid cartilage; and one at either side, which is continuous with the muscular fibers of the pharynx: as they descend they blend together, and form a uniform layer, which covers the outer surface of the tube.

  Accessory slips of muscular fibers pass between the esophagus and the left pleura, where the latter covers the thoracic aorta, or the root of the left bronchus, or the back of the pericardium.

  The circular fibers are continuous above with the Constrictor pharyngis inferior; their direction is transverse at the upper and lower parts of the tube, but oblique in the intermediate part.

  The muscular fibers in the upper part of the esophagus are of a red color, and consist chiefly of the striped variety; but below they consist for the most part of involuntary fibers.

 

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Section of the human esophagus. (From a drawing by V. Horsley.) Moderately magnified. The section is transverse and from near the middle of the gullet. a. Fibrous covering. b. Divided fibers of longitudinal muscular coat. c. Transverse muscular fibers. d. Submucous or areolar layer. e. Muscularis mucosæ. f. Mucous membrane, with vessels and part of a lymphoid nodule. g. Stratified epithelial lining. h. Mucous gland. i. Gland duct. m’. Striated muscular fibers cut across.

 

  The areolar or submucous coat (tela submucosa) connects loosely the mucous and muscular coats. It contains bloodvessels, nerves, and mucous glands.

  The mucous coat (tunica mucosa) is thick, of a reddish color above, and pale below. It is disposed in longitudinal folds, which disappear on distension of the tube. Its surface is studded with minute papillæ, and it is covered throughout with a thick layer of stratified squamous epithelium. Beneath the mucous membrane, between it and the areolar coat, is a layer of longitudinally arranged non-striped muscular fibers. This is the muscularis mucosæ. At the commencement of the esophagus it is absent, or only represented by a few scattered bundles; lower down it forms a considerable stratum.

  The esophageal glands (glandulæ æsophageæ) are small compound racemose glands of the mucous type: they are lodged in the submucous tissue, and each opens upon the surface by a long excretory duct.

 

Vessels and Nerves.—The arteries supplying the esophagus are derived from the inferior thyroid branch of the thyrocervical trunk, from the descending thoracic aorta, from the left gastric branch of the celiac artery, and from the left inferior phrenic of the abdominal aorta. They have for the most part a longitudinal direction.

  The nerves are derived from the vagi and from the sympathetic trunks; they form a plexus, in which are groups of ganglion cells, between the two layers of the muscular coats, and also a second plexus in the submucous tissue.

 

Learning objectives. After studying this lesson, you should be able to describe the structure of the stomach, small and large intestine.

Terms to remember: Stomach, Chyme, Doudenum, Jejunum, Ileum.

 

The main function of the stomach is to process and transport of the food. After feeding, the contractile activity of the stomach helps to mix, grind and eventually evacuate small portions of chyme into the small bowel, while the rest of the chyme is mixed and ground.

Anatomically the J-shaped stomach can be divided into some major regions: cardia with cardiac ostium, fundus or fornix, corpus and pylorus. Last contains antrum, pyloric canal and pyloric ostium. Externally stomach has anterior and posterior facies, which meet each other in greater and lesser curvaturae. Lesser curvature orients to the right and upward and carries angular incisura. 

Topography of the Stomach

·                     Holotopy: Stomach is disposed in left hypochondriac and proper epigastric areas;

·                     Skeletotopy: Stomach is related to the bodies of Th11 to L1 vertebrae. Cardiac ostium is disposed on level of the Th11 on the left from backbone, and pyloric ostium - on level of the Th12-L1 to the right side; stomach fundus reaches the 5th intercostal space on left medioclavicular line;

·                     Syntopy: the diaphragm, left liver lobe and anterior abdominal wall adjoin to anterior stomach wall. Posterior stomach surface adjoins to spleen, pancreas, and left kidney with left adrenal gland and transversal colon.

·                      Stomach is covered by peritoneum from all sides (intraperitoneally).

The stomach wall, like the wall of most other parts of the digestive canal, consists of three layers: the mucosa (the innermost), the muscularis and the serosal  - visceral sheet of peritoneum (the outermost). The mucosal layer itself can be divided into three layers: the mucosa (the epithelial lining of the gastric cavity), the muscularis mucosae (low density smooth muscle cells) and the submucosal layer (consisting of connective tissue interlaced with plexi of the enteric nervous system). Mucous membrane contains the gastric fields, which carry the gastric pits, where the ducts of gastric glands open. Lesser curvaturae carries group of longitudinal folds. Mucous membrane forms in area of pyloric ostium pyloric valve, which regulates transition of bolus of food into duodenum.

The second gastric layer, the muscularis, can also be divided into three layers: the longitudinal (the most superficial), the circular and the oblique. The thickness of the circular layer increases in the antrum and especially in the pyloric sphincter, which controls the rate of discharge of stomach contents into the duodenum. The longitudinal layer of the muscularis can be separated into two different categories: a longitudinal layer that is common with the oesophagus and ends in the corpus, and a longitudinal layer that originates in the corpus and spreads into the duodenum. The oblique layer of the muscularis is clearly seen in the fundus and near the lesser curvature of the corpus, but the oblique fibbers disappear distally (towards the antrum). The outermost main layer is the serosa. Double layer of peritoneum forms hepatogastric, gastrophrenic, gastrocolic and gastrolienal ligaments.

VIDEO

The stomach performs four major functions: (1) the bulk storage of ingested food, (2) the mechanical breakdown of ingested food, (3) the disruption of chemical bonds in food material through the action of acids and enzymes, and (4) the production of intrinsic factor, a glycoprotein whose presence in the digestive tract is required for the absorption of vitamin B12. The mixing of ingested substances with the secretions of the glands of the stomach produces a viscous, highly acidic, soupy mixture of partially digested food. This material is called chyme.

Anatomy of the Stomach

The stomach has the shape of an expanded J. A short lesser curvature forms the medial surface of the organ, and a long greater curvature forms the lateral surface. The anterior and posterior surfaces are smoothly rounded. The shape and size of the stomach are extremely variable from individual to individual and even from one meal to the next. In an "average" stomach, the lesser curvature has a length of approximately 10 cm (4 in.), and the greater curvature measures about 40 cm (16 in.). The stomach typically extends between the levels of vertebrae T7 and L3.

We can divide the stomach into four regions

 

1.                 The cardia. The cardia is the smallest part of the stomach. It consists of the superior, medial portion of the stomach within 3 cm (1.2 in.) of the junction with the esophagus. The cardia contains abundant mucous glands whose secretions coat the connection with the esophagus and help protect that tube from the acids and enzymes of the stomach.

VIDEO

 

2.     The fundus. The portion of the stomach superior to the junction between the stomach and esophagus is the fundus. The fundus contacts the inferior and posterior surface of the diaphragm.

3.     The body. The area between the fundus and the curve of the J is the body of the stomach. The body is the largest region of the stomach, and it functions as a mixing tank for ingested food and secretions produced within the stomach. Gastric glands (gaster, stomach) in the fundus and body secrete most of the acids and enzymes involved in gastric digestion.

4.     The pylorus. The pylorus is the curve of the J. The pylorus is divided into a pyloric antrum (antron, cavity), which is connected to the body, and a pyloric canal that empties into the duodenum, the proximal segment of the small intestine. As mixing movements occur during digestion, the pylorus frequently changes shape. A muscular pyloric sphincter regulates the release of chyme into the duodenum. Glands in the pylorus secrete mucus and important digestive hormones, including gastrin, a hormone that stimulates the activity of gastric glands.

The stomach's volume increases at mealtime, then decreases as chyme enters the small intestine. When your stomach is relaxed (empty), the mucosa is thrown into prominent folds called rugae (wrinkles). Rugae are temporary features that let the gastric lumen expand. As your stomach fills, the rugae flatten out. When your stomach is full, the rugae almost disappear. When empty, your stomach resembles a muscular tube with a narrow, constricted lumen. When full, it can expand to contain 1-1.5 liters of material.

 



Musculature of the Stomach

The muscularis mucosae and muscularis externa of the stomach contain extra layers of smooth muscle cells in addition to the usual circular and longitudinal layers. The muscularis mucosae generally contains an outer, circular layer of muscle cells. The muscularis externa has an inner, oblique layer of smooth muscle. The extra layer of smooth muscle strengthens the stomach wall and assists in the mixing and churning activities essential to the formation of chyme.

A simple columnar epithelium lines all portions of the stomach. The epithelium is a secretory sheet that produces a carpet of mucus that covers the interior surfaces of the stomach. The alkaline mucous layer protects epithelial cells against the acids and enzymes in the gastric lumen.

The stomach receives blood from (1) the left gastric artery; (2) the splenic artery, which supplies the left gastroepiploic artery; and (3) the common hepatic artery, which supplies the right gastric, gastroduodenal, and right gastroepiploic arteries.

Shallow depressions called gastric pits open onto the gastric surface. The mucous cells at the base, or neck, of each gastric pit are actively dividing, replacing superficial cells that are shed into the chyme. The continuous replacement of epithelial cells provides an additional defense against the acidic gastric contents. A typical epithelial cell has a life span of 3-7 days, but exposure to strong alcohol or other chemicals can increase the rate of cell turnover.


Gastric Glands

In the fundus and body of the stomach, each gastric pit communicates with several gastric glands that extend deep into the underlying lamina propria. Gastric glands are dominated by two types of secretory cells: (1) parietal cells and (2) chief cells. Together they secrete about 1500 ml of gastric juice each day.

Parietal Cells. Parietal cells are especially common along the proximal portions of each gastric gland. These cells secrete intrinsic factor and hydrochloric acid (HCl). Intrinsic factor is a glycoprotein that facilitates the absorption of vitamin B12 across the intestinal lining.

The parietal cells do not produce HCl in the cytoplasm, because it is such a strong acid that it would erode a secretory vesicle and destroy the cell. Instead, H+ and Cl-, the two ions that form HCl, are transported independently by different mechanisms.

Hydrogen ions are generated inside the cell as the enzyme carbonic anhydrase converts carbon dioxide and water to carbonic acid. The carbonic acid promptly dissociates into hydrogen ions and bicarbonate ions. The hydrogen ions are actively transported into the lumen of the gastric gland. The bicarbonate ions are ejected into the interstitial fluid by a countertransport mechanism that exchanges intracellular bicarbonate ions for extracellular chloride ions. The chloride ions then diffuse across the cell and through open chloride channels in the cell membrane to the lumen of the gastric gland.

The bicarbonate ions released by the parietal cell diffuse through the interstitial fluid into the bloodstream. When gastric glands are actively secreting, enough bicarbonate ions enter the circulation to increase the pH of the blood significantly. This sudden influx of bicarbonate ions has been called the alkaline tide.

The secretory activities of the parietal cells can keep the stomach contents at a pH of 1.5-2.0. This highly acidic environment does not by itself digest the chyme but has four important functions:

1.     The low pH of gastric juice kills most of the microorganisms ingested with food.

2.     The low pH denatures proteins and inactivates most of the enzymes in food.

3.     The acid helps break down plant cell walls and the connective tissues in meat.

4.     An acidic environment is essential for the activation and function of pepsin, a protease (protein-digesting enzyme) secreted by the chief cells.

Chief Cells. Chief cells are most abundant near the base of a gastric gland. These cells secrete pepsinogen, an inactive proenzyme. Pepsinogen is converted by the acid in the gastric lumen to pepsin, an active proteolytic (protein-digesting) enzyme. Pepsin functions most effectively at a strongly acidic pH of 1.5-2.0. The stomachs of newborn infants (but not of adults) also produce rennin, also known as chymosin, and gastric lipase, enzymes important for the digestion of milk. Rennin coagulates milk proteins; gastric lipase initiates the digestion of milk fats.

Pyloric Glands

Glands in the pylorus produce primarily a mucous secretion rather than enzymes or acid. In addition, several different types of enteroendocrine cells are scattered among the mucus-secreting cells. These cells produce at least seven different hormones, most notably the hormone gastrin. Gastrin is produced by G cells. G cells are most abundant in the gastric pits of the pyloric antrum. Gastrin stimulates (1) the secretion of both parietal and chief cells and (2) contractions of the gastric wall that mix and stir the gastric contents. The pyloric glands also contain D cells, which release somatostatin a hormone that inhibits gastrin release. D cells are continuously releasing their secretions into the interstitial fluid adjacent to the G cells. This inhibition of gastrin production can be overpowered by neural and hormonal stimuli when the stomach is preparing for digestion or is already engaged in the digestion of food.

 

Theme 3. small intestine, large intestine

Your stomach is a holding tank where food is saturated with gastric juices and exposed to stomach acids and the digestive effects of pepsin. These are preliminary steps, for most of the important digestive and absorptive functions occur in your small intestine, where the products of digestion are absorbed. The mucosa of the small intestine produces only a few of the enzymes involved. The pancreas provides digestive enzymes as well as buffers that assist in the neutralization of acidic chyme. The liver and gallbladder provide bile, a solution that contains additional buffers and bile salts, compounds that facilitate the digestion and absorption of lipids.

 

VIDEO

The small intestine is a convoluted tube, extending from the pylorus to the colic valve, where it ends in the large intestine. It is about 7 meters long, and gradually diminishes in size from its commencement to its termination. It is contained in the central and lower part of the abdominal cavity, and is surrounded above and at the sides by the large intestine; a portion of it extends below the superior aperture of the pelvis and lies in front of the rectum. It is in relation, in front, with the greater omentum and abdominal parietes, and is connected to the vertebral column by a fold of peritoneum, the mesentery. The small intestine is divisible into three portions: the duodenum, the jejunum, and the ileum.

 

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 The duodenum and pancreas.

 

  The Duodenum has received its name from being about equal in length to the breadth of twelve fingers (25 cm.). It is the shortest, the widest, and the most fixed part of the small intestine, and has no mesentery, being only partially covered by peritoneum. Its course presents a remarkable curve, somewhat of the shape of an imperfect circle, so that its termination is not far removed from its starting-point.

  In the adult the course of the duodenum is as follows: commencing at the pylorus it passes backward, upward, and to the right, beneath the quadrate lobe of the liver to the neck of the gall-bladder, varying slightly in direction according to the degree of distension of the stomach: it then takes a sharp curve and descends along the right margin of the head of the pancreas, for a variable distance, generally to the level of the upper border of the body of the fourth lumbar vertebra. It now takes a second bend, and passes from right to left across the vertebral column, having a slight inclination upward; and on the left side of the vertebral column it ascends for about 2.5 cm., and then ends opposite the second lumbar vertebra in the jejunum. As it unites with the jejunum it turns abruptly forward, forming the duodendojejunal flexure. From the above description it will be seen that the duodenum may be divided into four portions: superior, descending, horizontal, and ascending.

 

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Interior of the descending portion of the duodenum, showing bile papilla.

 

VIDEO

Relations.The superior portion (pars superior; first portion) is about 5 cm. long. Beginning at the pylorus, it ends at the neck of the gall-bladder. It is the most movable of the four portions. It is almost completely covered by peritoneum, but a small part of its posterior surface near the neck of the gall-bladder and the inferior vena cava is uncovered; the upper border of its first half has the hepatoduodenal ligament attached to it, while to the lower border of the same segment the greater omentum is connected. It is in such close relation with the gall-bladder that it is usually found to be stained by bile after death, especially on its anterior surface. It is in relation above and in front with the quadrate lobe of the liver and the gall-bladder; behind with the gastroduodenal artery, the common bile duct, and the portal vein; and below and behind with the head and neck of the pancreas.

  The descending portion (pars descendens; second portion) is from 7 to 10 cm. long, and extends from the neck of the gall-bladder, on a level with the first lumbar vertebra, along the right side of the vertebral column as low as the upper border of the body of the fourth lumbar vertebra. It is crossed in its middle third by the transverse colon, the posterior surface of which is uncovered by peritoneum and is connected to the duodenum by a small quantity of connective tissue. The supra- and infracolic portions are covered in front by peritoneum, the infracolic part by the right leaf of the mesentery. Posteriorly the descending portion of the duodenum is not covered by peritoneum. The descending portion is in relation, in front, from above downward, with the duodenal impression on the right lobe of the liver, the transverse colon, and the small intestine; behind, it has a variable relation to the front of the right kidney in the neighborhood of the hilum, and is connected to it by loose areolar tissue; the renal vessels, the inferior vena cava, and the Psoas below, are also behind it. At its medial side is the head of the pancreas, and the common bile duct; to its lateral side is the right colic flexure. The common bile duct and the pancreatic duct together perforate the medial side of this portion of the intestine obliquely, some 7 to 10 cm. below the pylorus; the accessory pancreatic duct sometimes pierces it about 2 cm. above and slightly in front of these.

  The horizontal portion (pars horizontalis; third or preaortic or transverse portion) is from 5 to 7.5 cm. long. It begins at the right side of the upper border of the fourth lumbar vertebra and passes from right to left, with a slight inclination upward, in front of the great vessels and crura of the diaphragm, and ends in the ascending portion in front of the abdominal aorta. It is crossed by the superior mesenteric vessels and the mesentery. Its front surface is covered by peritoneum, except near the middle line, where it is crossed by the superior mesenteric vessels. Its posterior surface is uncovered by peritoneum, except toward its left extremity, where the posterior layer of the mesentery may sometimes be found covering it to a variable extent. This surface rests upon the right crus of the diaphragm, the inferior vena cava, and the aorta. The upper surface is in relation with the head of the pancreas.

  The ascending portion (pars ascendens; fourth portion) of the duodenum is about 2.5 cm long. It ascends on the left side of the aorta, as far as the level of the upper border of the second lumbar vertebra, where it turns abruptly forward to become the jejunum, forming the duodenojejunal flexure. It lies in front of the left Psoas major and left renal vessels, and is covered in front, and partly at the sides, by peritoneum continuous with the left portion of the mesentery.

  The superior part of the duodenum, as stated above, is somewhat movable, but the rest is practically fixed, and is bound down to neighboring viscera and the posterior abdominal wall by the peritoneum. In addition to this, the ascending part of the duodenum and the duodenojejunal flexure are fixed by a structure to which the name of Musculus suspensorius duodeni has been given. This structure commences in the connective tissue around the celiac artery and left crus of the diaphragm, and passes downward to be inserted into the superior border of the duodenojejunal curve and a part of the ascending duodenum, and from this it is continued into the mesentery. It possesses, according to Treitz, plain muscular fibers mixed with the fibrous tissue of which it is principally made up. It is of little importance as a muscle, but acts as a suspensory ligament.

 BOWEL can be subdivided into small intestine and large intestine. Small intestine consists of duodenum (portion without mesentery), jejunum and ileum (mesenteric portion). Large intestine includes cecum and colon (ascending colon, transverse colon, descending colon, sigmoid colon) and rectum with anal channel.

SMALL INTESTINE. Topography: holotopy - small intestine lies in abdominal cavity and occupies epigastrium, mesogastrium and partly hypogastrium (pubic region). Skeletotopy - small intestine extends from the level of Th12 vertebral body till area of right iliac fossa. Syntopy - large intestine, duodenum, pancreas, liver, ductus choledochus, right kidney lie around coils of small intestine.

 

Horseshoe-shaped  Duodenum projected in umbilical area. The duodenum was given its name because it is usually 12 fingerbreadths long (about 25cm - 30 cm). It subdivides into superior part (ampoule on level of the Th 12 - L 1 vertebrae), descending (on the level of L 1-3 vertebrae), horizontal part (on level of L 3 vertebra) and ascending part (on the level of L 2-3 vertebrae). Transition of superior part into downward is called superior flexura of duodenum, transition of descending into horizontal part - inferior flexura of duodenum; transition of duodenum into jejunum generates duodenojejunal flexura, which fixed by suspensorius muscle of duodenum to ht eposterior abdominal wall (ligament of Treitz).

Duodenum adjoins above to quadrate lobe of liver and gallbladder, in inferiorly - to right kidney with adrenal gland and by internal surface girds head of pancreas. Duodenum is covered by peritoneum from one side (retroperineal position). Hepatoduodenal ligament passes from liver to intestine.

Structure of duodenal wall:

• external membrane (fibrous and in front - anterior serous /peritoneum/);

• middle membrane - muscular, which consists of external longitudinal and internal circular fibres;

• internal membrane - mucous membrane with well developed submucous  stratum. There are circular folds in all duodenal portiotns, medial wall of descending part contains longitudinal fold of duodenum, which carries major papilla (ampoule of ductus choledochus and pancreatic duct opens here) and minor papilla of duodenum, where accessories duct of pancreas opens.

The Jejunum occupies initial 2/5 part of mesenteric small intestine and has a structure typical for gastric-intestinal tract. The wall consists of: Serous membrane, muscular membrane, formed by longitudinal layer (stratum longitudinale) and circular layer (stratum circulare), and mucous membrane (tunica mucosa). Last forms the numerous of circular folds. Mucous membrane carries a numerous of specific finger-like processes that project from the surface of the mucosa into the lumen. They are fingerlike projections consisting of a core of reticular tissue covered by a surface epithelium. The connective tissue core contains numerous blood capillaries forming a plexus. The endothelium lining the blood capillaries is fenestrated thus allowing rapid absorption of nutrients into the blood. They are responsible for absorption of amino acids and carbohydrates, present in digested food. Some villi contain a central lymphatic vessel and called a lacteal (for absorption of fat). Solitary lymphatic follicles represent lymphoid apparatus of mucous membrane of the jejunum.

The Ileum occupies 3/5 terminal portion of small intestine and has a structure, analogic to jejunum. Lymphoid apparatus of mucous membrane of the ileum is represented by aggregated lymphatic follicles (Payer's patches). An ileal diverticulum (Meckel's diverticulum) is one of the most common malformations of the digestive tract. It occurs in 1 to 2% of people. This blind sac or fingerlike pouch is the remnant of the proximal part of the embryonic yolk stalk, jt is of clinical significance because it sometimes becomes inflamed and may cause symptoms that appendicitis.

Serous membrane /Peritoneum/ cover Jejunum and Ileum completely, and forms for them mesentery which contains blood and lymphatic vessels, nodes, nerves that supply the bowel also adipose tissue.

Vessels and Nerves.—The arteries supplying the duodenum are the right gastric and superior pancreaticoduodenal branches of the hepatic, and the inferior pancreaticoduodenal branch of the superior mesenteric. The veins end in the lienal and superior mesenteric. The nerves are derived from the cœliac plexus.

 

Jejunum and Ileum.—The remainder of the small intestine from the end of the duodenum is named jejunum and ileum; the former term being given to the upper two-fifths and the latter to the lower three-fifths. There is no morphological line of distinction between the two, and the division is arbitrary; but at the same time the character of the intestine gradually undergoes a change from the commencement of the jejunum to the end of the ileum, so that a portion of the bowel taken from these two situations would present characteristic and marked differences. These are briefly as follows:

  The Jejunum (intestinum jejunum) is wider, its diameter being about 4 cm., and is thicker, more vascular, and of a deeper color than the ileum, so that a given length weighs more. The circular folds (valvulæ conniventes) of its mucous membrane are large and thickly set, and its villi are larger than in the ileum. The aggregated lymph nodules are almost absent in the upper part of the jejunum, and in the lower part are less frequently found than in the ileum, and are smaller and tend to assume a circular form. By grasping the jejunum between the finger and thumb the circular folds can be felt through the walls of the gut; these being absent in the lower part of the ileum, it is possible in this way to distinguish the upper from the lower part of the small intestine.

 

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Section of duodenum of cat.

   The Ileum (intestinum ileum) is narrow, its diameter being 3.75 cm., and its coats thinner and less vascular than those of the jejunum. It possesses but few circular folds, and they are small and disappear entirely toward its lower end, but aggregated lymph nodules (Peyer’s patches) are larger and more numerous. The jejunum for the most part occupies the umbilical and left iliac regions, while the ileum occupies chiefly the umbilical, hypogastric, right iliac, and pelvic regions. The terminal part of the ileum usually lies in the pelvis, from which it ascends over the right Psoas and right iliac vessels; it ends in the right iliac fossa by opening into the medial side of the commencement of the large intestine. The jejunum and ileum are attached to the posterior abdominal wall by an extensive fold of peritoneum, the mesentery, which allows the freest motion, so that each coil can accommodate itself to changes in form and position. The mesentery is fan-shaped; its posterior border or root, about 15 cm. long, is attached to the posterior abdominal wall from the left side of the body of the second lumbar vertebra to the right sacroiliac articulation, crossing successively the horizontal part of the duodenum, the aorta, the inferior vena cava, the ureter, and right Psoas muscle. Its breadth between its vertebral and intestinal borders averages about 20 cm., and is greater in the middle than at its upper and lower ends. According to Lockwood it tends to increase in breadth as age advances. Between the two layers of which it is composed are contained bloodvessels, nerves, lacteals, and lymph glands, together with a variable amount of fat.

Meckel’s Diverticulum (diverticulum ilei).—This consists of a pouch which projects from the lower part of the ileum in about 2 per cent. of subjects. Its average position is about 1 meter above the colic valve, and its average length about 5 cm. Its caliber is generally similar to that of the ileum, and its blind extremity may be free or may be connected with the abdominal wall or with some other portion of the intestine by a fibrous band. It represents the remains of the proximal part of the vitelline duct, the duct of communication between the yolk-sac and the primitive digestive tube in early fetal life.

 Structure.—The wall of the small intestine is composed of four coats: serous, muscular, areolar, and mucous.

  The serous coat (tunica serosa) is derived from the peritoneum. The superior portion of the duodenum is almost completely surrounded by this membrane near its pyloric end, but is only covered in front at the other extremity; the descending portion is covered by it in front, except where it is carried off by the transverse colon; and the inferior portion lies behind the peritoneum which passes over it without being closely incorporated with the other coats of this part of the intestine, and is separated from it in and near the middle line by the superior mesenteric vessels. The rest of the small intestine is surrounded by the peritoneum, excepting along its attached or mesenteric border; here a space is left for the vessels and nerves to pass to the gut.

  The muscular coat (tunica muscularis) consists of two layers of unstriped fibers: an external, longitudinal, and an internal, circular layer. The longitudinal fibers are thinly scattered over the surface of the intestine, and are more distinct along its free border. The circular fibers form a thick, uniform layer, and are composed of plain muscle cells of considerable length. The muscular coat is thicker at the upper than at the lower part of the small intestine.

  The areolar or submucous coat (tela submucosa) connects together the mucous and muscular layers. It consists of loose, filamentous areolar tissue containing bloodvessels, lymphatics, and nerves. It is the strongest layer of the intestine.

  The mucous membrane (tunica mucosa) is thick and highly vascular at the upper part of the small intestine, but somewhat paler and thinner below. It consists of the following structures: next the areolar or submucous coat is a double layer of unstriped muscular fibers, outer longitudinal and inner circular, the muscularis mucosæ internal to this is a quantity of retiform tissue, enclosing in its meshes lymph corpuscles, and in this the bloodvessels and nerves ramify; lastly, a basement membrane, supporting a single layer of epithelial cells, which throughout the intestine are columnar in character. The cells are granular in appearance, and each possesses a clear oval nucleus. At their superficial or unattached ends they present a distinct layer of highly refracting material, marked by vertical striæ, the striated border.

 

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Villi of small intestine, showing bloodvessels and lymphatic vessels

 

  The mucous membrane presents for examination the following structures, contained within it or belonging to it:

Circular folds.

Duodenal glands.

Villi.

Solitary lymphatic nodules.

Intestinal glands.

Aggregated lymphatic nodules.

  The circular folds (plicæ circulares [Kerkringi]; valvulæ conniventes; valves of Kerkring) are large valvular flaps projecting into the lumen of the bowel. They are composed of reduplications of the mucous membrane, the two layers of the fold being bound together by submucous tissue; unlike the folds in the stomach, they are permanent, and are not obliterated when the intestine is distended. The majority extend transversely around the cylinder of the intestine for about one-half or two-thirds of its circumference, but some form complete circles, and others have a spiral direction; the latter usually extend a little more than once around the bowel, but occasionally two or three times. The larger folds are about 8 mm. in depth at their broadest part; but the greater number are of smaller size. The larger and smaller folds alternate with each other. They are not found at the commencement of the duodenum, but begin to appear about 2.5 or 5 cm. beyond the pylorus. In the lower part of the descending portion, below the point where the bile and pancreatic ducts enter the intestine, they are very large and closely approximated. In the horizontal and ascending portions of the duodenum and upper half of the jejunum they are large and numerous, but from this point, down to the middle of the ileum, they diminish considerably in size. In the lower part of the ileum they almost entirely disappear; hence the comparative thinness of this portion of the intestine, as compared with the duodenum and jejunum. The circular folds retard the passage of the food along the intestines, and afford an increased surface for absorption.

 

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 Vertical section of a human aggregated lymphatic nodule, injected through its lymphatic canals. a. Villi with their chyle passages. b. Intestinal glands. c. Muscularis mucosæ. d. Cupola or apex of solitary nodule. e. Mesial zone of nodule. f. Base of nodule. g. Points of exit of the lacteals from the villi, and entrance into the true mucous membrane. h. Retiform arrangement of the lymphatics in the mesial zone. i. Course of the latter at the base of the nodule. k. Confluence of the lymphatics opening into the vessels of the submucous tissue. l. Follicular tissue of the latter.

 

  The intestinal villi (villi intestinales) are highly vascular processes, projecting from the mucous membrane of the small intestine throughout its whole extent, and giving to its surface a velvety appearance. They are largest and most numerous in the duodenum and jejunum, and become fewer and smaller in the ileum.

  Structure of the villi.—The essential parts of a villus are: the lacteal vessel, the bloodvessels, the epithelium, the basement membrane, and the muscular tissue of the mucosa, all being supported and held together by retiform lymphoid tissue.

  The lacteals are in some cases double, and in some animals multiple, but usually there is a single vessel. Situated in the axis of the villus, each commences by dilated cecal extremities near to, but not quite at, the summit of the villus. The walls are composed of a single layer of endothelial cells.

  The muscular fibers are derived from the muscularis mucosæ, and are arranged in longitudinal bundless around the lacteal vessel, extending from the base to the summit of the villus, and giving off, laterally, individual muscle cells, which are enclosed by the reticulum, and by it are attached to the basement-membrane and to the lacteal.

  The bloodvessels form a plexus under the basement membrane, and are enclosed in the reticular tissue.

  These structures are surrounded by the basement membrane, which is made up of a stratum of endothelial cells, and upon this is placed a layer of columnar epithelium, the characteristics of which have been described. The retiform tissue forms a net-work in the meshes of which a number of leucocytes are found.

 

 

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Diagrams showing the arrangement and variations of the loops of the mesenteric vessels for various segments of the small intestine of average length. Nearest the duodenum the mesenteric loops are primary, the vasa recta are long and regular in distribution, and the translucent spaces (lunettes) are extensive. Toward the ileocolic junction, secondary and tertiary loops are observed, the vessels are smaller and become obscured by numerous fat-tabs. 

  

The intestinal glands (glandulæ intestinales [Lieberkühni]; crypts of Lieberkühn) are found in considerable numbers over every part of the mucous membrane of the small intestine. They consist of minute tubular depressions of the mucous membrane, arranged perpendicularly to the surface, upon which they open by small circular apertures. They may be seen with the aid of a lens, their orifices appearing as minute dots scattered between the villi. Their walls are thin, consisting of a basement membrane lined by columnar epithelium, and covered on their exterior by capillary vessels.

  The duodenal glands (glandulæ duodenales [Brunneri]; Brunner’s glands) are limited to the duodenum and are found in the submucous areolar tissue. They are largest and most numerous near the pylorus, forming an almost complete layer in the superior portion and upper half of the descending portions of the duodenum. They then begin to diminish in number, and practically disappear at the junction of the duodenum and jejunum. They are small compound acinotubular glands consisting of a number of alveoli lined by short columnar epithelium and opening by a single duct on the inner surface of the intestine.

  The solitary lymphatic nodules (noduli lymphatici solitarii; solitary glands) are found scattered throughout the mucous membrane of the small intestine, but are most numerous in the lower part of the ileum. Their free surfaces are covered with rudimentary villi, except at the summits, and each gland is surrounded by the openings of the intestinal glands. Each consists of a dense interlacing retiform tissue closely packed with lymph-corpuscles, and permeated with an abundant capillary network. The interspaces of the retiform tissue are continuous with larger lymph spaces which surround the gland, through which they communicate with the lacteal system. They are situated partly in the submucous tissue, partly in the mucous membrane, where they form slight projections of its epithelial layer.

  The aggregated lymphatic nodules (noduli lymphatici aggregati; Peyer’s patches; Peyer’s glands; agminated follicles; tonsillæ intestinales) form circular or oval patches, from twenty to thirty in number, and varying in length from 2 to 10 cm. They are largest and most numerous in the ileum. In the lower part of the jejunum they are small, circular, and few in number. They are occasionally seen in the duodenum. They are placed lengthwise in the intestine, and are situated in the portion of the tube most distant from the attachment of the mesentery. Each patch is formed of a group of solitary lymphatic nodules covered with mucous membrane, but the patches do not, as a rule, possess villi on their free surfaces. They are best marked in the young subject, become indistinct in middle age, and sometimes disappear altogether in advanced life. They are freely supplied with bloodvesselsm, which form an abundant plexus around each follicle and give off fine branches permeating the lymphoid tissue in the interior of the follicle. The lymphatic plexuses are especially abundant around these patches.

 

Vessels and Nerves.—The jejunum and ileum are supplied by the superior mesenteric artery, the intestinal branches of which, having reached the attached border of the bowel, run between the serous and muscular coats, with frequent inosculations to the free border, where they also anastomose with other branches running around the opposite surface of the gut. From these vessels numerous branches are given off, which pierce the muscular coat, supplying it and forming an intricate plexus in the submucous tissue. From this plexus minute vessels pass to the glands and villi of the mucous membrane. The veins have a similar course and arrangement to the arteries. The lymphatics of the small intestine (lacteals) are arranged in two sets, those of the mucous membrane and those of the muscular coat. The lymphatics of the villi commence in these structures in the manner described above. They form an intricate plexus in the mucous and submucous tissue, being joined by the lymphatics from the lymph spaces at the bases of the solitary nodules, and from this pass to larger vessels at the mesenteric border of the gut. The lymphatics of the muscular coat are situated to a great extent between the two layers of muscular fibers, where they form a close plexus; throughout their course they communicate freely with the lymphatics from the mucous membrane, and empty themselves in the same manner as these into the origins of the lacteal vessels at the attached border of the gut.

  The nerves of the small intestines are derived from the plexuses of sympathetic nerves around the superior mesenteric artery. From this source they run to the myenteric plexus (Auerbach’s plexus) of nerves and ganglia situated between the circular and longitudinal muscular fibers from which the nervous branches are distributed to the muscular coats of the intestine. From this a secondary plexus, the plexus of the submucosa (Meissner’s plexus) is derived, and is formed by branches which have perforated the circular muscular fibers. This plexus lies in the submucous coat of the intestine; it also contains ganglia from which nerve fibers pass to the muscularis mucosæ and to the mucous membrane. The nerve bundles of the submucous plexus are finer than those of the myenteric plexus.

 

The small intestine plays the primary role in the digestion and absorption of nutrients. The small intestine averages 6 m (20 ft) in length (range: 4.5-7.5 m; 14.8-24.6 ft) and has a diameter ranging from 4 cm (1.6 in.) at the stomach to about 2.5 cm (1 in.) at the junction with the large intestine. It occupies all abdominal regions except the right and left hypochondriac and epigastric regions. Ninety percent of nutrient absorption occurs in the small intestine, and most of the rest occurs in the large intestine. The small intestine has three subdivisions: (1) the duodenum, (2) the jejunum, and (3) the ileum.

The duodenum, 25 cm (10 in.) in length, is the section closest to the stomach. This portion of the small intestine is a "mixing bowl" that receives chyme from the stomach and digestive secretions from the pancreas and liver. From its connection with the stomach, the duodenum curves in a C that encloses the pancreas. Except for the proximal 2.5 cm (1 in.), the duodenum is in a retroperitoneal position between vertebrae L1 and L4.

A rather abrupt bend marks the boundary between the duodenum and the jejunum. At this junction, the small intestine reenters the peritoneal cavity, supported by a sheet of mesentery. The jejunum is about 2.5 meters (8 ft) long. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.

The ileum is the third and last segment of the small intestine. It is also the longest, averaging 3.5 meters (12 ft) in length. The ileum ends at a sphincter, the ileocecal valve, which controls the flow of materials from the ileum into the cecum of the large intestine.

The small intestine fills much of the peritoneal cavity, and its position is stabilized by a broad mesentery attached to the dorsal body wall.

 Movement of the small intestine during digestion is restricted by the stomach, the large intestine, the abdominal wall, and the pelvic girdle. Blood vessels, lymphatics, and nerves reach these segments of the small intestine within the connective tissue of the mesentery. The primary blood vessels involved are branches of the superior mesenteric artery and the superior mesenteric vein.

 

The intestinal lining bears a series of transverse folds called plicae, or plicae circulares. Unlike the rugae in the stomach, each plica is a permanent feature that does not disappear when the small intestine fills with chyme. There are roughly 800 plicae along the length of the small intestine, and their presence greatly increases the surface area available for absorption.

 

Intestinal Villi


The mucosa of the small intestine is thrown into a series of fingerlike projections, the intestinal villi. The intestinal villi are covered by a simple columnar epithelium that is carpeted with microvilli. Because the microvilli project from the epithelium like the bristles on a brush, these cells are said to have a brush border.

If the small intestine were a simple tube with smooth walls, it would have a total absorptive area of roughly 3300 cm2 (3.6 ft2). Instead, the mucosa contains plicae circulares, each plica supports a forest of villi, and each villus is covered by epithelial cells whose exposed surfaces contain microvilli. This arrangement increases the total area for absorption by a factor of more than 600, to approximately 2 million square centimeters, or more than 2200 square feet.

The lamina propria of each villus contains an extensive network of capillaries. These capillaries originate in a vascular network within the submucosa. They transport respiratory gases and carry absorbed nutrients to the hepatic portal circulation for delivery to the liver. The liver adjusts the nutrient concentrations of the blood before it reaches the general systemic circulation.

In addition to capillaries and nerve endings, each villus contains a lymphatic capillary called a lacteal (lacteus, milky). Lacteals transport materials that are unable to enter blood capillaries. For example, absorbed fatty acids are assembled into protein-lipid packages too large to diffuse into the bloodstream. These packets, called chylomicrons, reach the venous circulation by way of the thoracic duct, which delivers lymph into the left subclavian vein. The name lacteal refers to the pale, cloudy appearance of lymph that contains large quantities of lipids.

Contractions of the muscularis mucosae and smooth muscle cells within the villi move the villi back and forth, exposing the epithelial surfaces to the liquefied intestinal contents. This movement improves the efficiency of absorption, because local differences in the nutrient concentration of the chyme will be quickly eliminated. Movements of the villi also squeeze the lacteals, thus assisting in the movement of lymph out of the villi.

 

Intestinal Crypts


Between the columnar epithelial cells, goblet cells eject mucins onto the intestinal surfaces. At the bases of the villi are the entrances to the intestinal crypts (also known as intestinal glands or crypts of Lieberkuhn). These glandular pockets extend deep into the underlying lamina propria. Near the base of each intestinal gland, stem cell divisions produce new generations of epithelial cells. These new cells are continuously displaced toward the intestinal surface. Within a few days, they will have reached the tip of a villus, where they are shed into the intestinal lumen. This ongoing process renews the epithelial surface and the subsequent disintegration of the shed cells adds enzymes to the chyme.

Several important brush border enzymes enter the intestinal lumen in this way. Brush border enzymes are integral membrane proteins located on the surfaces of intestinal microvilli. The enzymes have important digestive functions: Materials in contact with the brush border are attacked by these enzymes, and the breakdown products are absorbed by the epithelial cells. Once the epithelial cells are shed, they disintegrate within the lumen, and the intracellular and brush border enzymes enter the chyme. There they continue to function until proteolytic enzymes break them apart. Enterokinase, also called enteropeptidase, is a brush border enzyme that enters the lumen in this way. Enterokinase does not directly participate in digestion, but it activates proenzymes secreted by the pancreas. (We shall consider the functions of enterokinase and other brush border enzymes in a later section.) Intestinal crypts also contain enteroendocrine cells responsible for theproduction of several intestinal hormones, including gastrin, cholecystokinin, and secretin.

 

Regional Specializations


The regions of the small intestine have histological specializations related to their primary functions. The duodenum contains few plicae; the villi are numerous but shorter and stumpier than those of the jejunum. There are numerous mucous glands in the duodenum, both within the epithelium and beneath it. In addition to the intestinal crypts, the submucosa contains submucosal glands, or Brunner's glands, which produce copious quantities of mucus when chyme arrives from the stomach. Mucus produced by these glands protects the epithelium from the acidic chyme. It also contains buffers that help elevate the pH of the chyme. Along the length of the duodenum, the pH of the chyme goes from 1-2 to 7-8. The submucosal glands also secrete the hormoneurogastrone, which inhibits gastric acid production. Urogastrone, or epidermal growth factor (EGF), stimulates the division of epithelial cells along the digestive tract as well as stem cell activity in other areas.

Jejunum. Plicae and villi are prominent over the proximal half of the jejunum. As materials approach the ileum, the plicae and villi become smaller and continue to diminish in size to the end of the ileum. This reduction parallels the reduction in absorptive activity; most nutrient absorption has occurred before ingested materials reach the ileum. One rather drastic surgical method of promoting weight loss is the removal of a significant portion of the jejunum. The reduction in absorptive area causes a marked weight loss, but the side effects can be very troublesome.

Ileum. The ileum adjacent to the large intestine lacks plicae altogether, and the scattered villi are stumpy and conical. The ileum also contains 20-30 masses of lymphoid tissue called aggregate lymphoid nodules, or Peyer's patches. The lymphocytes in these nodules protect the small intestine from bacteria that are normal inhabitants of the large intestine. Lymphoid nodules are most abundant in the terminal portion of the ileum, near the entrance to the large intestine.

Intestinal Movements. After chyme has arrived in the duodenum, weak peristaltic contractions move it slowly toward the jejunum. These contractions are myenteric reflexes not under CNS control. Their effects are limited to within a few centimeters of the site of the original stimulus. These short reflexes are controlled by motor neurons in the submucosal and myenteric plexuses. In addition, some of the smooth muscle cells contract periodically, even without stimulation, establishing a basic contractile rhythm that then spreads from cell to cell.

 

 

Theme 2. Structure, topography and functіon of large іntestіne

 

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

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

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 muscle—the 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 mesоcolic 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 anоrectal 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).

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.

 

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.

 

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Prepared by

Reminetskyy B.Y.