1. Organs of digestion. Postnatal development. Mouth cav³ty, walls and communications. Structure of permanent and m³lk teeth. structure and function of tongue and sal³vary glands
2. Development, structure and function of pharynx and oesophagus. Stomach, structure and topography
3. Structure, topography and functionof the small intestine
Lesson No 12
Organs of digestion. Postnatal development. Mouth cav³ty, walls and communications
SPLANCHNOLOGY is doctrine about viscera, which disposed in thoracic, abdominal and pelvic cavities, also in head and neck. Internal organs may be divided into digestive, respiratory, urinary and genital systems and endocrine glands.
The digestive system is a group of organs that work like wrecking equipment to break down the chemical components of food, through the use of digestive juices, into tiny nutrients which can be absorbed to generate energy for the body. Digestion begins in the mouth with the teeth, which grind the food into small particles; the tongue, a powerful muscle which detects "good" and "bad" flavours in food and manipulates the food between the teeth for chewing, and saliva, a watery fluid which lubricates chewing and swallowing and begins the process of digestion. The digestive system begins in the mouth, continues in the pharynx (throat) and oesophagus and into the "gut" region: the stomach, small and large intestines, the rectum and the anus. Food is chewed, pulped and mixed with saliva to become a soft mass which will easily travel down the oesophagus. The tongue traps the food and forces it into the throat, which is a mass of muscles and tissues which transports food into the gut system for final processing and distribution. The liver and the pancreas also secrete digestive juices that break down food as it passes through the digestive ducts. Not all that we eat can be digested, so the waste must be disposed of in an efficient way. It may not be a savoury ending for the food or drink we thought was so delicious in the mouth, but it is just as important for our health.
The apparatus for the digestion (Apparatus Digestorius; Organs Of Digestion) of the food consists of the digestive tube and of certain accessory organs.
The Digestive Tube (alimentary
canal) is a musculomembranous tube, about
The accessory organs are the teeth, for purposes of mastication; the three pairs of salivary glands—the parotid, submandibular, and sublingual—the 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.
Head end of human embryo of about thirty to thirty-one days.
Floor of pharynx of human embryo about twenty-six days old
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.
Floor of pharynx of human embryo about thirty days old.
The Palatine Tonsils.—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.
Sketches in profile of two stages in the development of the human digestive tube.
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.
Front view of two successive stages in the development of the digestive tube.
The primitive mesentery of a six weeks’ human embryo, half schematic.
Abdominal part of digestive tube and its attachment to the primitive or common mesentery. Human embryo of six weeks.
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 colon—the 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.
Reconstruction of a human embryo of
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 .
Diagrams to illustrate two stages in the development of the digestive tube and its mesentery. The arrow indicates the entrance to the bursa omentalis.
Final disposition of the intestines and their vascular relations. (Jonnesco.) A. Aorta. H. Hepatic artery. M, Col. Branches of superior mesenteric artery. m, m’. Branches of inferior mesenteric artery. S. Splenic artery
Schematic figure of the bursa omentalis, etc. Human embryo of eight weeks.
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 (Fig. 991).
FIG. 990– Diagrams to illustrate the development of the greater omentum and transverse mesocolon. (See enlarged image)
Tail end of human embryo from fifteen to eighteen days old.
Cloaca of human embryo from twenty-five to twenty-seven days old
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. 159
Tail end of human embryo, from eight and a half to nine weeks old.
Oral cavity is bordered up by palate, which separates the oral cavity from the nasal cavities and the nasal part of the pharynx or nasopharynx; in front and laterally – by cheeks, from below – by oral diaphragm (formed by mylohyoid muscle). 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 proper mouth cavity. Both portions communicate each other through the space behind last molars and through the fissure between upper and lower teeth. The vestibule is the slit like space between the lips, cheeks, the teeth and the gingivae. The vestibule communicates with the exterior through the orifice of the mouth - the opening, through which food and other substances pass into the oral cavity. Duct of parotid salivary gland opens into vestibule.
Cheeks have a muscular component - buccinator muscle. Superficial to the fascia covering this muscle is the buccal fat pad - Bisha body. It gives the cheeks their rounded contour, particularly in infants for sucking the milk. The lips and cheeks function as a unit (for example - during blowing, eating, sucking, and kissing). They act as an oral sphincter in pushing food from the vestibule to the oral cavity proper. Mucous membrane of the cheeks contains small buccal salivary glands.
Palate consists of two regions: 1. the anterior two-thirds or bony part - the hard palate. 2. the mobile posterior one-third or fibromuscular part – the soft palate. The hard palate formed by palatine processes of the maxillae and the horizontal plates of the palatine bones covered by mucous membrane, which contains small salivary glands. Posteriorly the hard palate is continuous with the soft palate. The soft palate contains a membranous aponeurosis and is a movable, fibromuscular fold that is attached to the posterior edge of the hard palate. The soft palate or velum palatinum extends posterior inferiorly to a curved free margin from which hangs a conical process - the uvula. It separates the nasopharynx superiorly from the oropharynx inferiorly. Laterally the soft palate is continuous with the wall of the pharynx and is joined to the tongue and pharynx by the palatoglossal and
palatopharyngeal arches, between which locate the palatine tonsil. Deep to the palatal mucosa are mucous glands. The soft palate is formad by 5 muscles:
Tensor veli palatini muscle – stretches velum palatine and widens aperture of uditory tube;
Levator veli palatini muscle – lifts soft palatine;
Uvulae muscle – lifts and shortens the uvula;
Palatoglossus muscle – lowers the velum palatinum, narrows the fauceus and lifts the lingual root;
Palatopharyngeus muscle –narrows the fauceus and lifts the pharynx.
Theme 2. Structure of permanent and m³lk teeth
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.
Permanent teeth of upper dental arch, seen from below.
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
6 – 9 months
7 – 9 years
16 - 20 months
10 – 13 years
10 – 12 years
11 –15 years
12 - 15 months
6 –7 years
20 – 24 months
13 – 16 years
Third Molar ("wisdom tooth")
18 – 30 years
Order of cutting of milk teeth:
• First molars;
• Second molars.
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;
• Second premolars;
• Second molars;
• Third molars (called "wisdom tooth", present not in all person).
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 3. structure and function of tongue and sal³vary glands
The tongue is situated partly in the mouth and partly in the oropharynx. It consists of three parts: apex, body and root. Also tongue has dorsum (upper surface), inferior surface, margin and median sulcus. The dorsum of the tongue carries V-shaped sulcus terminalis with foramen cecum at the apex of this sulcus. Sulcus terminalis divide dorsum linguae into anterior presulcal and posterior postsulcal parts. There is lingual tonsil on the mucous membrane of root.
Lingual frenulum attaches anterior presulcal portion to the floor of the mouth. Fimbriate plicae pass laterally from frenulum. Sublingual plica runs laterally and backward from frenulum base, also it carries sublingual caruncle. Ducts of submandibular and sublingual glands open there. The mucous membrane on the oral part of the tongue carries numerous of the papillae:
· The filiform papillae and conic papillae - contain afferent nerve endings that are sensitive to touch.
· The fungiform papillae - small and mushroom-shaped. They usually appear as pink or red spots. Contain taste receptors located in the taste buds.
The vallate papillae - are the
largest papillae (1 to
· The foliate papillae - are small lateral folds of the lingual margins. They contain taste receptors.
There are four extrinsic and four intrinsic muscles of tongue.
Extrinsic group contains four muscles:
1. THE GENIOGLOSSUS MUSCLE arises by a short tendon from the mental spine of the mandible. Insertion: enters the tongue inferiorly and its fibres attach to the entire dorsum of the tongue. Actions: depresses the tongue and its posterior part protrudes it.
2. THE HYOGLOSSUS MUSCLE arises from the body and greater horn of the hyoid bone. Insertion: the side and inferior aspect of the tongue. Actions: depresses the tongue, pulling its sides inferiorly.
3. THE STYLOGLOSSUS MUSCLE originates from the anterior border of the styloid process near its tip and from the stylohyoid ligament. Insertion: the side and inferior aspect of the tongue. Actions: lifting the tongue and curls its sides to create a trough during swallowing.
4. THE PALATOGLOSSUS MUSCLE starts from the palatine aponeurosis of the soft palate. Insertion: the side and the lateral part of the tongue. Actions: elevates the posterior part of the tongue.
The intrinsic muscles are mainly concerned with altering the shape of the tongue, making it broad or narrow. Their fibbers run in three directions.
2. THE INFERIOR LONGITUDINAL Muscle consists of a narrow band close to the interior surface of the tongue. Actions: curls the tip of the tongue inferiorly, making the dorsum of the tongue convex.
3.THE TRANSVERSE Muscle lies deep to the superior longitudinal muscle. Origin: the fibrous lingual septum. Insertion: submucous fibrous tissue. Actions: narrows and increases the height of the tongue.
4. Vertical Muscle originates in dorsum of the tongue. Insertion: site of the tongue. Actions: flattens and broadens the tongue; acting with the transverse muscle, it increases the length of the tongue.
The Tongue (lingua) is the principal organ of the sense of taste, and an important organ of speech; it also assists in the mastication and deglutition of the food. It is situated in the floor of the mouth, within the curve of the body of the mandible.
Its Root (radix linguæ base) is directed backward, and connected with the hyoid bone by the Hyoglossi and Genioglossi muscles and the hyoglossal membrane; with the epiglottis by three folds (glossoepiglottic) of mucous membrane; with the soft palate by the glossopalatine arches; and with the pharynx by the Constrictores pharyngis superiores and the mucous membrane.
Its Apex (apex linguæ tip), thin and narrow, is directed forward against the lingual surfaces of the lower incisor teeth.
Its Inferior Surface (facies inferior linguæ under surface) is connected with the mandible by the Genioglossi; the mucous membrane is reflected from it to the lingual surface of the gum and on to the floor of the mouth, where, in the middle line, it is elevated into a distinct vertical fold, the frenulum linguæ. On either side lateral to the frenulum is a slight fold of the mucous membrane, the plica fimbriata, the free edge of which occasionally exhibits a series of fringe-like processes.
The apex of the tongue, part of the inferior surface, the sides, and dorsum are free.
The Dorsum of the Tongue (dorsum
linguæ) (Fig. 1014) is convex and marked by a median sulcus, which divides it into
symmetrical halves; this sulcus ends behind, about
The Papillæ of the Tongue are projections of the corium. They are thickly distributed over the anterior two-thirds of its dorsum, giving to this surface its characteristic roughness. The varieties of papillæ met with are the papillæ vallatæ, papillæ fungiformes, papillæ filiformes, and papillæ simplices.
The mouth cavity. The apex of the tongue is turned upward,
and on the right side a superficial dissection of its under surface has been made
The papillæ vallatæ (circumvallate
papillæ) are of large size, and vary from eight to twelve in number.
They are situated on the dorsum of the tongue immediately in front of the
foramen cecum and sulcus terminalis, forming a row on either side; the two rows
run backward and medialward, and meet in the middle line, like the limbs of the
letter V inverted. Each papilla consists of a projection of mucous membrane
from 1 to
The papillæ fungiformes (fungiform papillæ), more numerous than the preceding, are found chiefly at the sides and apex, but are scattered irregularly and sparingly over the dorsum. They are easily recognized, among the other papillæ, by their large size, rounded eminences, and deep red color. They are narrow at their attachment to the tongue, but broad and rounded at their free extremities, and covered with secondary papillæ.
The papillæ simplices are similar to those of the skin, and cover the whole of the mucous membrane of the tongue, as well as the larger papillæ. They consist of closely set microscopic elevations of the corium, each containing a capillary loop, covered by a layer of epithelium.
Muscles of the Tongue.—The tongue is divided into lateral halves by a median fibrous septum which extends throughout its entire length and is fixed below to the hyoid bone. In either half there are two sets of muscles, extrinsic and intrinsic; the former have their origins outside the tongue, the latter are contained entirely within it.
The extrinsic muscles (Fig. 1019) are:
Extrinsic muscles of the tongue. Left side.
The Genioglossus (Geniohyoglossus) is a flat triangular muscle close to and parallel with the median plane, its apex corresponding with its point of origin from the mandible, its base with its insertion into the tongue and hyoid bone. It arises by a short tendon from the superior mental spine on the inner surface of the symphysis menti, immediately above the Geniohyoideus, and from this point spreads out in a fan-like form. The inferior fibers extend downward, to be attached by a thin aponeurosis to the upper part of the body of the hyoid bone, a few passing between the Hyoglossus and Chondroglossus to blend with the Constrictores pharyngis; the middle fibers pass backward, and the superior ones upward and forward, to enter the whole length of the under surface of the tongue, from the root to the apex. The muscles of opposite sides are separated at their insertions by the median fibrous septum of the tongue; in front, they are more or less blended owing to the decussation of fasciculi in the median plane.
The Hyoglossus, thin and quadrilateral, arises from the side of the body and from the whole length of the greater cornu of the hyoid bone, and passes almost vertically upward to enter the side of the tongue, between the Styloglossus and Longitudinalis inferior. The fibers arising from the body of the hyoid bone overlap those from the greater cornu.
The Chondroglossus is sometimes
described as a part of the Hyoglossus, but is separated from it by fibers of
the Genioglossus, which pass to the side of the pharynx. It is about
A small slip of muscular fibers is occasionally found, arising from the cartilago triticea in the lateral hyothyroid ligament and entering the tongue with the hindermost fibers of the Hyoglossus.
The Styloglossus, the shortest and smallest of the three styloid muscles, arises from the anterior and lateral surfaces of the styloid process, near its apex, and from the stylomandibular ligament. Passing downward and forward between the internal and external carotid arteries, it divides upon the side of the tongue near its dorsal surface, blending with the fibers of the Longitudinalis inferior in front of the Hyoglossus; the other, oblique, overlaps the Hyoglossus and decussates with its fibers.
The intrinsic muscles are:
The Longitudinalis linguæ superior (
Coronal section of tongue, showing intrinsic muscles.
The Longitudinalis linguæ inferior (Inferior lingualis) is a narrow band situated on the under surface of the tongue between the Genioglossus and Hyoglossus. It extends from the root to the apex of the tongue: behind, some of its fibers are connected with the body of the hyoid bone; in front it blends with the fibers of the Styloglossus.
The Transversus linguæ (Transverse lingualis) consists of fibers which arise from the median fibrous septum and pass lateralward to be inserted into the submucous fibrous tissue at the sides of the tongue.
The Verticalis linguæ (Vertical lingualis) is found only at the borders of the forepart of the tongue. Its fibers extend from the upper to the under surface of the organ.
The median fibrous septum of the tongue is very complete, so that the anastomosis between the two lingual arteries is not very free.
Nerves.—The muscles of the tongue described above are supplied by the hypoglossal nerve.
Actions.—The movements of the tongue, although numerous and complicated, may be understood by carefully considering the direction of the fibers of its muscles. The Genioglossi, by means of their posterior fibers, draw the root of the tongue forward, and protrude the apex from the mouth. The anterior fibers draw the tongue back into the mouth. The two muscles acting in their entirety draw the tongue downward, so as to make its superior surface concave from side to side, forming a channel along which fluids may pass toward the pharynx, as in sucking. The Hyoglossi depress the tongue, and draw down its sides. The Styloglossi draw the tongue upward and backward. The Glossopalatini draw the root of the tongue upward. The intrinsic muscles are mainly concerned in altering the shape of the tongue, whereby it becomes shortened, narrowed, or curved in different directions; thus, the Longitudinalis superior and inferior tend to shorten the tongue, but the former, in addition, turn the tip and sides upward so as to render the dorsum concave, while the latter pull the tip downward and render the dorsum convex. The Transversus narrows and elongates the tongue, and the Verticalis flattens and broadens it. The complex arrangement of the muscular fibers of the tongue, and the various directions in which they run, give to this organ the power of assuming the forms necessary for the enunciation of the different consonantal sounds; and Macalister states “there is reason to believe that the musculature of the tongue varies in different races owing to the hereditary practice and habitual use of certain motions required for enunciating the several vernacular languages.”
Structure of the Tongue.—The tongue is partly invested by mucous membrane and a submucous fibrous layer.
The mucous membrane (tunica mucosa linguæ) differs in different parts. That covering the under surface of the organ is thin, smooth, and identical in structure with that lining the rest of the oral cavity. The mucous membrane of the dorsum of the tongue behind the foramen cecum and sulcus terminalis is thick and freely movable over the subjacent parts. It contains a large number of lymphoid follicles, which together constitute what is sometimes termed the lingual tonsil. Each follicle forms a rounded eminence, the center of which is perforated by a minute orifice leading into a funnel-shaped cavity or recess; around this recess are grouped numerous oval or rounded nodules of lymphoid tissue, each enveloped by a capsule derived from the submucosa, while opening into the bottom of the recesses are also seen the ducts of mucous glands. The mucous membrne on the anterior part of the dorsum of the tongue is thin, intimately adherent to the muscular tissue, and presents numerous minute surface eminences, the papillæ of the tongue. It consists of a layer of connective tissue, the corium or mucosa, covered with epithelium.
The epithelium is of the stratified squamous variety, similar to but much thinner than that of the skin: and each papilla has a separate investment from root to summit. The deepest cells may sometimes be detached as a separate layer, corresponding to the rete mucosum, but they never contain coloring matter.
The corium consists of a dense felt-work of fibrous connective tissue, with numerous elastic fibers, firmly connected with the fibrous tissue forming the septa between the muscular bundles of the tongue. It contains the ramifications of the numerous vessels and nerves from which the papillæ are supplied, large plexuses of lymphatic vessels, and the glands of the tongue.
Structure of the Papillæ.—The papillæ apparently resemble in structure those of the cutis, consisting of cone-shaped projections of connective tissue, covered with a thick layer of stratified squamous epithelium, and containing one or more capillary loops among which nerves are distributed in great abundance. If the epithelium be removed, it will be found that they are not simple elevations like the papillæ of the skin, for the surface of each is studded with minute conical processes which form secondary papillæ. In the papillæ vallatæ, the nerves are numerous and of large size; in the papillæ fungiformes they are also numerous, and end in a plexiform net-work, from which brush-like branches proceed; in the papillæ filiformes, their mode of termination is uncertain.
Glands of the Tongue.—The tongue is provided with mucous and serous glands.
The mucous glands are similar in
structure to the labial and buccal glands. They are found especially at the
back part behind the vallate papillæ, but are also present at the apex
and marginal parts. In this connection the anterior lingual glands (Blandin or
Nuhn) require special notice. They are situated on the under surface of the
apex of the tongue (Fig. 1013), one on either side of the frenulum, where they are covered by a
fasciculus of muscular fibers derived from the Styloglossus and Longitudinalis
inferior. They are from 12 to
The serous glands occur only at the back of the tongue in the neighborhood of the taste-buds, their ducts opening for the most part into the fossæ of the vallate papillæ. These glands are racemose, the duct of each branching into several minute ducts, which end in alveoli, lined by a single layer of more or less columnar epithelium. Their secretion is of a watery nature, and probably assists in the distribution of the substance to be tasted over the taste area. (Ebner.)
The septum consists of a vertical
layer of fibrous tissue, extending throughout the entire length of the median
plane of the tongue, though not quite reaching the dorsum. It is thicker behind
than in front, and occasionally contains a small fibrocartilage, about
The hyoglossal membrane is a strong fibrous lamina, which connects the under surface of the root of the tongue to the body of the hyoid bone. This membrane receives, in front, some of the fibers of the Genioglossi.
Taste-buds, the end-organs of the gustatory sense, are scattered over the mucous membrane of the mouth and tongue at irregular intervals. They occur especially in the sides of the vallate papillæ. In the rabbit there is a localized area at the side of the base of the tongue, the papilla foliata, in which they are especially abundant (Fig. 1021). They are described under the organs of the senses (page 991).
Vertical section of papilla foliata of the rabbit, passing across the folia.
Vessels and Nerves.—The main artery of the tongue is the lingual branch of the external carotid, but the external maxillary and ascending pharyngeal also give branches to it. The veins open into the internal jugular.
The lymphatics of the tongue have been described on page 696.
The sensory nerves of the tongue are: (1) the lingual branch of the mandibular, which is distributed to the papillæ at the forepart and sides of the tongue, and forms the nerve of ordinary sensibility for its anterior two-thirds; (2) the chorda tympani branch of the facial, which runs in the sheath of the lingual, and is generally regarded as the nerve of taste for the anterior two-thirds; this nerve is a continuation of the sensory root of the facial (nervus intermedius); (3) the lingual branch of the glossopharyngeal, which is distributed to the mucous membrane at the base and sides of the tongue, and to the papillæ vallatæ, and which supplies both gustatory filaments and fibers of general sensation to this region; (4) the superior laryngeal, which sends some fine branches to the root near the epiglottis.
The Salivary Glands
—Three large pairs of salivary glands communicate with the mouth, and pour their secretion into its cavity; they are the parotid, submandibular, and sublingual.
Parotid Gland (glandula parotis).—The parotid gland, the largest of the three, varies in weight from 14 to 28 gm. It lies upon the side of the face, immediately below and in front of the external ear. The main portion of the gland is superficial, somewhat flattened and quadrilateral in form, and is placed between the ramus of the mandible in front and the mastoid process and Sternocleidomastoideus behind, overlapping, however, both boundaries. Above, it is broad and reaches nearly to the zygomatic arch; below, it tapers somewhat to about the level of a line joining the tip of the mastoid process to the angle of the mandible. The remainder of the gland is irregularly wedge-shaped, and extends deeply inward toward the pharyngeal wall.
Structures within the Gland.—The external carotid artery lies at first on the deep surface, and then in the substance of the gland. The artery gives off its posterior auricular branch which emerges from the gland behind; it then divides into its terminal branches, the internal maxillary and superficial temporal; the former runs forward deep to the neck of the mandible; the latter runs upward across the zygomatic arch and gives off its transverse facial branch which emerges from the front of the gland. Superficial to the arteries are the superficial temporal and internal maxillary veins, uniting to form the posterior facial vein; in the lower part of the gland this vein splits into anterior and posterior divisions. The anterior division emerges from the gland and unites with the anterior facial to form the common facial vein; the posterior unites in the gland with the posterior auricular to form the external jugular vein. On a still more superficial plane is the facial nerve, the branches of which emerge from the borders of the gland. Branches of the great auricular nerve pierce the gland to join the facial, while the auriculotemporal nerve issues from the upper part of the gland.
The parotid duct (ductus parotideus; Stensen’s duct) is about 7 cm. long. It begins by numerous branches from the anterior part of the gland, crosses the Masseter, and at the anterior border of this muscle turns inward nearly at a right angle, passes through the corpus adiposum of the cheek and pierces the Buccinator; it then runs for a short distance obliquely forward between the Buccinator and mucous membrane of the mouth, and opens upon the oral surface of the cheek by a small orifice, opposite the second upper molar tooth. While crossing the Masseter, it receives the duct of the accessory portion; in this position it lies between the branches of the facial nerve; the accessory part of the gland and the transverse facial artery are above it.
Structure.—The parotid duct is dense, its wall being of considerable thickness; its canal is about the size of a crow-quill, but at its orifice on the oral surface of the cheek its lumen is greatly reduced in size. It consists of a thick external fibrous coat which contains contractile fibers, and of an internal or mucous coat lined with short columnar epithelium.
Vessels and Nerves.—The arteries supplying the parotid gland are derived from the external carotid, and from the branches given off by that vessel in or near its substance. The veins empty themselves into the external jugular, through some of its tributaries. The lymphatics end in the superficial and deep cervical lymph glands, passing in their course through two or three glands, placed on the surface and in the substance of the parotid. The nerves are derived from the plexus of the sympathetic on the external carotid artery, the facial, the auriculotemporal, and the great auricular nerves. It is probable that the branch from the auriculotemporal nerve is derived from the glossopharyngeal through the otic ganglion. At all events, in some of the lower animals this has been proved experimentally to be the case.
Submandibular Gland (glandula submandibularis).—The submandibular gland is irregular in form and about the size of a walnut. A considerable part of it is situated in the submandibular triangle, reaching forward to the anterior belly of the Digastricus and backward to the stylomandibular ligament, which intervenes between it and the parotid gland. Above, it extends under cover of the body of the mandible; below, it usually overlaps the intermediate tendon of the Digastricus and the insertion of the Stylohyoideus, while from its deep surface a tongue-like deep process extends forward above the Mylohyoideus muscle.
Its superficial surface consists of an upper and a lower part. The upper part is directed outward, and lies partly against the submandibular depression on the inner surface of the body of the mandible, and partly on the Pterygoideus internus. The lower part is directed downward and outward, and is covered by the skin, superficial fascia, Platysma, and deep cervical fascia; it is crossed by the anterior facial vein and by filaments of the facial nerve; in contact with it, near the mandible, are the submandibular lymph glands.
The deep surface is in relation with the Mylohyoideus, Hyoglossus, Styloglossus, Stylohyoideus, and posterior belly of the Digastricus; in contact with it are the mylohyoid nerve and the mylohyoid and submental vessels.
The deep process of the gland extends forward between the Mylohyoideus below and externally, and the Hyoglossus and Styloglossus internally; above it is the lingual nerve and submandibular ganglion; below it the hypoglossal nerve and its accompanying vein.
The submandibular duct (ductus submandibularis; Wharton’s duct) is about 5 cm. long, and its wall is much thinner than that of the parotid duct. It begins by numerous branches from the deep surface of the gland, and runs forward between the Mylohyoideus and the Hyoglossus and Genioglossus, then between the sublingual gland and the Genioglossus, and opens by a narrow orifice on the summit of a small papilla, at the side of the frenulum linguæ. On the Hyoglossus it lies between the lingual and hypoglossal nerves, but at the anterior border of the muscle it is crossed laterally by the lingual nerve; the terminal branches of the lingual nerve ascend on its medial side.
Vessels and Nerves.—The arteries supplying the submandibular gland are branches of the external maxillary and lingual. Its veins follow the course of the arteries. The nerves are derived from the submandibular ganglion, through which it receives filaments from the chorda tympani of the facial nerve and the lingual branch of the mandibular, sometimes from the mylohyoid branch of the inferior alveolar, and from the sympathetic.
Sublingual Gland (glandula sublingualis).—The sublingual gland is the smallest of the three glands. It is situated beneath the mucous membrane of the floor of the mouth, at the side of the frenulum linguæ, in contact with the sublingual depression on the inner surface of the mandible, close to the symphysis. It is narrow, flattened, shaped somewhat like an almond, and weighs nearly 2 gm. It is in relation, above, with the mucous membrane; below, with the Mylohyoideus; behind, with the deep part of the submandibular gland; laterally, with the mandible; and medially, with the Genioglossus, from which it is separated by the lingual nerve and the submandibular duct. Its excretory ducts are from eight to twenty in number. Of the smaller sublingual ducts (ducts of Rivinus), some join the submandibular duct; others open separately into the mouth, on the elevated crest of mucous membrane (plica sublingualis), caused by the projection of the gland, on either side of the frenulum linguæ. One or more join to form the larger sublingual duct (duct of Bartholin), which opens into the submandibular duct.
Structure of the Salivary Glands.—The salivary glands are compound racemose glands, consisting of numerous lobes, which are made up of smaller lobules, connected together by dense areolar tissue, vessels, and ducts. Each lobule consists of the ramifications of a single duct, the branches ending in dilated ends or alveoli on which the capillaries are distributed. The alveoli are enclosed by a basement membrane, which is continuous with the membrana propria of the duct and consists of a net-work of branched and flattened nucleated cells.
The alveoli of the salivary glands are of two kinds, which differ in the appearance of their secreting cells, in their size, and in the nature of their secretion. (1) The mucous variety secretes a viscid fluid, which contains mucin; (2) the serous variety secretes a thinner and more watery fluid. The sublingual gland consists of mucous, the parotid of serous alveoli. The submandibular contains both mucous and serous alveoli, the latter, however, preponderating.
The cells in the mucous alveoli are columnar in shape. In the fresh condition they contain large granules of mucinogen. In hardened preparations a delicate protoplasmic net-work is seen, and the cells are clear and transparent. The nucleus is usually situated near the basement membrane, and is flattened.
In some alveoli are seen peculiar crescentic bodies, lying between the cells and the membrana propria. They are termed the crescents of Gianuzzi, or the demilunes of Heidenhainm and are composed of polyhedral granular cells, which Heidenhain regards as young epithelial cells destined to supply the place of those salivary cells which have undergone disintegration. This view, however, is not accepted by Klein. Fine canaliculi pass between the mucus-secreting cells to reach the demilunes and even penetrate the cells forming these structures.
In the serous alveoli the cells almost completely fill the cavity, so that there is hardly any lumen perceptible; they contain secretory granules imbedded in a closely reticulated protoplasm. The cells are more cubical than those of the mucous type; the nucleus of each is spherical and placed near the center of the cell, and the granules are smaller.
Both mucous and serous cells vary in appearance according to whether the gland is in a resting condition or has been recently active. In the former case the cells are large and contain many secretory granules; in the latter case they are shrunken and contain few granules, chiefly collected at the inner ends of the cells. The granules are best seen in fresh preparations.
The ducts are lined at their origins by epithelium which differs little from the pavement form. As the ducts enlarge, the epithelial cells change to the columnar type, and the part of the cell next the basement membrane is finely striated.
The lobules of the salivary glands are richly supplied with bloodvessels which form a dense net-work in the interalveolar spaces. Fine plexuses of nerves are also found in the interlobular tissue. The nerve fibrils pierce the basement membrane of the alveoli, and end in branched varicose filaments between the secreting cells. In the hilus of the submandibular gland there is a collection of nerve cells termed Langley’s ganglion.
Accessory Glands.—Besides the salivary glands proper, numerous other glands are found in the mouth. Many of these glands are found at the posterior part of the dorsum of the tongue behind the vallate papillæ, and also along its margins as far forward as the apex. Others lie around and in the palatine tonsil between its crypts, and large numbers are present in the soft palate, the lips, and cheeks. These glands are of the same structure as the larger salivary glands, and are of the mucous or mixed type.
Parotid gland is situated in retromandibular fossa: front and lower from auricle, laterally from ramus mandibulae and posterior margin of masseter muscle. This is – compound alveolar gland, which produces serous secret. Parotid duct opens on the cheeks into vestibule of mouth cavity opposite the second superior molar. Parotid gland has superficial part and deep part also can be additional parotid gland, which disposes on surface of masticator muscle closely to parotid duct.
Sublingual gland lies in the floor of the mouth between the mandible and the genioglossus muscle. This is compound alveolar-tubular gland, it produces mucous secret. Greater sublingual duct opens on sublingual papilla near submandibular duct (sometimes the ducts open together as one). Lesser sublingual ducts open along sublingual fold.
Development, structure and function of pharynx and oesophagus
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.
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 pharyngeî-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.
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.
The OESOPHAGUS is about 25-
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.
• pharyngeîesophageal 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.
esophagus or gullet is a muscular canal, about 23 to
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.
abdominal portion of the esophagus lies in the esophageal groove on the
posterior surface of the left lobe of the liver. It measures about
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.
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.
Stomach, structure and topography
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
he 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.
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
We can divide the stomach into four regions
The cardia. The
cardia is the smallest
part of the stomach. It consists of the superior, medial portion of the stomach
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.
5. 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.
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-
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.
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.
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. Structure, topography and functionof the small 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.
small intestine is a convoluted tube, extending from the pylorus to the
colic valve, where it ends in the large intestine. It is about
Duodenum has received its name from being about equal in length to the
breadth of twelve fingers (
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
of the descending portion of the duodenum, showing bile papilla. Relations.—The
superior portion (pars superior; first portion) is about
descending portion (pars descendens; second portion) is from 7 to
horizontal portion (pars horizontalis; third or preaortic or
transverse portion) is from 5 to
ascending portion (pars ascendens; fourth portion) of the
duodenum is about
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 -
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:
Jejunum (intestinum jejunum) is wider, its diameter being about
Section of duodenum of cat.
Ileum (intestinum ileum) is narrow, its diameter being
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
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.
The mucous membrane presents for examination the following structures, contained within it or belonging to it:
Solitary lymphatic nodules.
Aggregated lymphatic nodules.
Villi of small intestine, showing bloodvessels and lymphatic vessels
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
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.
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.
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
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.
small intestine plays the primary role in the digestion and absorption of
nutrients. The small intestine averages
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
is the third and last segment of the small intestine. It is also the longest,
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.
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.
the small intestine were a simple tube with smooth walls, it would have a total
absorptive area of roughly 3300 cm2 (
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.
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.
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, stablishing asic contractile rhythm that then spreads from cell to cell.