HUMAN ANATOMY – VOLUME 1

the stomach is empty it lies transversely. The length of the superior part is 4–6 cm. Its upper surface adjoins the posterior part of the quadrate lobe of the liver and crosses the proper hepatic artery and the common hepatic duct. At the bottom it touches the top of the head of the pancreas and transverse colon. Behind the superior part, inside the hepatoduodenal ligament lies the common bile duct, proper hepatic artery and portal vein.

The d e s c e n d i n g p a r t of the duodenum begins at its superior flexure, at the level of the L1 vertebra, and extends downwards along the right edge of the vertebral column. The descending part ends at the level of the third lumbar vertebra with as a sharp turn to the left, forming the inferior duodenal flexure. The length of the descending part is 8–10 cm. Behind it lies the hilus of the right kidney and the upper part of the ureter. The medial side of the descending part adjoins the inferior vena cava and, in the place of transition between the superior and descending parts, the right suprarenal gland.

In the front the descending part is covered by the peritoneum and is crossed by the root of the transverse colon. On the left it adjoins to the head of the pancreas and is firmly accreted with its capsule. Between the descending part and the head of the gland passes the end part of the common bile duct. Also passing through this space are the anatomized superior and inferior pancreaticoduodenal arteries.

The h o r i z o n t a l p a r t of the duodenum begins from its lower flexure. It extends horizontally to the left at the level of the L3 vertebra, then turns upward and continues into the ascending part. At this level it crosses the superior mesenteric artery and vein. Behind the horizontal part lie the inferior vena cava (right) and aorta (left). The anterior surface of the horizontal part is covered by the peritoneum and adjoins the loops of the small intestine.

The a s c e n d i n g p a r t begins in the place where the superior mesenteric artery and vein pass from beneath the lower margin of the pancreatic gland onto the front surface of the duodenum. The ascending part ends near the left edge of the L2 vertebra body with a sharp flexure downwards, forwards and to the left (d u o d e n o j e j u n a l f l e x u r e). This flexure is fixed to the diaphragm by the s u s p e n s o r y m u s c l e a n d l i g a m e n t o f t h e d u o d e n u m (Treiter’s muscle and ligament). Behind the ascending part lies the aorta, and in front to it — the parietal peritoneum.

Jejunum and Ileum

The jejunum and ileum form the mesenteric part of the small intestine. They form 14–16 loops, which are located primarily in the umbilical region. Part of its loops lies in the lesser pelvis. Loops of the jejunum are situated mainly in the upper left section, and loops of the ileum — in the

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Fig. 132. Relief of mucous membrane of small intestine. Region of intestine is longitudinally opened.

1 — wall of intestine; 2 — circular folds; 3 — serous coat.

lower right section of the abdominal cavity. There is no strict anatomical border between these two parts of the small intestine. In the front intestinal loops are covered by the greater omentum; in the back — by the parietal peritoneum, which lines the right and left mesenteric sinuses. The jejunum and ileum are fixated on the posterior abdominal wall by the mesentery, the root of which attaches to the right iliac fossa.

The wall of the small intestine is formed by the mucosa, submucosa, musc u l a r l a y e r and external lining. The mucosa is characterized by circular folds (Fig. 132). Their numbers reach 600–700. These folds take up more than two thirds of the circumference of the intestine. Their formation is promoted by the submucosa, and their sizes decrease in the direction of the large intestine. The average height of the circular folds is 8 mm. They increase the area of the mucosa by 3 times. The mucosa of the superior and descending parts of the duodenum also forms longitudinal folds. The largest of these is located on the medial wall of the descending part. Situated in the lower part of this fold is the m a j o r d u o d e n a l p a p i l l a of the duodenum (Vater’s papilla). On this papilla is the common opening of the common bile duct and the pancreatic duct. Above this papilla, situated on the longitudinal fold, is the m i n o r d u o d e n a l p a p i l l a of duodenum. On it are additional openings of the pancreatic duct.

The m u c o s a of the small intestine has numerous protrusions called i n t e r s t i n a l v i l l i (Fig. 133), the number of which reaches 4–5 millions. One square millimeter of the mucosa in the duodenum and jejunum carries 22–40 villi, and in the ileum — 18–31 villi. The average length of a villus is 0.7 mm. Their size tends to decrease in the direction of the large intestine. Villi may be foliate, lingual and fingerlike. The first two classes are orientated perpendicularly to the intestinal tube. The longest villi (approximately 1 mm)

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Fig. 133. Relief of mucous membrane of small intestine. Surface of mucous cover and layers of intestinal wall.

1 — circular folds; 2 — intestinal villi; 3 — mucous membrane; 4 — submucousa; 5 — circular layer of muscular coat; 6 — longitudinal layer of muscular coat; 7 — serous coat.

are mostly foliate. The beginning part of the jejunum contains primarily lingual villi. In more distal parts the villi become fingerlike. Their length decreases to 0.5 mm. The spaces between the villi measure 1–3 mm. The villi are formed by loose connective tissue, which is covered with epithelium. Inside them there are a lot of smooth myocytes, reticular fibers, lymphocytes, plasma cells and eosinophils. In the center each villus contains a lymphatic capillary (central lacteal), which is surrounded by blood vessels.

The surface of the mucosa, including the villi (Fig. 134), is covered with simple columnar epithelium, situated on a basal membrane. The majority of epitheliocytes (approximately 90 percent) are columnar enterocytes, characterized by a brush border on their apical membrane. The brush border is formed by microvilli. On the surface of the microvilli is the glycocalyx. The major function of columnar epithelium is absorption. The epithelial lining also contains numerous goblet cells, which secrete mucus. Approximately 0.5 percent of all epithelial cells are endocrinocytes. Inside the epithelium there are also lymphocytes, which permeate the stroma of villi and the basement membrane.

In the spaces between the villi are the opening of intestinal glands (Lieberkuhn’s crypts) (Fig.135). These are tubular glands with a round or

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reticular fibers, which form a thick network. Inside the lamina propria there are always eosinophils, lymphocytes and plasma cells. It also contains a large amount of s o l i t a r y l y m p h o i d n o d u l e s (in children there are 3–5 thousand). In the mesenteric part of the small intestine, especially in the ileum, there are 40–80 a g g r e g a t e d l y m p h o i d n o d u l e s called P e y e r’s p a t c h e s. Plaques are located primarily along the counter-me- senteric side of the intestine and have an oval shape (see «Organs of hemopoiesis and the immune system»).

The m u s c u l a r i s m u c o s a of the small intestine is about 40 mm thick. It has circular and longitudinal layers. Some individual myocytes separate off from the muscularis mucosa into the lamina propria.

The s u b m u c o s a of the small intestine contains lobules of adipose tissue. Inside it there are nerves and blood vessels, and various cell elements. The submucosa is also penetrated by groups of large lymph nodules. The submucosa of the duodenum also contains the secretory parts of the duodenal (Brunner’s) glands. Duodenal glands are compound tubular glands, which produce mucous secretion. Their ostiums are located between villi, on the surface epithelium or into the intestinal glands of the

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duodenum. The density of the duodenal glands and their sizes are greater in the beginning part of the duodenum than in its end.

The muscular coat of the small intestine consists of two layers. The internal (circular) layer is thicker than the external longitudinal one. The orientation of muscle fascicles is not strictly circular or longitudinal, but may be defined as spiral. In the external layer the volutions of the spiral are longer than in the internal layer. Between the two muscle layers there is loose connective tissue, which contains the nervous plexus and vessels.

The s e r o s a is situated over a subserosa layer. It covers the small intestine from all sides, except for the duodenum, which is covered only from the front.

I n n e r v a t i o n: duodenum — fibers from the vagus nerve and the gastric, hepatic and superior mesenteric

plexuses; jejunum and ileum — fibers of the vagus nerve and superior mesenteric plexus.

B l o o d s u p p l y: duodenum — anterior and posterior superior pancreaticoduodenal arteries (from gastroduodenal arteries), inferior pancreaticoduodenal artery (from superior mesenteric artery). Jejunum and ileum

— jejunal and ileal arteries (from superior mesenteric artery).

Ve n o u s o u t f l o w: along homonymous veins into the portal vein. Ly m p h o u t f l o w: duodenum — pancreaticoduodenal, superior me-

senteric, celiac and lumbar lymph nodes. Jejunum and ileum—mesenteric and ileocolic (from end part of ileum) lymph nodes.

LARGE INTESTINE

The large intestine (intéstinum crássum) continues from the small intestine. It is divided into the caecum, colon and rectum. The colon, in turn, consists of the ascending, transverse, descending and sigmoid colons. The main function of the large intestine is elimination of feces and absorption of water and vitamins. Its length is approximately 160 cm. In live

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persons this length is somewhat greater due to considerable elasticity of tissues. The length of the caecum makes up about 4.66 percent of the whole large intestine. The ascending colon constitutes approximately 16.17 percent, the transverse colon — 34.55 percent; the descending colon — 13,72 percent; and the sigmoid colon — 29.59 percent. The average diameter of the large intestine is 5–8 cm, and it tends to decrease in the direction of the rectum. An empty large intestine of an adult weights approximately 370 g.

The caecum (caecum)

continues from the ileum. It Fig. 136. Caecum with appendix. Anterior wall is is shaped like a pouch, which removed.

hangs downward. Occasionally the cecum can be coneshaped. Its length is 4–8 cm.

Its posterior surface adjoins the iliac and major psoas muscles. The front surface adjoins the anterior abdominal wall. The caecum does not have a mesentery, but is covered with peritoneum from all sides (intraperitoneal position). From the caecum extends the v e r m i f o r m a p p e n d i x, which pertains to the immune system. The appendix usually begins on the posteromedial surface of the cecum (Fig. 136). Its length varies between 2 and 20 cm (on average 8 cm). Its diameter is 0.5–1.0 cm. The appendix may begin on any other side of the caecum, and may extend in different directions. Usually it is situated within the right iliac fossa, its free end is turned down and medially, reaching the pelvic brim and sometimes entering the lesser pelvis. It can also be situated behind the cecum (retrocecal position) or may lie retroperitoneally. Normally the appendix has a mesentery, which connects it to the wall of the cecum and the end part of the ileum.

The ascending colon (cólon ascéndens) is 18–20 cm long. Its position may vary. Its posterior wall occupies the right lateral section of the

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posterior abdominal wall. The ascending colon extends upward, passing first in front of the quadrate muscle of the abdomen, and then over the right kidney, which is situated retroperitoneally. Near the inferior /visceral/ surface of the liver the ascending colon turns forward and to the left, forming the r i g h t c o l i c (h e p a t i c) f l e x u r e.

The transverse colon (cólon transvérsum) usually passes across the abdomen in a downward arch pattern. It begins in the right subcostal region, at a level of the tenth costal cartilage, and ends in the left subcostal region. Its length is on average 50 cm. (ranging between 40 and 50 cm). Within the left subcostal region, at a level of the ninth costal cartilage, the transverse colon forms the l e f t (s p l e n i c) f l e x u r e, which is shaped like a sharp angle. The transverse colon is covered from all sides by the peritoneum and has a mesentery, which fixes it to the posterior wall of the peritoneal cavity. The anterior surface of the transverse colon is fixed to the greater curvature of the stomach and the upper part of the duodenum by the gastrocolic ligament, which is formed by the upper part of the greater omentum.

The top of the right colic flexure adjoins to the liver, and the left colic flexure — to the stomach and spleen. Below the transverse colon lie loops of the small intestine; behind it lie the duodenum and pancreas. When the stomach is empty the front surface of the transverse colon adjoins the anterior abdominal wall, and when it is full, the colon is pushed back.

The descending colon (cólon descéndens) begins at the left colic flexure and continues into the sigmoid colon at the level of the iliac crest. The average length of the descending part is 25 cm. The descending colon projects within the left lumbar region. The front surface of the descending colon adjoins the anterior abdominal wall. To its right lie the loops of the jejunum, and to its left — the left abdominal wall. The descending colon is covered by the peritoneum from the front and sides (mesoperitoneal position).

The sigmoid colon (cólon sigmoídeum) begins at the level of the left iliac crest and ends at the level of the promontory, where it continues into the rectum. Its average length is 40–45 cm (ranging between 12 and 84 cm). The sigmoid colon forms 1–2 curves, which adjoin the front surface of the left iliac bone and partially descend into the pelvis. It lies intraperitoneally and has a mesentery. The mesentery allows this colon considerable mobility.

The rectum (réctum) is the terminal part of the large intestine. Its average length is 15 cm, and the diameter varies between 2.5 and 7.5 cm. The rectum consists of the ampoule and the anal canal. The r e c t a l a m p u l l a is situated within the lesser pelvis; the anal canal lies inside the perineum. Behind the ampoule are the sacrum and coccyx. In front of it, in males, lie the prostate, urinary bladder, seminal vesicles and ampoules of the right and left deferent ducts; in females there are the uterus and vagina. The a n a l c a n a l ends with the a n u s.

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The rectum forms flexures in the sagittal plane. The s a c r a l f l e x - u r e is protrudes backward, and corresponds to the curvature of the sacrum. The perineal flexure protrudes forward, and is situated inside the perineum (in front of the coccyx). Flexures of the rectum within the frontal plane are inconstant. The upper part of the rectum is covered with the peritoneum from all sides, the middle part — from only three sides, and the lower part does not have a serosa lining.

In the region of the anal canal a thickening of muscularis layer forms the internal (involuntary) anal sphincter. Directly beneath the skin of this region is the voluntary external anal sphincter, which is one of the muscles of the perineum. Both sphincters close the anal orifice, opening it during defecation.

An external characteristic of the large intestine is the presence of three muscle bands 3–6 mm wide, called taeniae coli (Fig. 137). The f r e e, m e s o c o l i c and o m e n t a l t a e n i a e c o l i begin near the base of the appendix and stretch up to the rectum. These bands form as a result of an unequal distribution of fascicles of the longitudinal muscle layer along the large intestine, which is thickened within these ribbons.

On the walls of the large intestine there are characteristic fingerlike, foliate or sack-like o m e n t a l a p p e n d i c e s, which are covered with by visceral peritoneum. Their average length is 3–4 cm, and their density increases in the direction of the rectum. These appendages may serve for amortization during peristalsis and act as fat depots. Because the taeniae coli are shorter than the actual length of the large intestine, the walls of the colons form bulging protrusions called haustra.

Fig. 137. Fragment of transverse colon.

1 — colonic wall; 2 — semilunar folds of colon; 3 — mesocolic taenia (band); 4 — transverse mesocolon; 5 — haustrae of colon; 6 — free taenia; 7 — omental appendices; 8 — greater omentum; 9 — omental taenia.

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The walls of large intestine are formed by the m u c o s a, s u b m u - c o s a, m u s c u l a r l a y e r and s e -

goblet cells and endocrinocytes. At the level of the anal canal simple epithelium passes into stratified cuboidal epithelium. Even further, it transforms abruptly into stratified squamous non-keratinizing, and then keratinizing epithelium.

The lamina propria of mucosa is formed by loose connective tissue. Inside it lie 7.5–12 million intestinal glands (crypts of Lieberkuhn), which perform both a secretory and absorptive functions. Approximately 4.5 percent of these glands are found in the cecum; 90 percent are in the colon; and 5.5 percent are in the rectum. The distribution of these glands has certain characteristics, for instance their concentration is higher (by 4–12 percent) at the levels of taeniae coli than between them. The glands situated on top of the semilunar folds and over sphincters tend to be larger. The walls of the glands are lined by simple epithelium, situated on a basement membrane. The epitheliocytes include primarily goblet cells and absorption cells. A constant element of the epithelium are stem cells. In some places there are also endocrinocytes. The number of endocrinocytes

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increases in the direction of the rectum. Among them are EC-cells, which produce serotonin and melatonin; A-cells, which produce glucagon; and D1-cells, which secrete the vasoactive intestinal peptide.

Along the length of the lamina propria of mucosa there are 5.5– 6 thousand solitary lymphoid nodules. Also there are lymphoid and mast cells, and sometimes a few eosinophils and neutrophils. The lamina propria contains blood and lymph capillaries and vessels, non-myelinated nerve cells of the intramural nervous plexus and nerve fibers.

The muscularis of the mucosa is formed by two layers of smooth muscles. The internal layer is circular, and the external layer is oblique and longitudinal. Some fascicles of smooth muscle cells (10–30 mm long and 0.2–2.0 mm in diameter) separate from the muscularis and penetrate the lamina propria. Thin muscle fascicles surround intestinal glands and participate in secretion.

The submucosa is formed by loose fibrous connective tissue. It contains solitary lymphoid nodules, the submucosal (Meissner’s) plexus, blood and lymph vessels, and mucous glands (at the level of the anal canal).

The muscularis of the large intestine increases in thickness in the direction of the rectum. It is formed by a circular (internal) and a longitudinal (external) layers of muscles. Between these layers lies the myenteric (Auerbach’s) plexus, which is formed by ganglion cells, glia cells (Schwann and satellite cells) and nerve fibers. Gangliocytes dominate in zones, which correspond to the taeniae coli. The internal part of the circular muscle layer is the zone of formation of peristaltic waves, which are generated by interstitial (Cajal’s) cells, located inside the submucosa, on the border with the muscle layer.

In some places of the large intestine, especially in transition segments between its different parts, the circular layer of the muscularis has thickenings. In these places its lumen may be narrowed, which serves to regulate the passage of chyme, or feces through the intestine during digestion. These muscular thickenings are called c o l o n i c s p h i n c t e r s. The cae- co-ascending sphincter is defined at the upper edge level of the ileocecal valve. The next sphincter (Girsh’s sphincter) is situated at the right colic flexure. There are three sphincters in the transverse colon. Near the beginning of the transverse colon is the right (Kennon’s) sphincter. The middle transverse and the left Kennon’s sphincters are located closer to the left colic flexure. Within the left colic flexure is the Payer’s sphincter. Another sphincter is located in the transition between the descending and sigmoid colons. Within the sigmoid colon there are the superior and inferior sigmoid sphincters. The sigmoido-rectal (O’Bernier’s) sphincter is situated between the sigmoid colon and rectum.

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