2Section 5 The Small Intestine The small intestine—is a convoluted tube, extending from the pylorus to the ileocecal valve, where it joins the large intestine. It is the longest part of the digestive tract, being 6～7 m.Division: The small intestine can be divided into three regions that are not otherwise distinct from each other:Duodenum: a short, curved section which is devoid of a mesentery and is named the duodenum.Jejunum, and Ileum: the long, greatly coiled part which is attached to the posterior abdominal wall by the mesentery, and of which the proximal 2/5 constitutes the jejunum, and the distal 3/5 the ileum.
4Ⅰ. The DuodenumIt is the shortest, widest and most fixed part of the small intestine, about 25 cm long. It has no mesentery, and thus is only partially covered with peritoneum.Its course presents a remarkably constant curve, somewhat of the shape of an incomplete circle, which encloses the head of the pancreas.It begins at the pylorus and ends opposite the second lumbar vertebra in the jejunum.For descriptive purpose it is divided into four parts i.e.the superior part,the descending part,the horizontal part,the ascending part.
6Ⅰ) The Superior Part (first part) It is about 3 cm long, and is the most movable of the four parts.It begins at the pylorus, and continues with its descending part at the area of the neck of the gallbladder, where it forms the superior duodenal flexure.
7Ⅱ) The Descending Part (second part) It is 8～10 cm long, descends from the superior duodenal flexure, along the right side of the vertebral column, and at the border of third lumbar vertebra continues with its horizontal part, where it forms the inferior duodenal flexure.The common bile duct from the liver and the pancreatic duct from the pancreas join together to form the hepatopancreatic ampulla (ampulla of Vater) , which empties into the duodenum at the major duodenal papilla. This opening is surrounded by a sphincter muscle called the hepatopancreatic sphincter (sphincter of Oddi) .
8Ⅲ) The Horizontal Part (inferior or third part) It is about 10 cm long, begins at the inferior duodenal flexure.
9Ⅳ) The Ascending Part (fourth part) It is about 2.5 cm long, ascends to the level of the upper border of the second lumber vertebra, where it turns ventrally at the duodenojejunal flexure and is continuous with the jejunum.The terminal part of the duodenum and the duodenojejunal flexure are usually described and fixed in position by the suspensory muscle of duodenum (suspensory muscle, or ligament of Treitz).
10Ⅱ. The JejunumThe next 2.5 m or so of the small intestine is the jejunum.This portion is suspended in the abdominal cavity by a mesentery.①It has a diameter of about 4 cm, and is thicker, redder and more vascular than the ileum.②The circular folds of its mucous membrane are large and thickly set, and its villi surpass those of the ileum in size.③The aggregated lymphatic follicles are almost absent in the upper part of the jejunum; in the lower part they are fewer and smaller than those in the ileum and tend to assume a circular form.④The most part of jejunum lies in the umbilical region, but it may extend into any of the surrounding areas.
12Ⅲ. The IleumThe ileum is the remaining 3.5 m or so of the small intestine. Like the jejunum, the ileum, is suspended from the posterior body wall by a mesentery.
13① It has a diameter of 3.5 cm, and its wall is thinner than that of the jejunum. ② A few circular folds are present in the upper part of the ileum, but they are small and disappear almost entirely towards its lower end.③ the aggregated lymphatic follicles are, however, large and more numerous than those in the jejunum.④ For the most part the ileum is situated in the hypogastric and pelvic regions. The lower portion of the ileum usually lies in the pelvis.It ends in the right iliac fossa by opening into the medial side of the junction of the cecum and ascending colon.The jejunum and ileum are attached to the posterior abdominal wall by an extensive fold of peritoneum, termed the mesentery, which allows of very free movement, so that each coil can accommodate itself to changes in form and position.
14Section 6 The Large Intestine The large intestine, which is about 1.5 m long, 6.5 cm in diameter, extends from the ileocecal valve to the anus. Its certain parts are attached to the posterior abdominal wall by its mesocolon.It is so named because its diameter in most regions is greater than that of the small intestine.
15The characteristics of the large intestine: The large intestine differs considerably in structure, appearance, size and arrangement from the small intestine: It has a greater caliber; for the most part, it is more fixed in position.The outstanding features are as the follows, with the exceptions of the cecum, the vermiform appendix and the rectum.
161. colic band: Its longitudinal muscular fibers form three colic bands. 2. haustras of colon: Since the colic bands are shorter than the circular muscular coat, the colon is puckered and sacculated, the sacculations being known as haustras of colon.3. epiploic appendices: Small, peritoneum—covered, adipose projections, termed epiploic appendices, are found scattered over the free surface of the whole of the large intestine.The large intestine is divided into several parts:the cecum,colon,rectum,anal canal.
18Ⅰ. The CecumThe large intestine begins as a blind pouch called the cecum, which receives the ileum of the small intestine.It is about 6 cm long and 7.5 cm wide. It is a blind pouch of the closed end is directed downward, and it opens above into the ascending colon.
20Ⅱ. The Vermiform Appendix It is a narrow, worm shaped tube, which springs from the posteromedial wall of the cecum, 2 cm or less below the end of the ileum. It is about 9 cm long and about 1.5 cm wide.The appendix is variable in position.Most commonly, the tip lies retrocecally or hangs over the brim of the lesser pelvis.The wall of the appendix contains numerous lymphatic nodules.
21Ⅲ. The Colon It may be considered in four parts: the ascending, transverse,descending,sigmoid.The large intestine extends upward from the cecum as the ascending colon.The ascending colon is not supported by a mesentery; instead, it lies tightly against the posterior wall of the abdomen.
22Just beneath the liver, the ascending colon bends sharply to the left (right colic flexure) and crosses the abdominal cavity as the transverse colon. This portion of the colon is suspended by a mesentery called the mesocolon.In the vicinity of the spleen, the transverse colon bends downward (left colic flexure) and forms the descending colon.
23The descending colon, like the ascending colon, is retroperitoneal. Where the descending colon reaches the left pelvic brim, it curves to the midplane via an S-shaped sigmoid colon.The sigmoid colon is closely surrounded by peritoneum, which forms a mesentery, the sigmoid mesocolon.
24The position and shape of the sigmoid colon vary very much, and depend on: ① its length,② the length and freedom of its mesocolon,③ the condition of distension,④ the condition of the rectum and bladder.Usually it is relatively free within the lesser pelvis below the small intestine
25Ⅳ. The RectumIt is continuous with the sigmoid colon at the level of the third sacral vertebra. It is about 12 cm long and its upper part has the same diameter as the sigmoid colon (about 4 cm in the empty state), but its lower part of the rectum is dilated to form the ampulla of rectum.
26Sacral flexure and perineal flexure of the rectum: From its origin it descends, following the concavity of the sacrum and coccyx, forming an anteroposterior curve known as the sacral flexure of the rectum. It thus passes at first downwards and backwards, then downwards, and finally downwards and forwards to become continuous with anal canal by passing through the pelvic diaphragm.
27The anorectal junction is situated 2～3 cm in front of and slightly below the tip of the coccyx. From this level, which in the male is opposite the apex of the prostate, the anal canal passes downwards and backwards from the lower end of the rectum, the backward bend of the gut at the anorectal junction being termed the perineal flexure of the rectum.
28The peritoneum covers the upper 1/3 of the rectum on its front and sides, the middle third on its front only, and does not cover anywhere of the lower third.In the empty state of the rectum, the mucous membrane of its lower part presents a number of longitudinal folds which are effaced by distension of the rectum. Besides, there are three permanent transverse folds of rectum (semilunar rectal folds).
29Ⅴ. The Anal CanalThe terminal 3 to 4 cm of the large intestine is called the anal canal. This region is located below the pelvic diaphragm and thus is outside the pelvis.
31Morphology of the anal canal: In the lower part of the canal the mucous membrane present 6～10 vertical folds, the anal columns.The lower ends of these columns are joined together by small crescentic valve-like folds of mucous membrane, the anal valves, above each of which lies a small recess or anal sinuses.The line along which the anal valves are situated is termed the dentate line.
32The succeeding part of the anal canal extends for about 1 cm below the anal valves, and is known as the anal pecten.Below the lower border of the anal pecten, there is the white line, it is the transitional zone between the anal mucous membrane and the anal skin. It can be felt in the digital examination of the anal canal because it lies at the interval between the sphincter ani internus and the sphincter ani externus.
33The walls of the anal canal are surrounded by a complex of muscular sphincters, which can be divided into internal and external parts. At the anorectal junction the circular muscle coat of the rectum becomes considerably thickened (5-8 mm) to form the sphincter ani internus. The sphincter ani externus surrounds the whole length of the anal canal; it is usually described as consisting of three parts and composed of striated muscle.