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Other Terms: Small bowel, Intestinum tenue, Intestin grêle, Kleiner Darm, Intestino pequeño
The small intestine is a convoluted tube. It extends from the pylorus to the colic valve, where it ends in the large intestine. It is about 7 meters long and gradually diminishes in size from its commencement to its termination. It is contained in the central and lower part of the abdominal cavity.
It is surrounded above and at the sides by the large intestine. A portion of it extends below the superior aperture of the pelvis and lies in front of the rectum. Anteriorly, it is in relation with the greater omentum and abdominal wall. It is connected to the vertebral column by the mesentery. The small intestine is divisible into three portions: the duodenum, jejunum, and the ileum. The wall of the small intestine is composed of four layers: tunica mucosa, tela submucosa, tunica muscularis, and the tunica serosa. The tunica mucosa is the innermost layer of the gut wall. It 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: circular folds, villi, and intestinal glands. The circular folds are large valvular flaps projecting into the lumen of the bowel. The two layers of the fold are 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. Some form complete circles and others have a spiral direction. The larger folds are about eight millimeters in depth at their broadest. There are more small folds than large folds. The larger and smaller folds alternate with each other. They are not found at the commencement of the duodenum, but begin to appear about 2.5 or 5 centimeters beyond the pylorus. In the lower part of the descending portion, they are very large and closely approximated. In the horizontal and ascending portions of the duodenum and upper half of the jejunum they are large and numerous. From this point on, they diminish considerably in size. In the lower part of the ileum they almost entirely disappear. The circular folds slow the passage of food along the intestines and provide an increased surface for absorption. The intestinal villi are highly vascular processes that project from the tunica mucosa throughout its entire extent through the small intestine. They are largest and most numerous in the duodenum and jejunum and fewer and smaller in the ileum. The essential parts of a villus are: the lymphatics, blood vessels, epithelium, basement membrane, and the muscular tissue of the mucosa. These are all supported and held together by lymphoid tissue. The lymphatics are situated in the axis of the villus. The walls of the lacteals are composed of a single layer of endothelial cells. The blood vessels form a plexus under the basement membrane and are enclosed in reticular tissue. The intestinal glands are found in large numbers over every part of the tunica mucosa. They consist of minute tubular depressions of the mucous membrane, arranged perpendicularly to the surface, upon which they open by small circular apertures. The tela submucosa connects the tunica muscularis and tunica mucosa. It consists of loose connective tissue containing blood vessels, lymphatics, and nerves. It is the strongest layer of the intestine. The tunica muscularis consists of inner circular and outer longitudinal layers. The longitudinal fibers are thinly scattered over the surface of the intestine and are more distinct along its free border. The circular fibers forma thick, uniform layer and are composed of plain muscle cells of considerable length. These muscles cause the wave like motion, peristalsis, that moves the chyme along the digestive tract. The tunica muscularis is thicker at the upper end than at the lower part of the small intestine. The tunica serosa is the surface of the intestine and is derived from peritoneum. The superior portion of the duodenum is almost completely surrounded by this membrane near its pyloric end. The descending portion is covered by it in front, except where it is carried off by the transverse colon. 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. It 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 except along its mesenteric border. Here a space is left for the vessels and nerves to pass to the gut.
The duodenum is supplied by branches originating from the celiac trunk .The jejunum and ileum are supplied by branches of the superior mesenteric artery. The intestinal branches of this artery reach the attached border of the bowel, run between the tunica serosa and muscularis. They 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 as the arteries.
The duodenum is the first, the shortest, and the widest portion of the small intestine. Its length, which is ten to twelve inches is less variable than that of the jejunum and ileum. It commences at the pyloric sphincter, and ends in the jejunum to the left of the body of the second lumbar vertebra.
The duodenum consists of four portions: first ascending, descending, transverse, and terminal ascending portion. The first, or first ascending, portion is nearly horizontal, and is about two inches long. Like the stomach, this portion is comparatively movable, more so than any of the other portions of the duodenum. It passes upward, laterally, and posteriorly from the pyloric end of the stomach to the neck of the gallbladder. Like the stomach, it is completely surrounded by peritoneum, which, in turning to the right, again forms the free border of the gastro-hepatic omentum just above it. Situated superior to it are the omental foramen and the lesser omentum; inferior, is the head of the pancreas; anterior to are the liver and the neck of the gall-bladder, and posterior are the common bile duct, the hepatic artery, the portal vein, and the gastro-duodenal artery. The second, or descending, portion is about three inches in length, and extends from the neck of the gallbladder to the level of the third lumbar vertebra. Its anterior surface is covered with peritoneum-the ascending layer of the transverse mesocolon except where the transverse colon passes anterior to it, and is firmly fixed to the head of the pancreas by connective tissue and peritoneum. anterior to it is the commencement of the transverse colon; posterior are the common bile duct, the medial border of the right kidney, the renal vessels, and the inferior vena cava; on its medial, or left, side are the common bile and the pancreatic ducts, both of which open into it, the superior and inferior pancreaticoduodenal arteries, and the head of the pancreas. After obliquely traversing the wall of the bowel, the pancreatic and the common bile ducts open into the postero-internal aspect of the lower part of this segment of the duodenum, on the summit of a papilla. The third, or transverse, portion, the longest and narrowest part of the duodenum, is about five inches in length; it passes obliquely upward and toward the left side, across the vertebral column, at the third lumbar vertebra, to the left of the second lumbar vertebra, where it terminates. A line drawn from a point three inches to the right of the umbilicus, to a point two inches to the left and above it, will nearly indicate its position. Its anterior surface, except where the superior mesenteric artery and vein cross it, is covered by peritoneum, the descending layer of the transverse mesocolon. anterior to it are the superior mesenteric artery and vein, the jejunum, and the lower layer of the transverse mesocolon; posterior are the crura of the diaphragm, the inferior vena cava, and the abdominal aorta; superior are the superior mesenteric artery and vein, the pancreas, and the inferior pancreaticoduodenal artery; inferior are parts of the mesentery and posterior parietal peritoneum. The fourth, or terminal ascending, portion is the shortest portion, being about one inch in length. It commences immediately to the left of the second lumbar vertebra, passes upward on the surface of the left psoas muscle, and terminates at the point where the mesentery begins, namely, at the origin of the jejunum. It is covered by peritoneum on the anterior surface and partly on the sides, and is firmly fixed to the left crus of the diaphragm and the connective tissue around the celiac artery by a band of fibrous tissue that contains some muscle tissue as well, and is called the suspensory ligament of the duodenum(ligament of Treitz). The junction of the terminal portion of the duodenum with the jejunum constitutes the duodenojejunal flexure, below and to the left of which is the duodenojejunal fossa.
The jejunum is wider. Its diameter is about four centimeters. It is thicker, more vascular, and of a deeper color than the ileum. The circular folds of its mucous membrane are large and thickly set. Its villi are larger than in the ileum. The aggregated lymph nodules are almost absent in the upper part of the jejunum. In the lower part, the lymph nodules are less frequently found than in the ileum and are smaller and tend to assume a circular form. The circular folds are absent in the lower part of the ileum. The jejunum mostly occupies the umbilical and left iliac regions. The jejunum and ileum are attaché to the posterior abdominal wall by mesentery. The mesentery allows the freest motion so that each coil can accommodate itself to changes in form and position.
The ileum is narrow; its diameter is about 3.75 centimeters. Its coats are thinner and less vascular than those of the jejunum. It possesses few circular folds and they are small and disappear entirely toward it lower end. Peyer’s patches are larger and more numerous. The ileum chiefly occupies the umbilical, hypogastric, right iliac, and pelvic regions. The terminal part of the ileum usually lies in the pelvis, from which it ascends over the right psoas and right iliac vessels. It ends in the right iliac fossa by opening into the medial side of the commencement of the large intestine. The jejunum and ileum are attaché to the posterior abdominal wall by mesentery. The mesentery allows the freest motion so that each coil can accommodate itself to changes in form and position. Between the two layers of mesentery are blood vessels, nerves, lacteals, lymph glands, and a variable amount of fat.