which is the longest part of the digestive tube, is divided into long, less distensible, small intestine, and shorter, more distensible large intestine. Food has to be digested, metabolized and stored for expulsion in the intestines. Intestines suffer from bacterial infection like typhoid, tuberculosis; parasitic infection, like roundworm, tapeworm, etc. in addition to diarrhea and dysentery. Good and healthy eating habits definitely prevent some of these conditions. The proximal one and a half parts of duodenum, including liver, gallbladder and pancreas, develop from fore gut. The distal two and a half parts of duodenum, jejunum, ileum, cecum, appendix, ascending colon and right two-thirds of transverse colon develop from mid gut. Lastly, the left one-third of transverse colon, descending colon, pelvic colon and proximal part of rectum develop from hind gut.
The small intestine extends from the pylorus to the ileocaecal junction. It is about 6 metres long. The length is greater in males than in females, and greater in cadavers, due to loss of tone than in the living. It is divided into: 1 An upper, fixed part, called the duodenum, which measures about 25 cm in length; and 2 Alower, mobile part, forming a very long convoluted tube. two-fifths of the mobile intestine are The upper known as the jejunum, and the lower three-fifths are known as the ileum (Fig. 20.1). The structure of the small intestine is adapted for digestion and absorption.
Large Surface Area
For absorption of digested food, a very large surface Warea is required. This is achieved by:
- The great length of the intestine.
- The presence of circular folds of mucous membrane, villi and microvilli.
The circular folds of mucous membrane, plicae circulares, or valves of Kerckring form complete or incomplete circles. These folds áre permanent, and are not obliterated by distension. They begin in the second part of the duodenum, and become large and closely set below the level of the major duodenal papilla. They continue to be closely set in the proximal half of the jejunum (Fig. 20.2), but diminish progressively in size and number in the distal half of the jejunum and in the proximal half of the ileum (Fig. 20.3).
|They are almost absent in the distal half of the ileum. Apart from increasing the surface area for absorption, the circular folds facilitate absorption by slowing down the passage of intestinal contents. The intestinal villi are finger-like projections of mucous membrane, just visible to the naked eye. They give the surface of the intestinal mucosa a velvety appearance. They are large and numerous in the duodenum and jejunum, but are smaller and fewer in the ileum. They vary in density from 10 to 40 per square millimeter, and are about 1 to 2 mm long. They increase the surface area of the small intestine about eight times (Fig. 20.4). Each villus is covered by a layer of absorptive columnar cells. The surface of these cells has a striated border which is seen, under the electron microscope, to be made of microvilli.Attachments areaReplyForward|