The small intestine which forms the continuation of the duodenum is composed of the jejunum and ileum. There is really no marked structural difference between the two, and it is therefore hardly possible to determine where one ends and the other begins. As a rule, the upper two-fifths are designated as the jejunum and the lower three-fifths as the ileum. The jejuno-ileum fills the greater part of the abdomen. It occupies the umbilical, hypogastric, iliac, and lumbar regions, and is more or less encircled by the large intestine. The coils formed by the jejunum and ileum are very movable and completely invested by the peritoneum. They are supported and attached to the posterior parietes by the mesentery. The latter extends from the end of the duodenum to the ileocaecal junction. The point at which the mesentery is attached above is on a level with the lower border of the pancreas and just to the left of the vertebral bodies. From this point of insertion the mesentery follows an oblique line running downward and to the right, crossing the great vessels and ending in the iliac fossa. The length of the mesentery from the spine to the intestines varies in different parts of the canal, its average being eight to nine inches.

It soon attains its full length, and within one inch of the end of the duodenum is already six inches long. The small intestine hangs on the mesentery in the form of coils, and the folds which the mesentery forms may be compared to those of a fan.

The small intestine including the duodenum has an average length of about twenty feet. The calibre of the small intestine is larger at its upper end and gradually diminishes in size until its entrance into the large bowel. Thus at the beginning the jejunum has a calibre of 17.5 cm., the ileum at its beginning of 11.5 cm., and at its end 9.5 cm. The ileum passes perpendicularly into the ascending part of the larger bowel just above the caecum, its mucosa forming a double valve, called valvula Bauhini. The jejuno-ileum is the most movable part of the intestinal tract. Wherever a free space is left it occupies it. It is therefore most often met with in hernias. During gravidity or when a tumor or ascites exists in the abdomen the small intestine moves up higher and thus escapes compression.

The small intestine receives its blood supply from the abdominal aorta. The arteria gastroduodenalis, a branch of the arteria hepatica, supplies the upper part of the duodenum; the lower part of the duodenum and the jejunum and ileum are supplied by the arteria mesenterica superior. The latter vessel branches off into a fine net of numerous small vessels which run through the intestinal wall. The end ramifications penetrate the submucosa and here again form a net. From the latter the finest ramifications penetrate the mucosa and form a capillary system of the villi and glands. The venous blood flows partly into the vena gastrica superior, partly into the vena mesenterica superior, and empties itself into the vena porta. The lymphatics form a continuous series which is divided into two sets, that of the mucous membrane and that of the muscular coat. The lymph vessels of both sets form a copious plexus. They run between the two folds of the mesentery and end in the mesenteric lacteals. They are provided with valves which prevent the current from flowing backward, the direction of which is into the truncus lymphati-cus intestinalis and finally into the thoracic duct.

The nerves of the small intestine originate principally from the plexus mesentericus superior or the sympathetic. The duodenum is supplied by the plexus hepaticus, a branch of the plexus coeliacus. The abdominal part of the vagus, namely, the plexus gastricus, anterior and posterior, also supplies the small intestine with nerves. The nerves, which are mostly non-medullary, enter the intestinal wall in connection with the branches of the arteria mesenterica superior and form a subserous net. They then penetrate the long muscular fibres and form between these and the circular muscular fibres ramifications which consist of numerous groups of multipolar cells (plexus mesentericus seu Auerbachii); fine branches of nerves arising here supply the muscularis. Others penetrate the circularis, reach the submucosa, and form the submucous nerve plexus, containing small groups of ganglion cells (Meissner's nerve plexus); fine ramifications also supply the muscularis mucosae, the muscles of the villi, and end in the remaining part of the mucosa.