1020 THE ORGANS OF DIGESTION. 



Jejunum and Ileum (Intestinum mesenteriale). 



Following the duodenum, about the upper two-fifths of the remaining small 

 intestine is called jejunum and the lower three-fifths ileum There is no mor- 

 phological line of distinction between these two, but there is considerable differ- 

 ence between the beginning of the jejunum and end of ileum. The diameter of 

 the first is about one and a half inches; of the latter, one and one-fourth inches; 

 the walls of the jejunum are thicker and a given length weighs more than the 

 same of the ileum ; the character of the mucous membrane and of the contents 

 markedly changes, but very gradually. The ileum possesses none or poorly- 

 formed valvulae conniventes. The jejunum is usually in the umbilical region and 

 left iliac fossa, while the coils of the ileum are more on the right side and right 

 iliac fossa and true pelvis. Both these parts of the intestine retain the mesentery, 

 which the duodenum does not. 



There is but little fixation to the loops of the small intestine ; the mesentery 

 allows the freest motion. Every moment the coil must accommodate itself to 

 changes in form and position of the peritoneal cavity or be prepared to fill some 

 hole. Contraction of the diaphragm and abdominal muscles, the filling and 

 emptying of viscera, presence of tumors, position of body, must all occasion 

 changes of position in the small intestines. With this great motility, no definite 

 shape can be ascribed for the coils, but frequently the upper loops of the jejunum 

 are transverse and the lower ofLthe ileum are more vertical. 



The terminal part of the ileum is more fixed than any other, as its mesentery 

 passing over the right Psoas muscle is very short. At the point of transition 

 from the duodenum to the jejunum or from the small to the large intestine the 

 various fossae have been noticed (p. 994). 



The vitelline duct coming from the original convexity of the intestinal loop 

 (Fig. 588) may persist in adult life ; it is then called MeckeVs diverticulum. It 

 is a blind intestine, having the same layers as the ileum, with the lumen of which 

 it directly communicates. It is two or three inches long (one-half to seven 

 inches), and rises about forty-three inches from the ileo-colic junction (from one 

 to ten feet) ; originally it passes toward the umbilicus, but usually hangs free in 

 the cavity. It may be connected with the umbilicus or other points by a solid 

 band, which attains great firmness and is the enlarged remains of the omphalo- 

 mesenteric vessels. It may be conical, cylindrical, or hour-glass in shape. It 

 occurs about once in fifty cases, and may cause surgical complications. 



Structure of the Wall of the Small Intestine. Like the stomach, the wall 

 is composed of four layers, serous, muscular, submucous, and mucous. It is 

 much thinner than that of the stomach, only | 1 mm. thick. 



The external or serous coat is peritoneum, which surrounds the whole of the 

 ileum and jejunum except along the little interspace left at the mesenteric border 

 of the intestine. Here a sort of linear hilus is left between the two layers where 

 vessels, nerves, veins, and lymphatics have their entrance or exit. In case of 

 the duodenum, each part is covered to a different degree. 



The muscular coat consists of two layers ; as usual for the alimentary canal, 

 the external is made of longitudinal fibres, and the internal of circular fibres. 

 The longitudinal fibres are best developed at the beginning of the duodenum and 

 end of the ileum, and are here closely attached to the serous coat. They are 

 most marked on the free border of the intestine, and may be wholly lacking on 

 the mesenteric attachment. The circular set is three times thicker than the 

 longitudinal, and consists of complete muscular rings which are pressed so closely 

 together as to only leave clefts for the passage of vessels and nerves to deeper 

 parts. This double coat gets thinner below, is pale, and made of unstriated mus- 

 cular tissue. It produces peristalsis, by which food is pushed onward. 



The submucous coat acts as a bed for the mucosa, is connected more closely with 

 it than with the muscular coat, and is made of areolar tissue. Here are lymphatic 

 vessels and nerve-plexuses, and the blood-vessels divide up for the mucosa. 



