The Intestines. 123 



3: In the mesenterial small intestine the following features may be 

 identified : 



(a) The lighter coloration, due to the thicker wall and greater 

 vascularity, of the first or duodenal portion, thus distin- 

 guished as the jejunum (intestinum jejunum). 



(b) The darker coloration, due to the thinner walls, which allow 

 the contents to show through, and diminished vascularity of 

 the terminal or caecal portion, thus distinguished as the 

 ileum (intestinum ileum) . The two portions are not distinctly 

 separable. Also the circular folds (plicae circulares), or 

 valvulae conniventes, of the mucous tunic, which in many 

 mammals contribute to the thickness of the wall in the 

 duodenum and jejunum, are in the rabbit not definitely 

 expressed. 



(c) The mesentery, the peritoneal support of the mesenterial 

 small intestine, is distinguished in its major portion by its 

 broad frill-like character, which allows great freedom of 

 movement to this portion of the digestive tube. Its terminal 

 portion, however, beginning at a point where the intestine 

 turns sharply forward on its way to the caecum, is adherent 

 to the mesocolon. 



(d) The mesenteric lymph glands (lymphoglandulae mesentericae) 

 are aggregated at the dorsal root of the mesentery, where 

 they form a compact mass surrounding the base of the 

 superior mesenteric artery. 



(e) The wall of the sacculus rotundus shows externally a hexagonal 

 pattern on account of the presence in it of a large number 

 of lymph follicles. Similar structures, forming oval areas 

 about 3 mm. in diameter and 5 mm. in length, may be found 

 along the intestinal wall (aggregated lymph nodules of Peyer). 



(f) The finger-like processes, or villi, of the mucous tunic of the 

 small intestine may be seen by making an incision of the 

 wall and examining its internal surface. A small portion of 

 the wall may be excised and examined under water. 



4. Trace the course of the large intestine, beginning at the sacculus 

 rotundus, as follows: Its first portion, the blind intestine or caecum 

 (intestinum caecum), distinguished by its great size, is connected with 

 the large intestine proper only in the region of the sacculus rotundus. 

 Its course is comparable to two turns of a spiral. Its closed end, formed 

 by the vermiform process (processus vermif ormis) , lies in a dorsal 

 position, and is directed backward. The second portion, the colon, 

 comprising the major portion of the large intestine proper, leaves the 

 caecum in the region of the sacculus rotundus, in which position it is 

 distinguished by its greatly sacculated walls. The third portion, the 

 straight intestine, or rectum, (intestinum rectum) is a small terminal 

 division situated in the middle line and enclosed for the most part by 

 the pelvis. It is scarcely distinguishable from the related portion of 

 the colon. 



