238 ANATOMY OF THE RABBIT 



sacculus rotundus, in which position it is distinguished by its 

 greatly sacculated walls. As noted above, the first part of the 

 colon of the rabbit has assumed the structure of the caecum, 

 constituting the ampulla caecalis coli, beyond which the structure 

 of the beginning of the colon appears suddenly. Such an ampulla 

 is not present in mammals which, like man, have not an extensively 

 developed caecum. 



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 dis- 

 tinguishable from the related portion of the colon, so that the point 

 of disappearance of the latter from the abdominal cavity may be 

 regarded for convenience as the dividing line between them. 



5. In the caecum the following features may be distinguished: 



(a) The wall is notably thin and, though otherwise smooth, is 

 divided by a spirally arranged constriction, the latter de- 

 noting the position, internally, of a fold of the mucous 

 tunic, the spiral valve. Both of these features are continued 

 into the ampulla caecalis coli. 



(6) The vermiform process is a narrow, light-coloured tube of 

 about five inches in length, the wall patterned externally 

 by lymph follicles, in the same way as that of the sacculus 

 rotundus, and greatly thickened in comparison with that 

 of the caecum proper. 



6. The colon, beyond the ampulla caecalis, is divisible into ascend- 

 ing, transverse, and descending portions, the relations of which 

 may be traced as follows: 



(a) The ascending colon (colon ascendens) passes from its 

 origin on the caecum to a point forward on the right side of 

 the dorsal body-wall. This portion is greatly elongated in 

 the rabbit and, instead of passing directly forward (i.e., 

 upward in man — see Fig. 51), follows more or less closeh' 

 the course of the caecum. It is composed of five principal 

 limbs, each of these being a portion which runs either for- 

 ward or backward and is united by a flexure to the next, 

 which has the opposite direction. Three of the limbs are 



