The Digestive System. 41 



In considering the divisions of the digestive tube in the rabbit, the 

 posterior, or post-cephalic portion, comprising the oesophagus and suc- 

 ceeding parts, may be distinguished from the anterior, or cephalic portion, 

 the latter comprising the oral cavity and pharynx. The former is a free 

 portion embracing the digestive tube proper, while the latter is a fixed 

 portion exhibiting a variety of general mammalian features connected 

 with the organization of the head. 



The oesophagus is a slender but greatly expansible tube leading from 

 the pharynx to the stomach. In its passage backward it traverses the 

 neck and the thorax, and in both regions occupies a median position. 

 In the thorax (Plate VII) it will be observed that it lies between the heart 

 and the dorsal aorta, thus exhibiting the original relation of the digestive 

 tube to the aortic portion of the vascular system. The succeeding 

 portions of the digestive tube are those associated with the peritoneal 

 cavity, and with the exception of the terminal portion, the rectum, are 

 displaced from a median position. Consequently, the divisions which 

 are recognized are based partly on the differential characters of the wall, 

 and partly on the position of structures more especially in relation to the 

 supporting peritoneum. Thus, the chief features of the stomach 

 depend on the expansion of the organ and the rotation of its pyloric end 

 forward and to the right. In the intestinal tract as a whole the chief 

 although by no -means most conspicuous feature of position depends on 

 the looping of the entire structure on itself, so that the terminal portion, 

 chiefly the transverse colon, crosses the ventral surface of the duodenum 

 and then turns backward on the dorsal surface of the mesenterial small 

 intestine. The duodenum is sharply marked off from the mesenterial 

 intestine as an extensive loop, containing the major part of the pancreas 

 and its duct, and lying on the right side of the dorsal wall of the abdomen. 

 The mesenterial intestine is a greatly convoluted portion, lying chiefly 

 on the left side of the abdominal cavity, and loosely supported by the 

 broad, frill-like mesentery. From the pylorus to the sacculus rotundus 

 there is no abrupt change in the character of the wall, although the first 

 portion of the mesenterial intestine, that designated as the jejunum, and 

 the duodenum may be considered together as a more vascular portion 

 with thicker walls in comparison with the second portion, the ileum, in 

 which the wall is less vascular and more transparent. 



T s he main portion of the large intestine, the colon, although greatly 

 specialized, may be considered to consist as in man of ascending, trans- 

 verse, and descending parts, that is to say, the ascending colon lies on the 

 right side of the body and passes in a general way from its point of origin 

 on the caecum forward to a point where it becomes flexed to the left as 

 the transverse colon ; the latter crosses the body and is flexed backward 

 as the descending coion. In the rabbit, however, that portion definable 

 as the ascending colon is greatly elongated, and is composed of five 

 principal limbs, united by flexures. Two of these, in dissection from the 

 ventral surface, are concealed by the base of the superior mesenteric artery, 

 since they lie on its right side. The descending colon is also only nom- 

 inally related to the left 'side of the body wall, its supporting peritoneum, 

 the descending mesocolon, being closely connected with the mesoduodenum 



