82 Anatomy of the Rabbit. 



fossae, and also with the middle ear through the internal auditory 

 tube, and a ventral or laryngeal portion, containing the aperture 

 of the larynx (Fig. 41). 



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 

 (Fig. 42) 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 duo- 

 denum 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 common bile duct enters its first portion immediately 

 beyond the pylorus. 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 mesen- 

 terial 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. 



The main portion of the large intestine, the colon, although 

 greatly specialized, may be considered to consist as in man of 



