100 



ANATOMY OF THE RABBIT 



Fig. 52. The nasopharynx and related parts 

 of the head as seen in median section (anterior 

 end to the left): 1, tongue; 2, hyoid; 3, tonsil; 

 4, epiglottis; 5, entrance to trachea; 6, entrance 

 to oesophagus; 7, basioccipital bone; 8, soft 

 palate; 9, pharyngeal aperture of auditory 

 (Eustachian) tube; 10, cranial cavity; 11, 

 ethmoturbinal scrolls; 12, nasal cavity; 13, nasal 

 septum; 14, hard palate; 15, oral cavity; 16, 

 nasopharynx. 



ventral or laryngeal portion, containing the aperture of the 

 larynx (Fig. 52). 



The oesophagus is a slen- 

 der but greatly expansible 

 tube leading from the pharynx 

 to the stomach. In its pas- 

 sage backward it traverses the 

 neck and the thorax, and in 

 both regions occupies a me- 

 dian position. In the thorax 

 (Plate VII) it will be obser- 

 ved to lie between the heart 

 and the dorsal aorta, thus 

 exhibiting the original rela- 

 tion of the digestive tube to 

 the aortic portion of the vas- 

 cular system. The function 

 of the oesophagus is that of a 



simple conveyer to the stomach. 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 dis- 

 placed 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 back- 

 ward as the descending colon on the dorsal surface of the mesenterial 

 small intestine (Fig. 51). In the development of this twisted 

 arrangement and its many variants in different mammals the 

 superior mesenteric artery has acted more or less as an axis of 

 rotation (Fig. 53). The duodenum is marked off from the mesen- 

 terial intestine as an extensive loop containing the major part 

 of the pancreas and its duct and lying on the right side of the 



