98 



ANATOMY OF THE RABBIT 



Form and Symmetry 

 In its most general features, the digestive system is significant 

 as an epithelial tube in which the food is modified, by solution or 

 otherwise, so that it is capable of being absorbed through the 

 epithelial surface. In the form of the digestive tube as seen in a 

 vertebrate, however, a number of gross mechanical features are 

 "^evident, such as, for example, the increase in capacity, or in ab- 



FiG. 51. Plan of successive embryonic stages in displacement of the 

 digestive tube and common mesentery from the mid-line position (man) : 

 a, tr, d, ascending, transverse, and descending colons; r, rectum; si, small 

 intestine; st, stomach. (Modified from figures by Toldt and Hertwig.) 



sorptive area, through the folding of the mucous membrane, or the 

 expansion of the wall; or again, the presence of a special muscular 

 tunic, and its modification at certain places, as in the oesophagus, 

 the pyloric limb of the stomach, and the first portion of the colon. 

 Moreover, many features of the abdominal portion of the tube, 

 and, indeed, certain of its recognized divisions, depend on its re- 

 lation to an extensive serous sac — in a mammal the peritoneal 

 cavity. In this connection it is to be considered that the digestive 

 tube is primarily a median structure. It has this relation in the 

 earlier stages of embryonic development (Figs. 22, 51), and in many 

 of the lower vertebrates it does not deviate to a great extent from a 

 median position. In all higher vertebrates, however, the tube 

 becomes greatly elongated in comparison with the cavity in which 

 it lies, and thus becomes extensively displaced to one side or other 

 of the median plane. This development, while advanced in all 

 mammals, may be said to reach an extreme in the herbivorous 



