The Digestive System. 



79 



the wall, The relatively great extent of the wall, including the 

 enormous development of the caecum in the rabbit and other 

 rodents, is related to the comparatively great bulk and low nutri- 

 tive quality of the ingested food. 



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 

 absorptive area, through the folding of the mucous membrane, 

 or the expansion of tl^e wall; or again, the presence of a special 

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



Fig. 40. Plan of successive embryonic stagesin displace - 

 ment of the digestive tube and common mesentery from the 

 midline position (man) : a, tr, d, ascending, transverse, and 

 descending colons; r, rectum; si, small intestine; st, stomach 

 Modified from figures by Toldt and Hertwig. 



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 

 vnrjTur awt> ^^ i^^ relation to an extensive serous sac — in a 



SYMMETRY. 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. 23, 40) , 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 



