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 the wall; or again, the presence of a special 
muscular tunic, and its modification at certain places, as in the 

Fic. 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 
FORM AND on its relation to an extensive serous sac—in a 
SYMMETRY. ‘™ammal 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 
