DIGESTIVE ORGANS. 207 
hepatic portions (fig. 209). From the anterior of these is formed 
part of the cavity of the mouth with the salivary glands, the pharynx, 
cesophagus, stomach, and duodenum; the post-hepatic portion gives 
rise to large and small intestines, rectum and cloaca, as well as to the 
urinary bladder. Of these parts the pharynx will be considered in 
connection with the respiratory organs, the bladder with the urogenital 
system. Mouth and pharynx belong primitively to the head, but by 
unequal growth the pharynx may be carried apparently to some 
distance behind the brain and other characteristically cephalic 
structures. 
Fic. 209.—Diagrams of the alimentary canal in embryos of 6 and 8 days of Gymnarchus 
niloticus, after Assheton. ab, air bladder; b, early diverticulum for air bladder; gb, gall 
bladder; /, liver; pa, pancreas; b, posterior part of air bladder; pc, pyloric ceca; ph, 
pharynx; s, stomach. 
In the following account stress is laid upon the epithelial lining 
(entoderm), the characteristic tissue of the digestive tract, but it must 
not be forgotten that the wall contains other tissues of mesenchymatous 
origin. That part of the canal which runs through the body cavity 
has the following layers. The lining epithelium is supported by a 
layer of connective tissue, containing the capillary absorbtive vessels; 
outside of this are two layers of smooth (involuntary) muscles, the inner 
with the fibres running in a circular, the other in a longitudinal direc- 
tion. By the action of these antagonistic muscles the peristalsis or 
movement of the digestive tract is effected, by which the food undergoing 
digestion is churned and thoroughly mixed with the digestive fluids, 
and all parts of it are brought into contact with the absorbtive surfaces. 
The outer surface of stomach, intestine and associated glands is covered 
with the serous coat, the lining of the peritoneal cavity, but this is 
lacking from those parts (pharynx, cesophagus, etc.) which are outside 
the region of the ccelom. 
