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, 

 oesophagus, 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. 



B 



FIG. 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; I, liver; pa, pancreas; pb, posterior part of air bladder; pc, pyloric oeca; 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 absorb tive 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, oesophagus, etc.) which are outside 

 the region of the ccelom. 



