222 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



to the head. This divides the alimentary canal into pre- and post- 

 hepatic portions (fig. 233). 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 posthepatic 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 

 connexion with the respiratory organs, the bladder with the uro- 

 genital 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. 



In the following account stress is laid upon the epithelial lining 

 (entoderiri), the characteristic tissue of the digestive tract, but it must 



Fig. 233. — Diagrams of the alimentary canal in embryos of 6 and 8 days of Gym- 

 narchus ntloticus, after Assheton. ab, air bladder; b, early diverticulum for air bladder; 

 gb, gall bladder; /, liver; pa, pancreas; pb, posterior part of air bladder; pc, pyloric 

 caeca; ph, pharynx; s; stomach. 



not be forgotten that the wall contains other tissues of mesenchyma- 

 tous 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 absorptive 

 vessels; outside of this are two layers of smooth (involuntary) mus- 

 cles, the inner with the fibres running in a circular, the other in a 

 longitudinal direction. 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. 



