200 



STRUCTURE OF THE VERTEBRATES 



as the reptiles and carnivorous mammals. The sphincter which 

 separates the esophagus from the stomach is poorly developed 

 in amphibia and reptiles, making regurgitation of food a simple 

 matter; but in mammals the opening is small and the constrictor 

 muscles are more completely under the control of the nervous 

 system. 



The stomach breaks the larger masses of food, and begins 

 the digestion of proteins. The acids and digestive enzymes de- 



A. Rat 



B. Pig 



C. Dog 



D. Sheep 



K Human 



Fig. 114. Structure of the Mammalian Stomach. The stomach is struc- 

 turally simple in most mammals. Herbivorous mammals show more ex- 

 treme modification. In (D), the sheep, the arrows show the course of 

 food in a cud-chewing animal. 



stroy the connective tissues which bind together the cells of 

 muscles, and hasten the stomach and intestinal digestion. In 

 correlation with this function the walls are generally thick and 

 muscular, churning the mass until it is mixed with the digestive 

 fluids. 



Anatomically, the simplest stomach is a slight enlargement, 

 and in the cyclostomes is continuous with the intestine. This 

 stomach-intestine is sufficient to digest the finely ground particles 

 of flesh which are swallowed. In the vertebrates generally the 



