SUBPHYLUM VERTEBRATA 323 



curd, while the pepsin partially digests proteins, changing them to pep- 

 tones. A sphincter muscle called the pyloric valve controls the passage of 

 the food from the stomach into the small intestine and prevents the 

 passing of any food which is not quite liquid. Thus as the food is gradu- 

 ally reduced to liquid form in the stomach it is passed in a series of spurts 

 or jets into the intestine. 



The small intestine is digestive and absorptive in function. Into it 

 empty the ducts of two glands, the liver and the pancreas. In the secre- 

 tion of the pancreas are several enzymes. These are (1) trypsin, which 

 completes the conversion of proteins into peptones and also changes the 

 peptones into amino acids, which can be absorbed; (2) amylopsin, which 

 forms soluble sugar from starches; and (3) steapsin, which changes fats 

 into soluble fatty acids and glycerin, both of which are capable of being 

 absorbed. The wall of the intestine also contains glands which secrete 

 enzymes capable of changing nonabsorbable sugars into those which are 

 absorbable. All of these enzymes act in an alkaline medium. The 

 secretions poured into the intestine serve to neutralize the hydrochloric 

 acid of the stomach and to render the food alkaline. 



In the small intestine the digested food is absorbed into lymphatics 

 and blood vessels which lie immediately under the lining epithelium. 

 The absorbing surface of the intestine is increased by its length, usually 

 much greater than that of the body, and also by the formation of folds 

 and of very numerous minute finger-like projections called villi (Fig. 8). 



The liver is the largest single organ in the body and has a variety of 

 functions. In it is secreted the hile, which is stored in the gall bladder 

 and passed into the intestine as needed. The bile assists in breaking up 

 the fats and preparing them for absorption and is also antiseptic in its 

 action. In addition to this the liver is a great storage organ where carbo- 

 hydrates are accumulated in the form of glycogen, so that though the 

 animal may be unable for some time to secure more, it has a supply 

 sufficient to maintain muscular activity. In the liver protein wastes are 

 broken up into urea, which is then carried by the blood to the kidneys 

 where it is eliminated. 



The la7^ge intestine contains mucous glands the secretions of which 

 lubricate the passage. In the anterior part of it digestion and absorption 

 are completed. In the posterior part, called the rectum, the undigestible 

 residue is accumulated. Sphincter muscles control its passage from the 

 body. In some vertebrates the anal opening is not upon the surface but 

 upon the wall of a cloaca, into which also open the ducts from the 

 excretory and reproductive systems. The cloaca in turn opens to the 

 outside. 



347. Respiratory System. — In the lower forms of vertebrates respira- 

 tion occurs through the walls of the gill slits or through gills, which are 

 branched projections from the walls of the slits. In terrestrial verte- 



