ALIMENTATION 111 



administering cod-liver oil, which is high in vitamin D. Since 

 rickets may also be controlled by ultraviolet radiation, it is 

 thought that radiation liberates vitamin D from substances in the 

 tissues that might be regarded as a kind of 'provitamin. This 

 substance is ergosterol, an unsaturated fat. The active vitamin 

 occurs in abundance in fish oils, butter fat, whole milk, and yolk 

 of eggs. Administration of irradiated ergosterol shows the same 

 antirachitic effects as the vitamin D of fish oils. Vitamin D is 

 involved in regulating the concentrations of phosphorus and 

 calcium in the blood and is concerned with the calcification of 

 bones and teeth. 



Vitamin E is fat-soluble and is necessary for fertility in repro- 

 duction. It is found in meat, cereal, lettuce, liver, and egg yolk. 

 Its omission from a diet results in sterility. 



Vitamin G or B 2 consists of two substances: (1) lavine, a 

 yellow pigment, which seems to be essential for growth, and (2) a 

 portion called B 6 that prevents and cures a dietary skin disease 

 in rats somewhat similar to pellagra in man. These vitamins 

 occur in yeast, whole cereals, milk, vegetables, and fruits. 



Salivary Digestion. — In the frog, food is swallowed without 

 mastication and salivary digestion is entirely lacking. In man 

 and most mammals, the saliva contains a starch-splitting enzyme, 

 ptyalin, secreted principally by the parotid gland. The flow 

 of saliva is a result of reflex stimulation of secretory nerves. 

 Thus the sight and odor of food cause a flow of saliva before the 

 food enters the mouth. In the mouth the taste of food also 

 stimulates salivary secretion. The action of saliva upon starch 

 may be demonstrated in a test tube, or by holding a small quan- 

 tity of boiled potato in the mouth for a short time. After a 

 few minutes sugar is found in the solution. The conversion of 

 starch into sugar is a step-by-step reaction, in which maltose is 

 split off leaving residues that become progressively simpler in 

 chemical structure until entirely converted into sugar. This 

 may be illustrated as follows: 



, maltose 

 Starchy maltose 



erythrodextrin^ , maltose 



achroodextrin^ ™ a ltose 



dextrin / 



maltose 



