FOOD AND DIGESTION 57 



many nerves, chiefly those of smell and taste, stimula- 

 tion of which will excite a flow of saliva. Dilatation of 

 the blood vessels in a gland will produce two chief ef- 

 fects. First it brings into contact with the gland cells 

 a larger quantity of blood containing the material from 

 which that particular secretion is made ; and, second by 

 increasing the pressure it will force a larger quantity of 

 the watery constituents of the blood through the vessel 

 wall and into the glandular structure. There are thus 

 two distinct acts, filtration, which is mechanical, and se- 

 cretion, or manufacture, which is a vital function per- 

 formed by living tissue. 



Mouth Digestion 



Saliva. The salivary secretion acts mechanically to 

 moisten dry food, thus getting it into a suitable condi- 

 .tion for swallowing; through the mucin contained to 

 lubricate the mass, and by the ptyalin, to digest some of 

 the starch. This is accomplished by causing the starch 

 molecules to take up water, in chemical combination, 

 thus converting it into maltose and dextrine, two forms 

 of sugar which are not absorbed as such but have to be 

 converted into dextrose in the intestines. Ptyalin diges- 

 tion, therefore, is simply preparatory. Food is retained 

 in the mouth for too short a time to be digested, but re- 

 cent evidence shows that it may be held in the fundus 

 of the stomach for more than an hour before the hydro- 

 chloric acid arrests ptyalin digestion. 



Stomach Digestion 



Gastric juice is secreted by glands in the mucous lin- 

 ing of the stomach. It is a watery solution containing 

 pepsin, and rennin, the first to act on proteins, the sec- 



