DIGESTION AND THE DIGESTIVE AGENTS. 329 



mouth. Very frequently, too, \vhite cells, probably identi- 

 cal with white blood corpuscles are present. These are 

 corpuscles which have probably wandered out from deeper 

 tissues along with the secretion. They have been called 

 salivary corpuscles, although now recognized as nothing 

 more than escaped leucocytes. Saliva is a little heavier 

 than water and is alkaline. The quantity secreted in a day 

 is, of course, subject to the widest variations, varying from 

 200 to 2,000 grains. Saliva contains the following constitu- 

 ents in 1,000 parts: 



Water 994-2 



Mucin 2.2 



Ptyalin 1.3 



Inorganic Salts 2.2 



The inorganic salts are largely chloride of potassium and 

 sodium, and phosphates of calcium and magnesium. These 

 inorganic salts in the saliva are interesting from the fact 

 that small amounts of them are deposited around the teeth 

 and give rise to the formation of tartar. The mucin pres- 

 ent in the saliva is ordinary mucus or phlegm, and is really 

 an addition to the saliva from the mucous glands in the 

 mouth. This mucus has no distinct value further than that 

 by its ropy and sticky nature it serves to hold bits of the 

 food together and so makes swallowing easier. Incidentally 

 along with the saliva it serves to keep the mouth moist. 

 The main and distinct constituent of saliva is the ptyalin. 

 This belongs to the class of substance called enzymes, or 

 ferments, and has the property of being able to convert 

 starch into sugar. 



THE THEORY OF HYDROLYSIS. 



As the term "enzyme" or ferment occurs repeatedly in 

 physiology (the pepsin in the stomach, the trypsin in the 

 pancreas, and others being ferments), a word in explana- 

 tion of these substances and the manner in which they are 

 supposed to act is given. 



There are two kinds of ferments; one, that of living fer- 

 ments, of which the many forms of bacteria and the yeast 



