148 PHYSIOLOGICAL CHEMISTRY 



Chemical substances may cause profuse secretion, for example 

 dilute acetic acid. "Weak alkalies do not stimulate secretion. 



Very interesting results have been obtained in studying the 

 psychic secretion. If a fasting animal is shown tempting food, 

 saliva flows copiously. If the animal is not allowed to eat the 

 food, however, and it is shown to him repeatedly, the psytehic 

 secretion 110 longer appears. Also, if an animal is first shown 

 something he does not care for, and which causes no secretion, 

 and then given something he is fond of, and the whole process 

 is repeated several times, finally saliva will be secreted at sight 

 .of the unappetizing food, the display of which always has been 

 followed by the feeding of something he likes. Evidently he has 

 learned to associate the unappetizing food with the appetizing, 

 a secretion resulting. 



The amount of saliva secreted in a day undoubtedly varies 

 within wide limits. 1500 c.c a day has been suggested as an 

 average amount. Many factors may greatly increase this 

 amount such as smoking, continuous chewing, mercury poison- 

 ing, or various drugs, notably pilocarpine. 



Composition of the Mixed Saliva. The mixed saliva is a thin 

 watery fluid (99.4% water) containing various salts, including 

 potassium sulphocyanate, and, as its most important constitu- 

 ents, mucin and an enzyme ptyalin. Other substances of vary- 

 ing nature are present in small amounts. The saliva usually is 

 somewhat turbid, due to cells or other material, and on stand- 

 ing the turbidity increases, due to the precipitation of calcium 

 carbonate. It often is thick and somewhat slimy in character 

 from the mucin present, most of which is secreted by the sub- 

 maxillary and sublingual glands. Mucin is a glycoprotein which 

 already has been discussed in the chapter on proteins. Saliva is 

 usually slightly alkaline in reaction, (H-ion concentration about 

 2X10' 8 ) but so slightly that it is acid to phenolphthalein. In 

 certain individuals the saliva is somewhat acid. Some recent 

 reports on the reaction of the saliva have been to the effect that 

 it normally is slightly acid in character. 



Functions of Saliva. The saliva moistens the mouth and food, 



