128 PHYSIOLOGICAL CHEMISTRY 



mechanical irritation of the mouth, by reflex action upon stimu- 

 lation of the end organs of taste or smell, by purely psychic in- 

 fluences, such as the thought or sight of food, or by a general 

 reflex, as in nausea preceding vomiting. Its flow may be in- 

 hibited by nervous stress or anxiety, a fact well known to 

 amateur public speakers or by those who have experienced dry- 

 ness of the throat in sudden fright. Some drugs stimulate, 

 others retard the flow of saliva. The composition of the secre- 

 tion is influenced by the nature of the stimulus producing it. 

 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, jnucin 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. The causes of 

 such a condition as yet are but imperfectly understood. 



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

 and by reason of the mucin present, serves as a lubricating agent 

 to aid in the manipulation and swallowing of the food. It also 

 cleans and preserves the teeth, both by washing away particles 

 of food which otherwise might decay, producing acids which 

 would attack the teeth, and also by neutralizing acids introduced 

 into the mouth. The saliva also performs an important digestive 



