USES OF SALIVA. 211 



tication are at rest, and the nerves of the mouth are subject to 

 no unusual stimulus, the quantity secreted is not more than 

 sufficient, with the mucus, to keep the mouth moist. But the 

 flow is much accelerated when the movements of mastication 

 take place, and especially when they are combined with the 

 presence of food in the mouth. It may be excited also, even 

 when the mouth is at rest, by the mental impressions produced 

 by the sight or thought of food ; also by the introduction of 

 food into the stomach. The influence of the latter circum- 

 stance was well shown in a case mentioned by Dr. Gairdner, 

 of a man whose pharynx had been divided : the injection of a 

 meal of broth into the stomach was followed by the secretion 

 of from six to eight ounces of saliva. 



Under these varying circumstances, the quantity of saliva 

 secreted in twenty-four hours varies also ; its average amount 

 is probably from two to three pints in twenty-four hours. In 

 a man who had a fistulous opening of the parotid duct, Mits- 

 cherlich found that the quantity of saliva discharged from it 

 during twenty-four hours, was from two to three ounces ; and 

 the saliva collected from the mouth during the same period, 

 and derived from the other salivary glands, amounted to six 

 times more than that from the one parotid. 



The purposes served by saliva are of several kinds. In the 

 first place, acting mechanically in conjunction with mucus, it 

 keeps the mouth in a due condition of moisture, facilitating 

 the movements of the tongue in speaking, and the mastication 

 of food. (2.) It serves also in dissolving sapid substances, and 

 rendering them capable of exciting the nerves of taste. But 

 the principal mechanical purpose of the saliva is (3) that by 

 mixing with the food during mastication, it makes it a soft 

 pulpy mass, such as may be easily swallowed. To this purpose 

 the saliva is adapted both by quantity and quality. For, 

 speaking generally, the quantity secreted during feeding is in 

 direct proportion to the dryness and hardness of the food : as 

 M. Lassaigne has shown, by a table of the quantity produced 

 in the mastication of a hundred parts of each of several kinds 

 of food, thirty parts suffice for a hundred parts of crumb of 

 bread, but not less than 120 for the crusts ; 42.5 parts of saliva 

 are produced for the hundred of roast meat; 3.7 for as much 

 of apples ; and so on, according to the general rule above stated. 

 The quality of saliva is equally adapted to this end. It is easy 

 to see how much more readily it mixes with most kinds of food 

 than water alone does ; and M. Bernard has shown that the 

 saliva from the parotid, labial, and other small glands, being 

 more aqueous than the rest, is that which is chiefly braided 

 and mixed with the food in mastication; while the more viscid 



