230 HAND-BOOK OF PHYSIOLOGY. 



The presence of potassium sulphocyanate (or tliiocyanate) {C N K S) 

 in saliva, may be shown by the blood-red coloration which the fluid gives 

 with a solution of ferric chloride (Fe 2 01 6 ), and which is bleached on 

 the addition of a solution of mercuric chloride (HgCl a ). 



Rate of Secretion and Quantity. The rate at which saliva is 

 secreted is subject to considerable variation. When the tongue and 

 muscles concerned in mastication 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. During actual 

 secretion the flow is much accelerated. 



The quantity secreted in twenty-four hours varies; its average amount 

 is probably from 1 to 3 pints (1 to 2 litres). 



Uses of Saliva. The purposes served by saliva are (1) mechanical and 

 (2) chemical. I. Mechanical. (1) 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 feed- 

 ing is in direct proportion to the dryness and hardness of the food. 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 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 mucous secre- 

 tion of the submaxillary, palatine, and tonsillitic glands is spread over 

 the surface of the softened mass, to enable it to slide more easily through 

 the fauces and oesophagus. II. Chemical. Saliva has the power of con- 

 verting starch into glucose or grape-sugar. When saliva, or a portion of 

 a salivary gland, is added to starch paste in a test-tube, and the mixture 

 kept at a temperature of 100 F. (37 -8 C.), the starch is very rapidly 

 transformed into grape-sugar. There is an intermediate stage in which a 

 part or the whole of the starch becomes dextrin. 



Test for Glucose. In such an experiment the presence of sugar is at 

 once discovered by the application of Trommer's test, which consists in 

 the addition of a drop or two of a solution of copper sulphate, followed 

 by a larger quantity of caustic potash. When the liquid is boiled, an 

 orange-red precipitate of copper suboxide indicates the presence of sugar; 

 and when common raw starch is masticated and mingled with saliva, and 

 kept with it at a temperature of 90 or 100 F. (30 37.8 C.), the 

 starch-grains are cracked or eroded, and their contents are transformed 

 in the same manner as the starch-paste. 



