338 HANDBOOK OF PHYSIOLOGY. 



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

 is subject to considerable variation. When the tongue and muscles con- 

 cerned 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, but is at least 2 

 pints (1 litre). 



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

 and (&) chemical. 



(a). Mechanical. (1) It keeps the mouth in a due condition of mois- 

 ture, facilitating the movements of the tongue in speaking, and the mas- 

 tication 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. Tor, speaking generally, the quantity secreted during feeding 

 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 resophagus. 



(b) Chemical. The chemical action which the saliva exerts upon the 

 food in the mouth is to convert the starchy materials which it contains into 

 soluble starch and then, partially, into sugar. This power the saliva 

 owes to one of its constituents, ptyalin, which is one of the enzymes, or 

 unorganized ferments. Certain investigators have of late asserted that 

 saliva contains another enzyme, known as glucose, which has the power 

 of splitting the disaccharides into monosaccharides, or maltose into dex- 

 trose. The action of this ferment is certainly very limited. The -conver- 

 sion of the starch under the influence of the ferment into sugar takes 

 place in several stages, and in order to understand it, a knowledge of the 

 structure and composition of starch granules is necessary. A starch 

 granule consists of two parts : an envelope of cellulose, which does not 

 give a blue color with iodine except on addition of sulphuric acid, and of 

 yranulose, which is contained within, and which gives a blue with iodine 

 alone. Brticke states that a third body is contained in the granule, which 

 gives a red with iodine, viz., erythro-granulose. On boiling, the granu- 

 Itse swells up, bursts the envelope, and the whole granule is more or less 



