DIGESTION. 235 



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 mois- 

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

 see how much more readily it mixes with most kind of foods 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 secretion 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. The chemical action which the saliva exerts upon the 

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

 into some kind of sugar. This power the saliva owes to one of its con- 

 stituents ptyalin, which is a nitrogenous body of uncertain composition. 

 It is classed among the unorganized ferments, which are substances of 

 uncertain composition capable of producing changes in the composition 

 of other bodies with which they come into contact, without themselves 

 undergoing change of suffering diminution. The conversion of the 

 starch under the influence of the ferment into sugar takes place in sev- 

 eral stages, and in order to understand it, a knowledge of the structure 

 and composition of starch granules is necessary. A starch granule con- 

 sists 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. 

 Briike states that a third body is contained in the granule, which gives 

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

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

 pletely converted into a paste or gruel, which is called gelatinous starch. 



When ptyalin or other amylolytic ferment is added to boiled starch, 

 sugar almost at once makes its appearance in small quantities, but in ad- 

 dition there is another body, intermediate between starch and sugar, 

 called erythro-dextrin, which gives a reddish-brown coloration with 

 iodine. As the sugar increases in amount, the erythro-dextrin disappears, 

 hut its place is taken in part by another dextrin, achroo-dextrin, which 



