242 ELEMENTARY PHYSIOLOGY 



increased secretion takes place. No clearer proof could 

 be desired to show that when the gland secretes it is 

 because the impulses tohich reach it along the nerve exert a 

 direct influence on its cells. These impulses make the 

 cells take up water and discharge it, together with some 

 of their stored up cell-subst«ince, as saliva which passes 

 as the secretion of the gland into the ducts. It is thus 

 not the increased blood-supply which causes the secretion, 

 although of course it is necessary if the cells are to con- 

 tinue to secrete, for it is from tlie blood alone that they 

 can obtain all that they require for the manufacture of 

 their secretion. 



Saliva is ordinarily secreted in increased quantity as 

 soon as food is introduced into the mouth or even smelt. 

 This result is brought about refle.vly. The food stimulates 

 the ends of certain nerves (Vth and IXth cranial, see 

 Les.S()ri XI.), wliich supply the inside of tlie walls of the 

 mouth or the Olfactory nerve which supplies the nose. 

 Impulses pass up these nerves to the brain, and from this 

 other impulses pass out down to the submaxillary gland 

 and make its cells secrete. 



7. The Composition and Action of Saliva.— The 

 mixed saliva from the several glands consists chiefly of 

 water, holding in solution a small amount of proteid 

 matter, some inorganic salts to which its faintly alkaline 

 reaction is due, a small amount of mucin, which gives to 

 saliva its well-known sliminess, and a small quantity of a 

 peculiar substance called ptyalin, which has certain very 

 remarkable properties. It does not act on proteids or 

 fats, but if a little saliva, i.e. ptyalin, be mixed with 

 ordinary starch-paste and warmed to the temperature of 

 the body, it turns that starch into a sugar identical with 

 that obtained from malt in brewing and hence known as 

 maltose. 



But although this chemical change is without doubt of 

 some use to the body, its imj)ortance must not be over- 

 estimated. For in many animals the action of their saliva 

 on starch is very slight, and moreover (see p. 264) the larger 



