DIGESTION. 231 



Saliva from the parotid is less viscid, less alkaline, clearer, and more 

 watery than that from the submaxillary. It has, moreover, a less power- 

 ful action on starch. Sublingual saliva is the most viscid, and contains 

 more solids k than either of the other two, but does not appear to be so 

 powerful in its action. 



The salivary glands of children do not become functionally active till 

 the age of 4 to 6 months, and hence the bad effect of feeding them before 

 this age on starchy food, corn-flour, etc., which they are unable to render 

 soluble and capable of absorption. 



Action of Saliva on Starch. This action is due to the presence 

 in the saliva^ of the body called ptyalin. It is a nitrogenous body, and 

 belongs to the order of ferments, which are bodies whose exact chemical 

 composition is unknown, and which are capable of producing by their 

 presence changes in other bodies, without themselves undergoing change. 

 Ptyalin is called a liydrolytic ferment, that is to say, it acts by adding a 

 molecule of water to the body changed. The reaction is supposed to be 

 as follows: 



3 C.H 10 6 + 3 H,0 = C,H,,0 6 + 2 (C.H, S ) + 2 H 5 = 3 O.H,,0, 



Starch + Water. Glucose Dextrin Glucose 



But it is not unlikely that the action is by no means so simple. In 

 the first place, recent observers believe that a molecule of starch must be 

 represented by a much more complex formula; next, that the stages in 

 the reaction are more numerous and extensive; and thirdly, that the pro- 

 duct of the reaction is not true glucose, but maltose. Maltose is a sugar 

 more akin to cane than grape sugar, of very little sweetening power, and 

 with less reducing power over copper salts. Its formula is C 12 H 22 O n . 



The action of saliva on starch is facilitated by: (a) Moderate heat, 

 about 100 F. (37'8 C.). (b) A slightly alkaline medium, (c) Removal 

 of the changed material from time to time. Its action is retarded by: (a) 

 Cold; a temperature of 32 F. (0 C.) stops it for a time, but does not 

 destroy it, whereas a high temperature above 140 F. (60 C.) destroys 

 it. (b) Acids or strong alkalies either delay or stop the action altogether. 

 (c) Presence of too much of the changed material. Ptyalin, in that it 

 converts starch into sugar, is an amylolytic ferment. 



Starch appears to be the only principle of food upon which saliva acts 

 chemically: it has no apparent influence on any of the other ternary prin- 

 ciples, such as sugar, gum, cellulose, or on fat, and seems to be equally 

 destitute of power over albuminous and gelatinous substances. 



Influence of the Nervous System. The secretion of saliva is 

 under the control of the nervous system. It is a reflex action, and in 

 ordinary conditions is excited by the stimulation of the peripheral 

 branches of two nerves, viz., the gustatory or lingual branch of the in- 



