DIGESTION 303 



(3) The functions of the saliva are twofold : — 



(1) Mechanical. — Saliva moistens the mouth and gullet, 

 and thus assists in chewing and swallowing. Since 

 the salivary glands are absent from aquatic mammals, 

 and since in carnivorous animals saliva has no chemical 

 action, it would appear that this is the important 

 function. 



(2) Chemical. — Under the action of the ptyalin of the 

 saliva, starches are broken down into sugar. The starch is 

 first changed into the dextrins, first into erythrodextrins and 

 then into achroodextriiis, and lastly into the disaccharid 

 maltose (see p. 286), {Chemical Physiology). Like other 

 enzyme actions, tlie process requires the presence of water 

 and a suitable temperature, and it is stopped by the presence 

 of stronoc acids or alkalies, bv various chemical substances, 

 and by a temperature of over 60° C, while it is temporarily 

 inhibited by reducing the temperature to near the freezing 

 point. During the short time the saliva acts on the food in 

 the mouth, the conversion is by no means complete. 



2. Physiology of Salivary Secretion. — (1) The changes 

 which the secretinof cells undergo during the so-called resting^ 

 state of the gland and during secretion have been already 

 considered (p. 35). 



(2) The nervous nnechanism of secretion. — In order to 

 study the influence of different factors upon salivary secretion, 

 a cannula may be inserted into the duct of one of the glands, 

 and the rate of flow of saliva or the pressure of secretion may 

 be thus measured. In this way, it may be shown that in 

 ihe dog the taking of food, or simply the act of chewing, 

 or, in some cases, the mere sight of food, causes a flow of 

 saliva. This shows that the process of secretion is presided 

 over by the central nervous system, a fact which is further 

 illustrated in man by the decrease in the secretion of saliva 

 which accompanies some emotional conditions. 



The submaxillary and sublingual glands are supplied — 

 (1) By branches from the lingual division of the fifth cranial 

 nerve ; and (2) by branches of the perivascular sympathetic 

 fibres coming from the superior cervical ganglion. The 



