THE FOOD AND DIGESTION 329 



slum, and in some pathological conditions this may be 

 markedly increased. 



The functions of the saliva are twofold : 



1. Mechanical, to moisten the mouth and gullet, and thus 

 to assist in speaking, chewing and swallowing. Since the 

 salivary glands are absent from aquatic mammals, it would 

 appear that this is the more important function. 



2. Chemical Under the action of the ptyalin of the 

 saliva, polysaccharids, like the starches, are broken down into 

 sugars. Like other enzyme actions the process requires the 

 presence of water and a suitable temperature, and it is 

 stopped by the presence of strong acids or alkalies, by 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 the conversion is by no means complete. 

 The starch is first changed into the dextrins, giving a brown 

 colour with iodine, and hence called erythrodextrins, then into 

 dextrins which give no colour with iodine, achroodextrins, 

 and lastly into the disaccharid maltose (see p. 317). (Chemical 

 Physiology, p. 18.) 



Physiology of Salivary Secretion. In order to study 

 the physiology of salivary secretion, a canula may be inserted 

 into the duct of any of the salivary glands and the flow of 

 saliva or pressure of secretion may be thus measured. In this 

 way it may be shown that the taking of food, or simply the 

 act of chewing, and 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. 



The submaxillary and sublingual 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 parotid 

 gland is supplied by the auriculo-temporal division of the 

 fifth and also by sympathetic fibres (Fig. 146). 



The influence of these nerves has been chiefly studied on 

 the submaxillary and sublingual glands. 



It has been found that when the lingual nerve is cut the 

 reflex secretion of saliva still takes place, but that, when the 

 chorda tympani (Ch. T.), a branch from the seventh nerve 



