FOOD AND DIGESTION 345 



calcic and magnesic phosphates, and generally numerous 

 bacteria. Its specific gravity is low generally about 1003. 

 In reaction it is neutral or faintly alkaline. 



Chemically it is found to contain a very small proportion 

 of solids, and the proportion of these varies with the stimulus 

 which causes the secretion. In addition to mucin, traces of 

 proteins are present, and with these proteins in certain 

 animals, but not in the dog, is associated the active constituent 

 or enzyme of the saliva ptyalin. 



Saliva generally contains traces of potassium sulpho-cyanide. 



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, 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 

 when this is present, polysaccharids, like the starches, are 

 broken down into sugars. Like other enzyme actions the pro- 

 cess 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. 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 di- 

 saccharid maltose (see p. 330). (Chemical Physiology). 



Physiology of Salivary Secretion. In order to study the 

 physiology of salivary secretion, a canula may be inserted into 

 the duet of any of the salivary 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 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 glands are supplied (1) By 

 branches from the lingual division of the fifth cranial nerve ; 

 and (2) by branches of the perivascular sympathetic fibres 

 coining from the superior cervical ganglion. The parotid gland 



