298 COMPARATIVE PHYSIOLOGY. 



scribed can be easily sbown by holding a solution of starch in 

 the mouth for a few seconds, and then testing it for sugar, 

 when it will be invariably found. 



While salivary digestion is not impossible in a neutral me- 

 dium, it is arrested in an acid one even of no great strength 

 (less than one per cent), and goes on best in a feebly alkaline 

 medium, which is the condition normally in the mouth. Though 

 a temperature about equal to that of the body is best adapted 

 for salivary digestion, it will proceed, we have ourselves found 

 at a higher temperature than digestion by any other of the 

 juices, so far as man is concerned — a fact to be connected, in all 

 probability, with his habit for ages of taking very warm fluids 

 into the mouth. 



The active principle of saliva is ptyalin, a nitrogenous body 

 which is assumed to exist, for it has never been perfectly iso- 

 lated. It belongs to the class of unorganized ferments, the 

 properties of which have been already referred to before (page 

 163). 



Characteristics of the Secretion of the Different Glands,— 

 Parotid saliva is in man not a viscid iluid, but clear and limpid, 

 containing very little mucin. Submaxillary saliva in most 

 animals and in man is viscid, while the secretion of the sub- 

 lingual gland is still more viscid. 



Comparative. — Saliva difiPers greatly in activity in different 

 animals ; thus saliva in the dog is almost inert, that of the 

 parotid gland quite so ; in the cat it is but little more effective ; 

 and in the horse, ox, and sheep, it is known to be of very feeble 

 digestive power. 



In man, the Guinea-pig, the rat, the hog, both parotid and 

 submaxillary saliva are active ; while iu the rabbit the sub- 

 maxillary saliva, the reverse of the preceding, is almost in- 

 active, and the parotid secretion very powerful. 



An aqueous or glycerin extract of the salivary glands has 

 digestive properties. The secretion of the different glands may 

 be collected by passing tubes or cannulas into their ducts. 



The saliva, normally neutral or only faintly acid, may be- 

 come very much so in the intervals of digestion. The rapid 

 decay of the teeth occurring during and after certain diseases 

 seems in certain cases to be referable in part to an abnormal 

 condition of the saliva. 



The tartar which collects on the teeth consists largely of 

 earthy phosphates. 



