318 THE AMYLOLYTIC FERMENT. [Book ii. 



The salient features of this body, this amylolytic agent, which 

 we may call ptyalin, are then : — 1st, its presence in minute and 

 almost inappreciable quantity. 2nd, the close dependence of its 

 activity on temperature. 3rd, its permanent and total destruction 

 by a high temperature and by various chemical reagents. 4th, the 

 want of any clear proof that it itself undergoes any change during 

 the manifestation of its powers ; that is to say, the energy neces- 

 sary for the transformation which it effects does not come out of 

 itself ; if it is all used up in its action, the loss is rather that 

 of simple wear and tear of a machine than that of a substance 

 expended to do work, f.th, the action which it induces is probably 

 of such a kind (splitting up of a molecule with assumption of 

 water) as is affected by that particular class of agents called 

 " hydrolytic." 



These features mark out the amylolytic active body of saliva 

 as belonging to the class of ferments; 1 and we may henceforward 

 speak of the amylolytic ferment of saliva. The fibrin-ferment 

 (§ 20) is so called because its action in many ways resembles that 

 of the ferment of which we are now speaking. 



§ 178. Mixed saliva, whose properties we have just discussed, 

 is the result of the mingling in various proportions of saliva from 

 the parotid, submaxillary, and sublingual glands with the secretion 

 from the buccal glands. These constituent juices have their own 

 special characters, and these are not the same in all animals. 

 Moreover in the same individual the secretion differs in composition 

 and properties according to circumstances ; thus, as we shall see in 

 detail hereafter, the saliva from the submaxillary gland secreted 

 under the influence of the chorda tympani nerve is different from 

 that which is obtained from the same gland by stimulating the 

 sympathetic nerve. 



In man pure parotid saliva may easily be obtained by introducing a 

 fine cannula into the opening of the Stenonian duct, and submaxillary 

 saliva, or rather a mixture of submaxillary and sublingual saliva, by 

 similar catheterisation of the Whartonian duct. In animals" the duct 

 may be dissected out and a cannula introduced. 



Parotid saliva in man is clear and limpid, not viscid ; the reaction 

 of the first drops secreted is often acid, the succeeding portions, 



1 Ferments may, for the present at least, be divided into two classes, commonly 

 called orqnmsfd and unorganised. Of the former, yeast may be taken as a well- 

 known example The fermentative activity of veast which leads to the conversion 

 of sugar into alcohol, is dependent on the life of the yeast-cell. Unless the yeast- 

 cell be livinsr and functional, fermentation does not take place; when the yeast- 

 cell dies fermentation ceases ; and no substance obtained from the fluid parts of 

 yenst, by precipitation with alcohol or otherwise, will give rise to alcoholic fermen- 

 tation. The salivary ferment belongs to the laltor class ; it is a substance, not a 

 livinsr organism like yeast. It mav be added however that nossibly the organised 

 ferment, the yeast for instance, produces its effect bv means of an ordinary 

 unorganised ferment which it generates, but which is immediately made away with. 



