250 THE TISSUES AND MECHANISMS OF DIGESTION. 



be allowed, convert into sugar a very large, one might almost say an indef- 

 inite, quantity of starch. Whether the particular constituent on which the 

 activity of saliva depends is at all consumed in its action has not at present 

 been definitely settled. 



On what constituents do the amylolytic virtues of saliva depend ? 



If saliva, filtered and thus freed from much of its mucin and from other 

 formed constituents, be treated with ten or fifteen times its bulk of alcohol, 

 a precipitate is formed containing, besides other substances, all the proteid 

 matters. Upon standing under the alcohol for some time (several days), the 

 proteids thus precipitated become coagulated and insoluble in water. Hence, 

 an aqueous extract of the precipitate, made after this interval, contains very 

 little proteid material ; yet it is exceedingly active. Moreover, by other 

 more elaborate methods there may be obtained from saliva solutions which 

 appear to be almost entirely free from proteids and yet are intensely amylo- 

 lytic. But even these probably contain other bodies beside the really active 

 constituent. Whatever the active substance be in itself, it exists in such 

 extremely small quantities that it has never yet been satisfactorily isolated ; 

 and indeed the only clear evidence we have of its existence is the manifesta- 

 tion of its peculiar powers. 



The salient features of this body, this amylolytic agent, which we may call 

 ptyalin, are then : 1st, its presence in minute and almost inappreciable quan- 

 tity. 2d, the close dependence of its activity on temperature. 3d, its perma- 

 nent 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 manifestations of its power that is to say, the energy 

 necessary for the transformation which it effects does not come out of itself; 

 if it is at 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. 5th, the 

 action which it induces is probably of such a kind (splitting up of a mole- 

 cule with assumption of water) as is effected by that particular class of agents 

 called " hydrolytic." 



These features mark out the amylolytic active body of saliva as belong- 

 ing 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. 



186. 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 circum- 

 stances ; thus, as we shall see in detail hereafter, the saliva from the sub- 

 maxillary 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. 



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

 organised and unorganised. Of the former, yeast may be taken as a well-known example. 

 The fermentative activity of yeast which leads to the conversion of sugar into alcohol, 

 is dependent on the life of the yeast-cell. Unless the yeast-cell be living and functional, 

 fermentation does not take place ; when the yeast-cell dies fermentation ceases ; and no 

 substance obtained from the fluid parts of yeast, by precipitation with alcohol or other- 

 wise, will give rise to alcoholic fermentation. The salivary ferment belongs to the latter 

 class ; it is a substance, not a living organism like yeast. It may be added, however, 

 that possibly the organized ferment, the yeast for instance, produces its effect by means 

 of an ordinary unorganized ferment which it generates, but which is immediately made 

 away with. 



