CHAP, i.] DIGESTION. 237 



The action of saliva is hampered by the presence in a concen- 

 trated state of the product of its own action, that is, of sugar. If 

 a small quantity of saliva be added to a thick mass of boiled starch, 

 the action will after a while slacken, and eventually come to almost 

 a stand-still long before all the starch has been converted. On 

 diluting the mixture with water, the action will recommence. If 

 the products of action be removed as soon as they are formed, a 

 small quantity of saliva will, if sufficient time be allowed, convert 

 into sugar a very large, one might almost say an indefinite, 

 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 constituent do the amylolytic virtues of saliva depend ? 



If saliva, filtered and thus freed from mucus and other formed 

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

 a precipitate takes place containing besides other substances all 

 the proteid matters. Upon standing under the alcohol for some 

 time (several days, or, better, weeks), 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, and yet 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 amylolytic. But even these probably 

 contain other bodies besides 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 evidence we have of its existence is the manifesta- 

 tion of its peculiar powers. 



The salient features of this body, which we may call ptyalm, 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 necessary for the transformation which 

 it effects does not come oat 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 molecule with assumption of water) as is effected by the agents 

 called catalytic, and by that particular class of catalytic 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. 



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- 



