CHAP.I.J TISSUES AND MECHANISMS OF DIGESTION. 317 



temperature of the atmosphere. The lower the temperature the 

 slower the change, and at about C. the conversion is indefinitely 

 prolonged. After exposure to this cold for even a considerable 

 time the action recommences when the temperature is again 

 raised. Increase of temperature up to about 35 40, or even a 

 little higher, favours the change, the greatest activity being said 

 to be manifested at about 40. Much beyond this point, however, 

 increase of temperature becomes injurious, markedly so at 60 or 

 70 ; and saliva which has been boiled for a few minutes not only 

 has no action on starch while at that temperature, but does not 

 regain its powers on cooling. By being boiled, the amylolytic 

 activity of saliva is permanently destroyed. 



The action of saliva on starch is most rapid when the reaction 

 of the mixture is neutral or nearly so ; it is hindered or arrested 

 by a distinctly acid reaction. Indeed the presence of even a very 

 small quantity of free acid, at all events of hydrochloric acid, at 

 the temperature of the body not only suspends the action but 

 speedily leads to permanent abolition of the activity of the juice. 

 The bearing of this will be seen later on. 



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, by 

 dialysis for example, 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 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 amylolytic. But even these probably 

 contain other bodies besides the really active constituent. What- 

 ever 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 

 manifestation of its peculiar powers. 



