DIGESTION. 139 



digestion. It appears from these experiments, in general terms, that 

 each digestive fluid has not only an action of its own, but that each 

 one of its ingredients contributes in a special way to the digestive 

 process. 



Beside the use of artificial fistula? there is still another method for 

 the experimental study of the digestive fluids. It is based on the fact 

 that the principal organic ingredient of a secretion is in most instanct > 

 produced in the solid substance of the gland, and may be extracted 

 by proper solvents from its tissue. To the solution thus obtained, 

 the needed accessory ingredients, such as saline matters, or dilute acids 

 or alkalies, are added, and an artificial digestive fluid thus produced, 

 similar in most respects to the natural one. It is then subjected to 

 examination in regard to its influence on alimentary substances. This 

 method has received a wide extension of late years with the use of 

 glycerine as a convenient menstruum for the extraction of glandular 

 products. It has been the source of much important information, but 

 its results need to be verified, in every instance, by examination of the 

 normal secretion in the living animal. 



The digestive fluids and their mode of action are especially charac- 

 terized by the presence of ferments. In every instance where their 

 digestive function is plainly evident, its dependence on the activity of 

 a ferment is equally unmistakable. In experimental digestions, with 

 either the normal secretion, or artificial extracts, all the conditions of 

 moisture, temperature, degree of concentration, and the like, requisite 

 for the operation of organic ferments, must be maintained ; and when 

 such an experiment is smve>sfully carried out, the quantity of ali- 

 mentary material digested is far greater than that of the organic 

 ingredient which produces the effect. 



The nature of the change caused by digestion in the alimentary 

 substances is partly physical and partly chemical. But although this 

 change is indispensable for the absorption of these substances in due 

 quantity, it does not consist in any profound alteration of their chemi- 

 cal characters. The alimentary materials are not decomposed, nor 

 converted into substances of a different kind. They are simply trans- 

 formed into soluble materials of the same class with themselves. The 

 carbohydrates after digestion remain carbohydrates, the albnmenoid 

 matters are still albumenoids. and the fatty substances retain the chem- 

 ical properties of the fats. The transformation of starch into glucose 

 by the digestive process is an act of hydration, which may be 

 accomplished by continued boiling with water and a mineral acid 

 outside the body. Albuminous matters, in digestion, are converted 

 into peptones. This change is also regarded as a hydration, and it 

 has further been shown that albumen may be made to undergo a sim- 

 ilar transformation by long boiling in acidulated water, or by boiling 

 at a high temperature under pressure. Thus the animal ferments, in 

 the alimentary canal, act by inducing rapidly, at the temperature of 

 the body, changes which would otherwise require a longer time or more 



