322 DIGESTION. 



of a practical character, and accurately to determine the digesti- 

 bility of every article of food, or at all events to furnish sharply- 

 defined rules for its estimation ; and some have even gone so far as 

 to expect that this branch of our science should serve in every 

 respect as a guide for a system of dietetics. But even if our 

 physiological premises were sufficient for this purpose, and if 

 our positive facts were less deficient than they are, we should 

 consider a text-book of physiological chemistry as an inappropriate 

 place for a comprehensive exposition of the relations involved in 

 this department of science : purely scientific inquiry is bounded 

 by definite limits ; the application to practical life of the facts 

 discovered by science must be left to the kindred but less 

 strictly scientific branches of knowledge ; all that is required for 

 practical application must be supplied by the methods peculiar to 

 the so-called practical sciences. Thus, for instance, there exists a 

 large amount of material in reference to the digestibility of the 

 different nutrient matters, which appertains exclusively to a 

 medical inquiry. For we do not concur with those who denounce 

 as unfounded every fact which has not been obtained by the exact 

 methods of physical science, and who consequently often arbi- 

 trarily cast aside many striking hypotheses which may be ad- 

 vanced by physicians in reference to dietetic conditions ; far from 

 participating in such views, we do justice to the fruits which 

 practical experience is able to furnish, and we are fully aware that 

 a physician may do a great deal by the bedside towards the intro- 

 duction of correct dietetics, without our being able to refer his 

 mode of practice to scientific grounds. Thus, for instance, we know 

 that a number of substances, which present considerable analogy 

 to one another in a physical and chemical point of view, exhibit 

 great differences in reference to their digestibility under certain 

 morbid conditions ; there are a number of still more mysterious 

 phenomena known to every attentive physician, which, although 

 they owe their recognition to no exact scientific method, are yet 

 as firmly established as if they were mathematical propositions. 

 Yet the solution of these mysteries the formation of compre- 

 hensive scales for the calculation of the digestibility of different 

 articles of food in accordance with their chemical nature or 

 preparation and the determination of the causes by which a 

 substance which is in itself easy of digestion may become less 

 digestible under different external or internal relations (for the 

 simple digestibility of a substance must be distinguished from the 

 facility' with which a patient is able to digest it) these are all 



