CHAP, v.] . NUTRITION. 839 



The above refers to what may be called rough digestibility, but 

 besides this there are other circumstances to be considered. The 

 same food-stuff in two articles of food, though actually digested, 

 that is to say taken up by the alimentary canal, may, even while 

 still within the alimentary canal, undergo changes in the one case 

 differing from those in the other. A proteid may for instance in one 

 case tend to be entirely converted into peptone, or to break up into 

 leucin &c., or in other cases to undergo other changes ; and a 

 carbohydrate may in one case be absorbed as maltose, and in 

 another give rise to lactic acid. Indeed, when we speak of the 

 digestibility or the indigestibility of this or that article of food, we 

 do not in many cases so much mean the relative amount of the 

 substance taken up in some way or other by the alimentary canal, 

 as the characters advantageous or otherwise of the changes which 

 it undergoes in being so taken up. 



Hence the purely chemical statement of the amount of poten- 

 tial energy present in an article of food is no safe guide of the 

 physiological value of the substance. A chunk of cheese stands 

 very high on, generally at the top of, a table of the nutritive value 

 of articles of food drawn up on exclusively chemical principles, 

 according to the units of energy present in a unit of the material ; 

 but it is very low down in a corresponding physiological table. 

 And similarly a dish of old peas has a very different physiological 

 function from a plate of fresh meat even when both contain the 

 same amount of nitrogen. 



In thus correcting for digestion the nutritive value of a diet it 

 must also be borne in mind that the alimentary canal, while 

 chiefly a receptive organ, is also to some extent, 284, an excretory 

 organ : a free passage through the canal is needed not only for 

 carrying off undigested matter but also for getting rid of excreted 

 matter; and the presence of the former, up to certain limits, 

 assists the discharge of the latter. Were it possible to prepare a 

 diet every jot and tittle of which could be digested and absorbed, 

 the use of such a diet would probably bring about disorder in the 

 economy, through the absence of a sufficiently rapid discharge of 

 the matters excreted into the alimentary canal. Hence cellulose 

 and like substances even when unutilized through absorption, are 

 not without their use, and experience shews that digestion may 

 be promoted by eating undigestible things. 



553. The several food-stuffs of a diet may be drawn from 

 the animal or from the vegetable kingdom. Vegetable proteids 

 appear to undergo the same changes in the alimentary canal as do 

 animal proteids, and the main effects on the body of proteids from 

 the two sources seem to be the same. Our knowledge at present 

 however is too imperfect to enable us to decide whether the 

 functions of the two are exactly the same, whether the body 

 behaves exactly the same upon a diet in which the proteids 

 are exclusively of vegetable origin, as upon a diet in which, 



