664 IMPORTANCE OF DIGESTIBILITY. [Book n. 



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 corre- 

 sponding 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, § 234, 

 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 pos- 

 sible 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 ali- 

 mentary 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. 



§ 443. 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, otherwise the same, the proteids are partly of animal 

 origin also. Nor have we much better knowledge of the rela- 

 tive nutritive value of vegetable and animal fats. And as we 

 have already said, we possess little or no exact knowledge as 

 to the part played by those extractives in respect to the amount 

 and nature of which animal food strikingly differs from vege- 

 table food. In attempting therefore a judgment from a purely 

 physiological point of view as to the value of an exclusively 

 vegetarian diet compared with a diet of both animal and vege- 

 table origin, we can do little more at present than inquire 

 whether the former supplies the several food-stuffs in adequate 

 quantity, in proper proportion, and in such a form as to be 

 economically utilized by the body. 



The careful examination during three separate periods of 

 several days each of the ingesta and egesta of a man, 28 years 

 old, weighing 57 kilos, who had for three years lived on an 

 exclusively vegetable diet, viz. bread, fruit and oil, gave the 

 following results. 



