398 NUTRITION. 



only by something similar or identical. This restitution must be 

 regulated according to the expenditure. Although these proposi- 

 tions are almost too self-evident to be advanced, yet it required the 

 most extensive, laborious, and exact investigations of the most 

 intelligent inquirers to arrive at the result which now seems to us 

 to be so simple and so easy of attainment, namely, that the re- 

 quirements of the animal organism in the accomplishment of the 

 vital functions, must correspond with those four groups of sub- 

 stances which, as we have already delineated in the above sketch of 

 the motions of animal matter, constitute their essential and funda- 

 mental elements. But this need scarcely excite our wonder, since 

 it has required all the researches and investigations embraced 

 within the entire compass of this work to effect this object. Is any 

 one link of this long chain of so slight an importance that it does 

 not stand in the closest relation to the general process of vitality ? 

 It is only by means of all these introductory propositions that we 

 are able to comprehend how those substances, which we have learnt 

 to know as the most important adjuncts in the metamorphosis of 

 animal matter, must also be contained in those matters which the 

 animal body takes up ,for the renovation and restoration of that 

 which has been lost or rendered effete (whether it be a fluid or a 

 tissue, a ponderable or an imponderable matter), and for the regu- 

 lar accomplishment of the vital phenomena. 



There could scarcely be a doubt that the albuminous matters, 

 the fats, the carbo-hydrates and salts, which we have learnt to 

 know as the bases of the metamorphosis of animal matter, must at 

 the same time serve as the nutrient matters of the animal body ; yet 

 the opinion that one of these groups was more important than 

 another to the vital process has led to many errors in reference to 

 the physiological value of the nutrient matters. It was long before it 

 could be conclusively shown by experiment, that the value of any of 

 those bodies could only be expressed by the combination of these 

 four categories. How far we were removed, even a short time 

 since, from comprehending this simple truth, is sufficiently proved 

 by the various experiments prosecuted under the direction of differ- 

 ent learned societies, in which animals were fed exclusively on 

 nitrogenous matters, as for instance, albumen or animal jelly ; and 

 the result of these observations excited general surprise, when it 

 was found that in those experiments, which had been conducted 

 with the greatest exactness, the animals perished under symptoms 

 of inanition, and exactly as if they had been deprived of all food. 



