STOMACH AND INTESTINE. 



385 



exsudes directly from this fluid. It is the main 

 component of the muscles, which execute 

 the various movements of the body. It is an 

 equally important ingredient in the tissues 

 of both the central and peripheric parts of the 

 nervous system. It is probably the- source of 

 the gelatinous * tissues ; which, in herbivorous 

 animals, can only be derived from a kind of 

 degradation or regression of the albuminous 

 substances. And, finally, its large amount in 

 the structures of the foetus proves that it is just 

 as important to the evolution and growth of 

 the animal, as it is to its maintenance. In 

 short, in protein and its various kindred sub- 

 stances, we recognize the principle, which 

 forms the material exponent of all the struc- 

 tures and functions, and is the chief sub- 

 stantive agent of the chemistry of life. 



The quantity of protein necessary for the 

 proper maintenance of the healthy animal can 

 only be estimated from very indirect and ap- 

 proximate calculations. 



In milk, the albuminous compounds are 

 chiefly represented by casein, which forms 

 about 3^ per cent, of its total quantity. But 

 we can scarcely guess how much milk is 

 daily consumed by the sucking animal, or 

 what proportion this amount bears to the 

 weight of its whole body. And we are 

 justified in assuming, that a large fraction of 

 the protein thus introduced into the system, 

 is applied to exigencies of growth and deve- 

 lopment which have little or no place in the 

 adult animal. 



Assuming an exact maintenance of the 

 adult organism, without increase or decrease, 

 we might expect that an examination of 

 its various azotized excretions would teach 

 us how much nitrogen had been discharged 

 from the system within a given time : and 

 hence that, by comparing this quantity with 

 the known elementary composition of protein, 

 we might be enabled to calculate how large a 

 quantity of the azotizedf constituent of the 

 food ought to be added to the system, in order 

 to replace its daily loss. 



But here we are met by a difficulty con- 

 nected with the process of nutrition itself: 

 \\ith that chain of events of which food and 

 waste constitute only the extreme links. The 

 amount of nitrogen given oflfby the body does 

 not depend solely upon the quantity excreted 

 by its \vaste, but also varies in close corre- 

 spondence with the quantity taken in its 

 food. It is therefore greater in carnivorous, 

 and less in herbivorous, animals. 



Hence the true or essential waste of the 

 organism, in respect of this constituent, can 

 only be determined from an analysis of the ex- 

 cretions of animals which have been kept for a 

 day or two, either without food, or on a diet 

 altogether devoid of nitrogen. In both cases 

 the results are the same. The nitrogen of 



* Ignorant as we are, both of the nature of this 

 metamorphosis, and of the various stages through 

 which it is conducted, there are reasons for con- 

 jecturing that the formation of the chondrin radicle 

 generally precedes that of the substance which yields 

 gelatin by boiling. 

 Supp. 



the egesta drops to a certain minimum ; at 

 which it remains for a considerable period. 



The quantity of nitrogen evolved by the 

 lungs and skin is at any rate so small, as 

 scarcely to form an important element of cal- 

 culation. And even the larger quantity 

 excreted in the biliary resin, hardly deserves 

 notice. It is in the uric acid, and above all 

 in the urea, of the renal secretion, that this 

 element is chiefly dismissed from the body as 

 an effete compound. And hence it is from 

 the urea found in such experiments that we 

 may best deduce the probable rate of daily 

 waste in the albuminous tissues ; and the cor- 

 responding quantity of protein which there- 

 fore has to be supplied in the daily food. 



From observations of this kind on the 

 human subject, we may infer that, in Man, 

 the albumen of the adult organism under- 

 goes a loss of about 2 ounces daily ; a quan- 

 tity which corresponds to scarcely more than 

 T5 Voth of the weight of the body. While if 

 we suppose that a new-born infant, weighing 

 six or seven pounds, consumes daily about ten 

 or twelve ounces of milk, containing 3* per 

 cent, of casein, the quantity of protein thus 

 introduced into its alimentary canal would 

 amount to the larger proportion of about ^th 

 of its total bodily mass. 



The larger proportion of albumen thus 

 consumed by the infant probably depends 

 upon at least two causes. As a smaller* 

 animal, it is subject to a more energetic waste 

 of substance. And as a growing animal, 

 it not only lays aside in its body a constant 

 surplus of its income over its waste; but 

 possibly undergoes a more active metamor- 

 phosis, that still further increases the propor- 

 tion of its effete materials. 



But, apart from the influence of age or 

 size, there is no donbt that a careful compari- 

 son of the azotized ingesta and egesta would 

 always show a marked disproportion between 

 the two. There are indeed obvious reasons, 

 why the nitrogenous constituent of any 

 suitable food should always greatly exceed 

 that quantity which is required by the strict 

 exigencies of the organism. A part of the 

 casein which is contained in the milk taken 

 by the sucking-child, is often found to pass 

 through the alimentary canal without being 

 absorbed into the blood. And in the case of 

 many other varieties of food, the insoluble 

 state of the protein-compounds actually 

 present affords a still greater obstacle to their 

 absorption. In a proper mixed diet, how- 

 ever, we may detect some approximation be- 

 tween the presumable gain and loss. Thus the 

 daily rations of the British soldier on home 

 service include little more than 5 ounces of 

 albuminous substance ; a quantity which is 

 therefore little more than double the amount 



* From researches by Frerichs, Lehmann, Bidder, 

 Schmidt, Boussingault, Valentin, and others, we 

 may estimate the daily waste of albuminous com- 

 pounds, relatively to the whole body, in the under- 

 mentioned animals, as follows: Rabbit, T ] th; 

 Cat, T } g th; Dog, 3 ^th; Horse, ^th. 



c c 



