80 



ANIMAL PHYSIOLOGY. 



Iii fact, it may be said, that in a diet sufficient in quantity, 

 if the nitrogenous constituents be too plentiful, unnecessary 

 work is thrown on the kidneys, while, if they be too scanty, an 

 unnecessary load is thrown on the intestines. Moreover, it 

 has been found by experiment that no amount of gelatinoid 

 substance will suit instead of the albuminoids; animals can 

 be supported on lean meat, but they die when fed on jelly 

 alone, even when it is pleasantly flavoured, and at first 

 relished. It appears, therefore, that animals have no power 

 of building up albuminoid matter from simpler chemical sub- 

 stances. They have no power of manufacturing organic 

 matter from the materials found in inorganic nature, but 

 feed either directly on the vegetable world, or on other 

 animals which have fed on vegetables; and there is no proof 

 that in any instance they have the power of acting on the 

 simpler organic substances, so as to produce from them the 

 more complex. Further, it appears from the researches of 

 botanists, that even in plants the power of building organic 

 matter is confined to the green parts. The statement may 

 therefore be ventured on, that so far as observation has yet 

 proceeded, it would appear that the presence of chlorophyll is 

 as necessary for the production of organic matter in organisms, 

 as the presence of protoplasm is necessary for growth. 



With regard to gelatin, the question is often asked how it 

 happens, if it be incapable of sustaining life, that in conjunc- 

 tion with other things, it is useful as an article of diet, and a 

 favourite in the sick room. Perhaps that question is suffi- 

 ciently answered by pointing out that carbonaceous substances 

 are likewise insufficient by themselves to support life, and 

 that in the formation of urea from gelatin, five-sixths of the 

 carbon is unused, and therefore combines with oxygen to 

 form carbonic acid, as does the carbon of carbonaceous food; 

 also, that gelatin requires little digestion, and is at once de- 

 composed on entering the circulation. 



All carbonaceous food serves the economy sooner or later 

 by undergoing oxidation into carbonic acid and water, and 

 thereby supporting the temperature of the body, or assisting 

 its vital processes, as we have seen that it does in muscular 

 action. Oil may be temporarily stored up in the shape of 

 adipose tissue; and the carbohydrates may be stored as 



