PHYSIOLOGICAL 285 



should be able to interpret. Thus the relative weight is less in 

 marine fishes than in freshwater forms, and carnivorous mammals 

 generally have a larger spleen than the vegetarians. But this must 

 not be used as an argument either for or against humanitarian diet I 



THE BIOLOGY OF EXCRETION— A living creature is assuredly 

 more than an engine, for it exhibits activities which cannot be 

 adequately described in terms of mechanics and dynamics. Organ- 

 ism transcends mechanism. And this conclusion stands firmly on 

 legs of its own, apart from the obvious fact that in higher animals 

 there is a mind that counts for much, even in everyday affairs. 

 None the less it is very useful to think of the living body as a 

 number of interlinked engines that work into each other's hands, 

 so to speak; for a living body is engine-Hke in being a material 

 system adapted for the economical transformation of matter and 

 energy. Let us pursue the analogy in regard to the waste-products 

 that are formed by engine and by body alike. 



A hard-worked engine soon accumulates some unused or un- 

 usable remains of the fuel, and these have to be reckoned with. But 

 there is also a subtler kind of waste that is due to the wear and 

 tear of the active parts. This is not so readily dealt with, and here 

 the living body is far ahead of any engine in having remarkable 

 powers of self-repair. In the living body there is often much quite 

 unused fuel or food, and there are also the results of the chemical 

 disruption of this fuel. Thus carbon dioxide is the familiar gaseous 

 waste-product that results from the ceaseless combustion or oxida- 

 tion of the nutritive carbon compounds. Everyone knows how it 

 is got rid of on the internal Surface of the lungs in higher animals, 

 and in other ways in lower animals, for instance, by gills or by 

 the skin. 



But if we keep by itself the getting rid of the poisonous carbonic 

 acid gas, and also the getting rid of undigested or undigestible food 

 in the alimentary canal, we may narrow our inquiry to the nitro- 

 genous waste-products which have to be filtered out of the animal, 

 or otherwise summarily dealt with, if life is to continue smoothly. 

 The nitrogenous waste is partly due to the wear and tear of the 

 protein framework of the cells, and might be compared to the 

 minute particles of steel found in the lubricating oil of the engine. 

 But it is also, and in great part, due to a surplus of unused, 

 though digested, nitrogenous food which has passed into the blood. 

 This part of the nitrogenous waste might be compared to the 

 products formed in an oil-engine when the combustion is not 

 thorough enough, or when the proportions of the factors in the 

 combustion are not well adjusted. To the filtering out of the nitro- 

 genous waste-products of twofold origin, physiologists usually 

 restrict the term excretion. 



