THE METABOLISM OP THE BODY. 459 



are facts wliich seem to indicate that the uric-acid metabolism 

 is the older, from an evolutionary point of vie^, and that in 

 mammals, and especially in man, as the results of certain errors 

 there may be a physiological (or pathological) reversion. Hip- 

 puric acid, as replacing uric acid in the herbivora, may be re- 

 garded in a similar light. 



Our knowledge of the metabolism of the spleen, beyond its 

 relations to the formation of blood-cells and their disintegra- 

 tion, is in the suggestive rather than the positive stage. It 

 seems highly probable that this organ plays a very important 

 part, the exact nature of which is as yet unknown. 



When an animal starves, it may be considered as feeding on 

 its own tissues, the more active and important utilizing the 

 others. Notwithstanding, organs with a very active metabo- 

 lism, as the muscles and glands, lose weight to a large extent. 

 The presence of urea to an amount not very greatly below the 

 average in health, shows that there is an active proteid metabo- 

 lism then as at all times in progress. 



General experience and exact experiments prove that, while 

 an animal's diet may be supplied with special regard to fatten- 

 ing, to increase working power, or simply to maintain it in 

 health, as evidenced by breeding capacity, form, etc., in all 

 cases there must be at least a certain minimum quantity of each 

 of the food-stuffs. No one food can be said to be exclusively 

 fattening, heat-forming, or muscle-forming. 



A carbohydrate diet tends to production of fat ; proteid food 

 to supply muscular energy, but the latter also produces fat, and 

 a diet of proteid mixed with fat or gelatin will serve the pur- 

 poses of the economy better than one containing a very much 

 larger quantity of proteid alone. Muscular energy, as is to be 

 inferred from the excreta, is not the result of nitrogenous me- 

 tabolism alone ; and in arranging any diet for man or beast the 

 race and the individual must be considered. Animals can not 

 be treated as machines, like engines using similar quantities of 

 fuel ; though this holds far more of man than the lower ani- 

 mals — i.e., the results may be predicted from the diet with far 

 more certainty in their case than for man. 



Food is related to excreta in a definite way, so that all that 

 enters as food must sooner or later appear as urea, salts, car- 

 bonic anhydride, water, etc. These are individually to be re- 

 garded as the final links in a long chain of metabolic processes, 

 or rather a series of these. Fats and carbohydrates are repre- 



