THE METABOLISM OP THE BODY. 439 



age. It has been estimated that strong adults die when they 

 lose ^ of the body-weight. Well-fed animals lose weight more 

 rapidly at first than afterward. 



Diet. — All experiments and observations tend to show that 

 an animal can not remain in health for any considerable period 

 without having in its food proteids, fats, carbohydrates, and 

 salts ; indeed, sooner or later deprivation of any one of these 

 will result in death. 



Estimates based on many observations have been made of 

 the proportion in which these substances should enter into a 

 normal diet. 



For the herbivora from 1 to 8-9 (some claim 1 to 5i) is the 

 estimated ratio of nitrogenous to non-nitrogenous foods; and 3 

 of the former to 1 of fat. 



One conclusion that is obvious from analysis of foods is that, 

 in order to obtain the amount of proteids needed from certain 

 kinds, enormous quantities must be eaten and digested ; and as 

 there would be in such cases an excess of carbohydrates, fats, 

 etc., unnecessary work is entailed upon the organism in order 

 to dispose of this; so that to feed a working horse entirely on 

 grass, a dog wholly on porridge, or a man on bread would be 

 very unwise. 



FEEDING EXFERIMENTS (Ingeata and Eigesta). 



If all that enters the body in any form be known, and all 

 that leaves it be equally well known, conclusions may be drawn 

 in regard to the metabolism the food has undergone. The pos- 

 sible sources of fallacy will appear as we proceed. 



The ingesta, in the widest sense, include the respired air as 

 well as the food ; though from the latter must be subtracted 

 the wa.ste (undigested) matters that appear in the faeces. The 

 ingesta when analyzed include carbon, hydrogen, oxygen, ni- 

 trogen, sulphur, phosphorus, water, and salts, their source being 

 the atmosphere and the food-stuffs. 



The egesta, the same, and chiefly in the form of carbonic an- 

 hydride, of water from the lungs, skin, klimentary canal, and 

 kidneys, of salts and water from the skin and kidneys, and of 

 nitrogen, chiefly as urea almost wholly from the kidneys. Usu- 

 ally in experimental determinations the total quantity of the 

 nitrogen of the urine is estimated. If free nitrogen plays any 

 part in the metabolic processes it is unknown. 



