THE TOTAL EXCHANGES OF THE BODY 615 



composition published by the Agricultural Board of the United States. 

 Since, however, the foods vary in composition, especially in water content, 

 from time to time, a calculation of the total income of proteins, fats, and 

 carbohydrates from data given by workers in other lands must present a 

 considerable margin of error. In order to attain greater accuracy some 

 observers have made a complete food in the form of biscuits or of preserve 

 which is prepared in large quantities at the beginning of the experiment and 

 used as the sole food throughout the experiment. Pniiger, for instance, con- 

 verted the horse-flesh, with which he desired to feed his dogs in a metabolism 

 experiment, into sausage meat which was sealed up in cases and sterilised. 

 The sausage meat having been analysed at the beginning of the experiment, 

 it was only necessary thereafter to weigh the amount eaten by the dog in 

 order to know accurately the total amount of protein, fat, and carbohydrate 

 ingested by the animal. In experiments on man it has been endeavoured to 

 obtain the same result by limiting the food to a few articles of diet which could 

 be accurately analysed in each case. The monotony of such a diet tends 

 to interfere with the success of the experiment, since the subject of the 

 experiment loses his appetite and his processes of nutrition are not normally 

 carried out. It is usually possible to steer a middle course between the two 

 extremes of too much and too little variation of diet, and so to obtain values 

 for the composition of the ingesta which cannot differ very largely from 

 their true composition. 



The material output of the body consists of the products of combustion 

 of the food-stuffs, which are turned out by the various channels of excretion, 

 namely, the kidneys, the alimentary canal, the lungs, and the skin. These 

 excreta must therefore be collected and analysed. In addition to the main 

 sources of excretion, small quantities of material are lost by the shedding 

 of the cuticle, by the growth and cutting of the hair and nails, and so on. In 

 most cases the losses in this way are so small that they may be disregarded. 

 The nitrogen of the food-stuffs and that derived from the disintegration of 

 the tissues of the body is excreted almost exclusively in the urine, a small 

 amount being thrown out by the alimentary canal. The total nitrogen must 

 be therefore determined both in the faeces and in the urine. The nitrogen in 

 the faeces is derived from two sources. Part represents those nitrogenous 

 constituents of the tissues which have resisted the digestive processes of the 

 alimentary canal. There is in addition a certain amount derived from the in- 

 testine itself. During complete starvation faecal masses are formed in the 

 intestine, and it has been calculated that in a normal individual about one 

 gramme of nitrogen a day is excreted by the mucous membrane of the gut and 

 contributes to the formation of the faeces. It is usual therefore to regard 

 one gramme of the nitrogen of the faeces as belonging to the output of the 

 body and representing the result of nitrogenous metabolism, while the 

 balance is taken as belonging to undigested food-stuffs, and is subtracted 

 from the total nitrogen of the latter in reckoning the real income of the body. 

 A small amount of nitrogen is also lost by sweat, but this can be disregarded 

 unless the sweating is profuse, when the loss of nitrogen by this channel may 



