

THE TOTAL EXCHANGES OF THE BODY 661 



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 in the form of biscuits or of preserve a complete food which is prepared 

 in large quantities at the beginning of the experiment and used as the sole diet through- 

 out the experiment. Pfliiger, for instance, converted 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 but. 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 foodstuffs, 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 foodstuffs 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 

 intestine 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 foodstuffs, 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 

 rise to as much as 4 per cent, of the total nitrogenous output of the body. 

 Although a trace of ammonia has been described as occurring in the expired 

 air, the amount is so minute that any loss of nitrogen by the lungs can be 

 neglected. That the loss both by lungs and skin under ordinary circum- 

 stances can be disregarded is shown by the fact that it is possible to account 

 directly for the whole nitrogen of the body by a comparison of the compo- 

 sition of the food with that of the urine and faeces. If, for instance, an animal 

 is kept on a sufficient diet which contains a perfectly regular amount of 



