THE TOTAL EXCHANGES OF THE BODY 687 



means difficult, the total analysis of the food during a metabolism 

 experiment may become extremely tedious on account of the very 

 large number of analyses which have to be performed. The labour 

 is lightened by the fact that nearly all the ordinary food-stuffs have 

 been subjected to analysis and their average 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. Pfliiger, for instance, converted 

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

 bolism 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 com- 

 bustion 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 dis- 

 integration 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 



