EXCRETA OF THE BODY. 823 



tive to the weight of the body, taken and absorbed in a given time is 

 compared with the quantity of final metabolic products which leave the 

 organism at the same time. Researches of this kind have been made by 

 investigators, but above all should be mentioned those made by BISCHOFF 

 and VOIT, by PETTEXKOFER and VOIT, and by VOIT and his pupils, by 

 RUBXER, ZUXTZ and by ATWATER. 



It is absolutely necessary in researches on the exchange of material to 

 be able to collect, analyze, and quantitatively estimate the excreta of 

 the organism, so that they may be compared with the quantity and com- 

 position of the nutritive bodies ingested. In the first place, one must 

 know what the habitual excreta of the body are and in what way these 

 bodies leave the organism. One must also have trustworthy methods 

 for their quantitative estimation. 



The organism ma}', under physiological conditions, be exposed to 

 accidental or periodic losses of valuable material such losses as occur 

 only in certain individuals, or in the same individual only at a certain 

 period; for instance, the secretion of milk, the production of eggs, the ejec- 

 tion of semen or menstrual blood. It is therefore apparent that these losses 

 can be the subject of investigation and estimation only in special cases. 



The regular and constant excreta of the organism are of the very 

 greatest importance in the study of metabolism. To these belong, in the 

 first place, the true final metabolic products carbon dioxide, urea (uric 

 acid, hippuric acid, creatinine, and other urinary constituents), and a part 

 of the water. The remainder of the water, the mineral bodies, and those 

 secretions or tissue constituents mucus, digestive fluids, sebum, perspira- 

 tion, and epidermal formations which are either poured into the intestinal 

 tract, or secreted from the surface of the body, or broken off and thereby 

 lost to the body, also belong to the constant excreta. 



The remains of food, sometimes indigestible, sometimes digestible but not acted 

 upon, which are contained in the feces, and which vary considerably in quantity 

 and composition with the nature of the food, also belong to the excreta of the 

 organism. Even though these remains, which are never absorbed and therefore 

 are never constituents of the animal fluids or tissues, cannot be considered as 

 excreta of the body in a strict sense, still their quantitative estimation is absolutely 

 necessary in certain experiments on the exchange of material. 



The determination of the constant loss is in some cases accompanied with the 

 greatest difficulties. The loss from the detached epidermis, from the secretion of 

 the sebaceous glands, etc., cannot be determined with exactness without difficulty, 

 and therefore as they do not occasion any appreciable loss because of their small 

 quantity they need not be considered in quantitative experiments on metabolism. 

 This also applies to the constituents of the mucus, bile, pancreatic and intestinal 

 juices, etc., occurring in the contents of the intestine, and which, leaving the body 

 with the feces, cannot be separated from the other contents of the intestine and 

 therefore cannot be quantitatively determined separately. The uncertainty which, 

 because of the intimated difficulties, attaches itself to the results of the experiment, 

 is very small as compared to the variation which is caused by different individu- 

 alities, different modes of living, different foods, etc. Only approximate values can 

 therefore be given for the constant excreta of the human body. 



