EXCRETA OF THE ORGANISM. 879 



Such knowledge can be attained only by a series of systematic and 

 thorough observations, in which the quantity of nutritive material, rela- 

 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 PETTENKOFER and VOIT, and by VOIT and his pupils, by 

 RUBNER, ZUNTZ 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 

 composition 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 may, 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 

 ejection 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, 

 perspiration, 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 constitutents of the animal fluids or tissues, cannot be con- 

 sidered 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 dif- 

 ficulty, 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 



