EXCHANGE OF MATERIAL. 437 



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

 or tissue-constituents MUCUS, DIGESTIVE FLUIDS, SEBUM, SWEAT, 

 and EPIDERMIS FORMATIONS which are either poured into the 

 intestinal tract, or secreted from the surface of the body, or broken 

 off and thereby lost for the body, also belong to the constant excreta. 

 The remains of food, sometimes indigestible, sometimes digest- 

 ible but not acted upon, contained in the faeces, which vary 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 experiments on the exchange of material. 



The determination of the constant loss is in some cases accom- 

 panied 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 mentionable loss because of their small 

 quantity they need not be considered in quantitative experiments 

 on the exchange of material. This also applies to the constituents 

 of the mucus, bile, pancreatic and intestinal juices, etc., occurring 

 in the contents of the intestines and which, leaving the body with 

 the faeces, cannot be separated from, the other contents of the 

 intestines and therefore cannot be quantitatively determined sepa- 

 rately. The uncertainty which, because of the intimated difficulties, 

 attaches itself to the results of the experiments is very small as 

 compared to the variation which is caused by different individuali- 

 ties, different modes of living, different foods, etc. No general but 

 only approximate value can therefore be given for the constant 

 excretions of the human body. 



The following figures represent the quantity of excreta for 

 24 hours, with a mixed diet, of a grown man weighing 60-70 kilos. 

 The numbers are compiled from the results of different investi- 

 gators. Grammes. 



Water 2500-3500 



Salts (with the urine) 20-30 



Carbon dioxide 750-900 



Urea 20-40 



Other nitrogenized urinary constituents 2-5 



Solids in the excrements . . 30-50 



