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CHAPTER VI. 

 EXCKETIOK. 



WE have now followed the ingoing tide of gaseous, liquid' 

 and solid substances within the physiological surface of the 

 body. There we leave them for the present, and turn to the 

 consideration of the channels of outflow, and the waste 

 products which pass along them. In a body which is 

 neither increasing nor diminishing in mass the outflow must 

 exactly balance the inflow ; all that enters the body must 

 sooner or later, in however changed a form, escape from it 

 again. In the expired air, the urine, the secretions of the 

 skin, and the faeces, by far the greater part of the waste pro- 

 ducts is eliminated. Thus the carbon of the absorbed solids 

 of the food is chiefly given off as carbon dioxide by the lungs ; 

 the hydrogen, as water by the kidneys, lungs and skin, along 

 with the unchanged water of the food ; the nitrogen, as urea 

 by the kidneys. The faeces in part represent unabsorbed 

 portions of the food. A small and variable contribution to 

 the total excretion is the expectorated matter, and the secre- 

 tions of the nasal mucous membrane and lachrymal glands. 

 Still smaller and still more variable is the loss in the form of 

 ead epidermic scales, hairs and nails. The discharges from 

 the generative organs are to be considered as excretions 

 with reference to the parent organism, and so is the milk, 

 and even the foetus itself, with respect to the mother. 



Excretion by the lungs and in the faeces has been already 

 dealt with. All that is necessary to be said of the expectora- 

 tion and the nasal and lachrymal discharges is that the 

 first two generally contain a good deal of mucin, and are 

 produced in small mucous and serous glands, the cells of 



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