CHAPTER IX 

 EXCRETION 



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 products is elimin- 

 ated. 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 secretions of the nasal mucous membrane and 

 lachrymal glands. Still smaller and still more variable is the loss 

 in the form of dead epidermic scales, hairs, and nails. The dis- 

 charges from the generative organs are to be considered as excre- 

 tions 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 expectoration 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 which are of the same general type 

 as those of the mucous and serous salivary glands. The lachrymal 

 glands are serous like the parotid; and the tears secreted by them 

 contain some albumin and salts, but little or no mucin. The sexual 

 secretions and milk will be best considered under reproduction 

 (Chap. XIX.), so that there remain only the urine and the secre- 

 tions of the skin to be treated here. 



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