CHAPTER VI 

 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 considera- 

 tion 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 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 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 discharges from the 

 generative organs are to be considered as excretions with refer- 

 ence 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 trte 

 same general type as those of the mucous and seious 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. XIV.), so that there remain 

 only the urine and the secretions of the skin to be treated here 



435 28 2 



