No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 345 



9. The Mechanisms for Elimination of Waste Products 

 The elimination of waste products occurs promptly and 

 surely. The liver of higher animals not only separates 

 out the useless portions of the hemoglobin derived from 

 worn-out corpuscles (with retention of the iron-bearing 

 part), in the form of bile pigment, but also transforms 

 the ammonia compounds arising from nitrogenous metab- 

 olism to the relatively non-poisonous urea. The known 

 chemical functions of the liver 37 are numerous. The kid- 

 neys promptly remove the urea and uric acid from the 

 blood as these and other products of metabolism accumu- 

 late following activity of the cells. Certain mineral salts, 

 e. g., iron, are excreted by the intestinal mucosa. Some 

 fat, in addition to that which is not absorbed from the 

 food, is also excreted from the intestine. 



The attempt to tabulate the various excretions of the 

 body reveals the fact that, although the qualitative com- 

 position of the bile and the urine is relatively constant, 

 the quantitative variations are very wide under different 

 conditions. The quantity of carbon dioxide exhaled dur- 

 ing twenty-four hours depends upon the diet and upon 

 the amount of muscular activity daring the day. Al- 

 though somewhat disconcerting at first sight, in view of 

 the constancy of internal conditions, this very inconstancy 

 of the excretions is to be regarded as a consequence of the 

 maintenance of constant internal conditions. For it is 

 only by the conservation of those necessary elements or 

 substances whose supply is limited, and the free elimina- 

 tion of waste products or of other substances in excess 

 that the constancy of internal conditions can be main- 

 tained. Intake and outgo of matter and energy are 

 such as to maintain relatively constant physico-chemical 

 conditions within the organism. The greater the energy 

 requirements of the organism, the greater must be the 

 intake to meet these requirements, and the greater the 



