Xo.582] SIGNIFICANCE OF INTERNAL CONDITIONS 357 



nection has been shown between stimulus and response. 

 And we may hope that here as elsewhere in biology the 

 limits to our knowledge of nature will gradually be broken 

 down. 



The accumulation of waste products in the blood or body 

 fluids through increase of their concentration in these 

 fluids, leads to modified activity of the excretory and 

 other organs. We cite a few examples. 



The kidney, in addition to the elimination of water, fol- 

 lows the law of mass action in other ways. The volume 

 of urinary secretion, other things being equal, is propor- 

 tional to the volume of blood flow through the kidneys in 

 unit time. The greater volume of blood carries with it a 

 greater volume of waste products in unit time, and hence 

 a greater volume of secretion is the result to be expected if 

 urinary secretion is a physico-chemical process following 

 the general provision of the mass law. 



The accumulation of waste products arising from the 

 slow reactions in the cells gives rise to the phenomena of 

 fatigue, and the general slowing down of the cell proc- 

 esses, just as the accumulation of the end products of 

 any slow reversible reaction decreases the amount of 

 chemical transformation in unit time, in accordance with 

 the mass law. 



Excess of carbon dioxide, a typical waste product, even 

 of the activity in nerves, 49 decreases or abolishes the con- 

 ductivity of a nerve fiber. A stimulus (geotropic) may be 

 applied to a plant in an oxygen-free atmosphere, but the 

 responses will not occur until the plant is moved to an 

 atmosphere containing oxygen. 50 



But even waste products in a certain concentration 

 may be necessary for the optimum conditions of activity 

 of an organ. Baglioni 51 points out that the selachian 

 heart maintains its activity better in a solution of the 

 inorganic salts containing two per cent, urea— the normal 



