KATABOLISM. 



435 



Below these the renal organ has not been detected. 

 Probably this function has not yet been differentiated 

 from others. 



Observe, then : (i) Only a little way down the verte- 

 brate scale (birds) the distinction of cortex and medulla 

 is lost. (2) Only a little farther down the animal scale 

 (insects) the functions of liver and kidneys are merged, 

 although they are found separate even lower down. 

 (3) A little lower down (crustacea) the function of the 

 kidneys is incompletely performed — i. e., a part of the 

 waste escapes decomposition and is thus wasted by not 

 being utilized as fuel. (4) A little lower down — i. e., 

 below mollusks — this organ has not been certainly 

 found. Probably the function is not yet differentiated. 



SECTION IV. 

 The Skin and its Function. 



We have spoken of the lungs and kidneys as the two 

 great organs which share the elimination of katabolic 

 products between them. But there is still a third, the 

 skin, which, though less important, must not be neg- 

 lected. 



Function. — All organs and functions are sympa- 

 thetically related to one another, so that the importance 

 of healthy action of an organ is not to be measured by 

 that of its more obvious and distinctive function. The 

 distinctive function of the skin is the moderation of the 

 Mood heat produced by continual combustion, by the 

 elimination of excess of water, and its evaporation on an 

 exposed surface. The elimination of water it shares with 

 the kidneys and is complementary with it, but it is 

 peculiar in the elimination of water in such wise as 

 thereby to cool the blood. 



But elimination of water by the skin, as also by the 



