THE DIGESTION 



235 



417. In summer, when 

 the circulation in the skin 

 is active, the secretion of 

 the kidneys diminishes in 

 amount. On the other hand, 

 cold drives the blood from 

 the surface to the internal 

 organs, and the secretion of 

 the kidneys is increased, 

 although probably but little, 

 if any, more urea passes off. 



418. The blood leaving 

 the active kidney is still 

 of a bright red color. It is 

 probably the purest blood 

 in the body. Can you ex- 

 plain why this is so. 



FIG. 186.- Section of the Kidney. 



Ct, cortex; AT, medulla; .Py, pyramid; 

 P, pelvis; U, ureter; RA, renal 

 artery. 



