SKIN AND KIDNEYS. 



165 



emptying their secretion into the cavity of the kidney. As 

 the blood flows through the tuft of capillaries in the Capsule 

 at the end of the tube, a good deal of water, together with salt 

 and some other substances, passes through the thin partition 

 into the cavity of the capsule, and thence down the tube. The 

 walls of the tube are thicker than, and its cells are different 

 from, those of the capsule. These cells take the urea and 

 some other substances from the blood, and pass them into the 

 tube to join the more watery material from the capsule. 



Comparison of the Skin and the Kidneys. The kid- 

 neys, then, are not very different from the skin. Imagine a 

 piece of skin rolled up 

 with its outer surface 

 turned inward. Its glands 

 then would pour their se- 

 cretions into a cavity where 

 they might accumulate in- 

 stead of evaporating as 

 fast as they are poured 

 out. Of course the 

 kidneys have a 



Urinary Capsule 



Artery 



Vein 



somewhat different 

 work from the skin, 

 but in its general 

 plan of working we 

 might say they are skin 

 turned outside in. The 

 kidney unit (the tubular 

 gland) has branches ; i.e., is compound. The kidney is a com- 

 pound gland of excretion, internal in position. Both skin and 

 kidneys excrete a good deal of water, with salt and some 

 other matter in common. There is a very immediate relation 

 between the work of the kidneys and that of the skin. In 



Urinary, Tube 

 Fig. 54. Urinary Cone, Enlarged. (Diagram.) 



