222 ADVANCED PHYSIOLOGY 



material. There are, then, two places where material from 

 the blood vessels can be set free into the tubule; through the 

 walls of the capsule which surrounds the glomerulus, and 

 through the walls of the convoluted part of the tubule. By 

 a series of delicate tests it has been found that water and 

 some common salts in solution leave the blood through the 

 walls of the capsule. Although this process is, doubtless, 



MalpKfhian 



Tubule 

 FIG. 111. SHOWING THE DISTRIBUTION OF THE CAPILLARIES 



AROUND A SINGLE KIDNEY TUBULE 

 The blood capillaries in the capsule form a glomerulus. 



largely one of filtration, it is unlike ordinary filtering through 

 paper in that not all substances in solution will pass through. 

 The cells of a Malpighian capsule allow some substances to go 

 through, while they prevent others. This, like the absorption 

 of food through the intestinal walls, can only be explained 

 by saying that the cells making up the membranes are alive. 



Where the capillary blood vessels spread out over the con- 

 voluted part of the tubule, the cells of the tubule are true 

 secreting cells, selecting from the blood waste organic ma- 

 terials, chiefly urea, but also some other substances (pig- 

 ment, phosphoric and sulfuric acids, sodium, chlorine, ammo- 

 nia, etc.). These pass into the tubule; there the water coming 

 down from the capsule dilutes the secretion and carries it 

 into the pelvis cavity. 



The entire kidney is a compact mass of thousands of these 

 tubules, each having an irregular course and each opening 

 into the reservoir of the pelvis. Inasmuch as the cortex is 

 so richly supplied with blood capillaries, this surface layer is 

 redder in appearance than the deeper parts which border on 



