KIDNEYS, SKIN, AND GENERAL EXCRETION. 391 



ing tubules, into which other zig-zag tubules enter, and by 

 means of this large collecting tubule which runs from cor- 

 tex and medulla, the secretion is finally poured into the 

 pelvis of the kidney. These large collecting ducts are 

 called the ducts of Bellini. The whitish glistening threads 

 so characteristic of the medulla are such ducts of Bellini. 

 The course of the uriniferous tubule as just given permits, 

 of course, of certain variations, but is with a surprising 

 regularity the usual one. 



Most of the cortex and even some of the upper portions 

 of the medulla consists of these closely packed tubules with 

 their accompanying blood-vessels. On cross sections each 

 tubule shows a single layer of columnar epithelial cells rest- 

 ing on a small basement membrane, and it is these epithelial 

 cells which play the physiological role of the kidney. It is 

 these cells which pick out of the blood the nitrogenous 

 waste products and pass them into their lumen. At the 



Fig. 131. TUBULES FROM A SECTION OF THE DOG'S KIDNEY. (After Klein and Noble 



Smith.) 



a, capsule, enclosing glomerulus; n, neck; c, c, convoluted tubules; /, from the limb 

 of Henle (in the medulla) ; d, a. collecting tubule. 



Malpighian corpuscle, that is, at the very head of the urin- 

 iferous tubule, water, and some salts of course, are -being 

 continually poured into the tubule, and this stream serves 

 to wash out other substances passed into it. The fact that 

 urea and many other nitrogenous compounds are with diffi- 

 culty soluble in water at ordinary temperatures accounts for 

 the large volume of water required by the kidney to wash out 

 this product. We have therefore to look upon the function 



