THE EXCRETORY SYSTEM 



203 



rounding membrane, or Bowman's capsule. The artery, 

 vas afferens, entering the capsule breaks up into several 

 capillaries which, after forming a few coils, emerge as the 

 efferent blood vessel from the same opening. Bowman's 

 capsule is an exceedingly thin membrane ; there is an inner 

 fold closely applied to the glomerulus which is continuous 

 with the outer wall at the point where the blood vessels enter 

 the capsule. Bowman's capsule is 

 simply the thinned out and expanded 

 end of a uriniferous tubule which has 

 become pushed by the glomerulus as 

 one might push in the end of a finger 

 of a glove. The capsule, however, 

 has grown around the glomerulus and 

 closely surrounds the afferent and effe- 

 rent vessels. At the dorsal side of the 

 capsule, and usually opposite the point 

 where the blood vessels enter, the outer 

 wall passes into the neck of the urinif- 

 erous tubule. The very thin cells of 

 this wall shade off gradually into cells 

 of columnar epithelium which for a 

 short distance carry very large cilia. 

 Beyond the neck, which is somewhat 

 narrower than the rest of the tubule, the cells are lined with 

 much shorter cilia. Each tubule is lined with a single layer 

 of cells which varies in character in the different parts. 

 The course of each tubule is quite complicated. At first it 

 runs dorsally, where it forms a more or less complicated coil, 

 then it proceeds to the ventral side of the kidney, forms a 

 second coil, and finally runs dorsally again, emptying into 

 one of the collecting canals which extend transversely across 

 the dorsal surface of the kidney from the inner margin to 



/ 



Fig. 53. — A urinifer- 

 ous tubule, c, col- 

 lecting tubule, m, 

 Malpighian body; 

 /, uriniferous tubule 

 leading from the lat- 

 ter to the collecting 

 tubule. (After Nuss- 

 baum.) 



