KIDNEYS OF HIGHER ANIxMALS 



106 



emptied into the central cavity. In most instances the 

 inner end of the kidney tube is provided with heavy ciha 

 whose continued beating drives a steady current of body 

 cavity fluid through the canal and thus washes the wastes 

 to the exterior. 



Kidneys of Higher Animals. — The kidneys in the 

 vertebrates are relatively large paired organs attached 

 within the body cavity on each side of the backbone. A 

 tube, passing from each organ, leads to the bladder or to 

 the exterior, and on the other hand communicates with a 

 relatively small cavity within the kidney itself (Fig. 21). 



Fig. 21. — A, longitudinal section through human kidney, showing 

 radiating tubes opening into the main cavity communicating with 

 the bladder. B, kidney tubes enlarged. C, terminal section of tube 

 to show cup-like depression filled with a capillary loop which extends 

 as a network over the adjacent section. 



Into this space the wastes are poured from the main 

 kidney mass through small openings each of which com- 

 municates with a canal, that, as it makes its way into the 

 interior, breaks up into a large number of very small 

 subdivisions. The kidney is thus made up of many tree- 

 like though hollow branches, opening on the one hand 

 into the main kidney cavity and on the other terminating 



