106 RESPIRATION AND DISPOSAL OF WASTES 



blindly near the outer surface of the organ. The terminal 

 section of each tube thus developed is the active agent 

 in removing excretions and calls for a brief description. 



Each of the thousands of kidney tubes ends in an en- 

 largement, resembling the rubber tube and terminal bulb 

 of a camera with one side of the bulb pushed in to form 

 a cup-like depression. In the kidney this cup is filled 

 with a twisted capillary loop, and the tubular section for 

 a considerable distance is covered with a network of 

 capillaries. 



Mode of Operation. — Not only does each terminal 

 cup with its included capillary loop differ widely in struc- 

 ture from the adjacent tubular section, but the processes 

 carried on in each are equally unlike. In the cup a 

 stream, consisting essentially of lymph, is continually 

 filtering from the capillary into the cavity of the tube 

 down which it flows to the exterior. This is a purely 

 physical process. 



At the same time the cells of the tubular section are 

 absorbing from the blood of the capillary network cer- 

 tain waste substances, chiefly nitrogenous, that are un- 

 able to filter through the terminal cup. These are passed 

 through the cells of the tube to the central cavity where 

 they dissolve in the before-mentioned lymph stream and 

 are washed away in it. 



