OUTPUT OF WASTE MATTER 131 



saline matters, chiefly chloride of sodium 

 (common salt) and phosphates of the alkalies 

 (potash and soda), and of the alkaline earths 

 (lime and magnesia) ; various nitrogenous 

 substances, more especially urea, uric acid, 

 creatinin, etc. ; and pigmentary matter. 

 The kidneys also take a slight part in the 

 elimination of carbonic acid. These matters 

 are separated from the blood mainly by the 

 activity of the epithelial cells lining the 

 uriniferous tubules. In the cortical part of 

 the kidney there are remarkable structures 

 known as the Malpighian Bodies, consisting 

 of a glomerulus or ball formed by a network 

 of capillaries, surrounded by the dilated end 

 of a uriniferous tubule, so as to form a capsule 

 lined by a peculiar form of epithelium. The 

 end of the tubule is infolded over the capillary 

 nodule so that a double wall surrounds the 

 nodule. Thus a somewhat complex membrane 

 is formed like a kind of cap, and three layers, 

 blended together, separate the blood from the 

 urine namely, the wall of the capillaries and 

 a double wall formed by the infolded end of 

 the tubule. This was once supposed to 



