418 



COMPARATIVE PHYSIOLOGY. 



other opening into a long common e£Ferejlt tube or duct. The 

 glomerulus is, however, peculiar to the vertebrate kidney. 

 The graded complexity in arrangement, etc., of the tubes is 

 represented well in the figure below. It is a significant fact 



Fig. 318. — Diagrammatic rei>reBentation of dietiibntion of tnbnleB of l^idney (after 

 HuxleyV C, cortical region; B. boundary zone, containing large part of Henle's 

 loops; P, papillary zone, in which are the main outflow tubules. 



that the kidney of the human subject is lobulated in the embryo, 

 which condition is persistent in some mammals (ruminants, etc.). 

 As the lungs are the organs employed especially for the 

 elimination of carbonic anhydride, so the kidneys are above 

 all others the excretors of the nitrogenous waste products of the 

 body chiefly in the form of uric acid or urea. Before treating of 

 secretion by the kidney it will be well to examine into the phys- 

 ical and chemical properties of urine with some detail, especially 

 on account of its great importance in the diagnosis of disease. 



