STRUCTURE OF THE URINARY TUBULES. 95 



As the tubuli uriniferi of all Mammals present a general 

 accordance, in their structure, course, and relations, with the 

 above description, the kidneys of the different classes would 

 assimilate in the closest manner, were the transverse and 

 longitudinal proportions of the canals similar. The only differ- 

 ence that could then exist would be in the number of the 

 canals participating in the formation of the organ. This, how- 

 ever, it is well known, is not the case. The obvious differences 

 which exist between the kidneys of different Mammals show 

 that in the several classes, in reference to the proportions of 

 the primary urinary passages, a great field of variation exists. 



Fig. 149*. Varieties of urinary tubules, 

 taken from Hiifner's work.) 



(All these figures have been 



The most superficial examination of preparations obtained by 

 section or by breaking up the tissue of the organ, shows that not 

 only the absolute length and thickness of the kidneys vary in 

 different specimens, but that a similar variation exists in the 

 length of the different segments of the canals. Apart from all 

 other evidence, this is sufficiently proved by the unequal pro- 

 portion presented by different kidneys in the amount of the 

 cortical and medullary proportions of their substance. It will 

 be impossible to arrive at any very precise conclusion in regard 

 to this undoubtedly important functional relation of the cor- 



