140 FROM FISH TO PHILOSOPHER 



and cold spell for cold spell, with the more sluggish 

 reptiles. Into whatever area the uric acid-excreting rep- 

 tiles could migrate, the urea-excreting mammals could 

 follow them; and, when winter forced the reptiles to 

 hibernate, the warm-blooded mammals remained active 

 and alert. It is therefore reasonable to suggest that the 

 long delay in mammalian evolution— from the Permian 

 to the Cenozoic, a period of one hundred and fifty mil- 

 lion years— was in part attributable to the slow evolution 

 of the warm-blooded state and the concomitant evolu- 

 tion of the mammaUan concentrating kidney. 



The unique functional feature of the mammahan kidney 

 —its concentrating power— is accompanied by only two 

 notable anatomical changes. The first of these is in the 

 structure of the tubule. The narrow neck, which in the 

 amphibian nephron is interposed between the proximal 

 and distal segments, has lost its ciha and is variably 

 elongated into what is known as the 'thin segment'— so 

 that we can now speak of three segments: 'proximal,' 

 'thin,' and 'distal,' The tubule as a whole is bent into a 

 hairpin loop, called the loop of Henle: after the proximal 

 segment undergoes extensive convolutions near the glo- 

 merulus, it descends in a more or less straight course to- 

 ward the interior of the kidney; ultimately it turns 

 sharply to return to the glomerulus of origin, and after 

 further convolutions (distal segment) it joins a collect- 

 ing duct. The thin segment may be confined to the de- 

 scending limb of Henle's loop, or (as is shown in Fig- 

 ure 10 ) it may extend around the loop and for some 

 distance up the ascending distal limb. 



The second change concerns the disappearance of the 

 renal-portal system. We have emphasized the impor- 

 tance of this ancient venous blood supply in maintain- 

 ing tubular function in the fishes, Amphibia, and reptiles, 

 at times when the glomerular circulation had to be cur- 

 tailed. In the mammals, however, the renal-portal sys- 

 tem has been completely discarded and the tubules are 



