THE KIDNEY 43 



man) it degenerates into a small vascular network, the 

 'pampiniform plexus/ surrounding the speimatic duct— 

 itself a vestige of the archinephric duct long ago stolen 

 from the kidney by the testis. This circulatory plexus 

 now serves but to puzzle the unhistorically minded stu- 

 dent of anatomy, and on occasion to undergo varicose 

 enlargement into the pathological condition known as 

 varicocele. 



The microscopic structure of the renal tubule is much 

 the same in all vertebrates. This structure, as we know 

 its details today, throws little light on the many remark- 

 able physiological operations which the tubule carries 

 out. In all forms below the mammals the tubule is dif- 

 ferentiated into only two anatomically distinct segments, 

 a proximal ( = near) segment adjoining the glomerulus, 

 and a distal ( = remote) segment leading into the col- 

 lecting ducts. In the lower vertebrates these two seg- 

 ments are connected by a short, narrow intermediate 

 segment which carries whiphke hairs or ciha that, by 

 beating downstream, help propel the urine from one seg- 

 ment to the other. 



By a variety of techniques modem renal physiology 

 has greatly advanced our knowledge of the function of 

 these segments in man as well as in experimental ani- 

 mals. It is known that the proximal segment reabsorbs 

 many of the valuable constituents of the glomerular fil- 

 trate, perhaps nearly all of those substances which we 

 have enimcierated earHer as being conserved by the kid- 

 ney. In the mammals, the proximal segment also reab- 

 sorbs approximately seven-eighths of the salt and water 

 of the filtrate, and in this operation imreabsorbed waste 

 products in the tubular urine are concentrated eight- 

 fold. 



The proximal segment is also responsible for the ex- 

 cretion of a large variety of both naturally occurring and 

 foreign substances. The reason why some substances are 

 excreted by the tubule and others are not is poorly un- 

 derstood, but it appears that this process helps to rid the 



