42 FROM FISH TO PHILOSOPHER 



functioned simultaneously. The simple coelomate tubule 

 had required for its reabsorptive and excretory opera- 

 tions a supply of blood, but since there was no need for 

 the blood to have a high pressure (as for filtration), it 

 was supplied in a random sort of way from miscellane- 

 ous veins draining organs in the posterior part of the 

 body— the tail muscles, ovaries, and others. (Figure 5a.) 

 This blood had already traversed the capillary bed of 

 these organs before it was distributed to the capillaries 

 of the renal tubules. The presentation of blood in this 

 manner to two capillary systems in succession is classi- 

 cally exhibited between the intestine and the hver, 

 where the nutriment absorbed into the capillary plexus 

 of the one is delivered directly to the capillaries of the 

 other; ever since the days of the anatomist, Galen, the 

 vein by which the blood makes this transit has been 

 called the 'portal vein (from porta = gate). Anatomical 

 names are sometimes as conservative as organisms, and 

 the venous blood supply to the kidney came, by analogy, 

 to be called the 'renal-portal vein.' 



When the glomeruli were evolved they had to be sup- 

 pUed with arterial blood at a high pressure in order to 

 efiFect filtration, but this blood was made to do a double 

 duty: after leaving the glomerulus it, too, was spread 

 out around the tubules in a network of capillaries where 

 it could sustain tubular reabsorption and excretion. (Fig- 

 ure 5b.) Thus the tubules acquired a double blood sup- 

 ply, the old one carrying venous blood, the new one, 

 arterial blood, but with no confusion: the capillaries of 

 the two systems simply fused with each other so that 

 it is impossible in any capillary to tell which blood is 

 which. 



This double blood supply persists in the kidney of all 

 vertebrates below the mammals. Li the latter the tubules 

 are supplied with blood only from the glomeruli (Figure 

 5d), and the renal-portal system appears only transiently 

 in the embryo as a flashback into history, and then (in 



