EXCRETION 455 



boundary layer, the blood passing away by straight veins (venae 

 rectae) which join the larger veins accompanying the arterial arches. 

 The greater part of the blood going through the kidney has to pass 

 through two sets of capillaries, one in the glomeruli, the other around 

 the tubules. Even the portion of it which does not go through the 

 glomeruli has for the most part a long route to traverse in narrow 

 arterioles and venules to and from its capillary distribution. And 

 the mean circulation-time through the kidney has been found to be 

 longer than that through most other organs (p. 125). 



Theories of Renal Secretion. To come back to our problem 

 of the nature of renal secretion, the anatomical structure of the 

 kidney might be expected to throw light upon the question. 

 And, indeed, it was on a purely histological foundation that 

 Bowman established his famous ' vital ' theory of renal secre- 

 tion. Impressed with the resemblance between the renal 

 epithelium and the epithelial cells of other glands, and with the 

 distribution of the bloodvessels in the kidney, he came to the 

 conclusion that the characteristic constituents of urine, in- 

 cluding urea, were secreted from the blood by the tubules. To 

 the Malpighian bodies he assigned what he doubtless considered 

 the humbler office of separating water from the blood for the 

 solution of the all-important solids. To Ludwig, on the other 

 hand, with his whole attention fastened on the mechanical 

 factors by which the flow of urine could be influenced, the 

 tubules seemed of secondary importance, while the glomeruli 

 appeared a complete apparatus for filtering urine from the blood 

 into Bowman's capsule. He saw that the efferent vessel was 

 smaller than the afferent ; that it was therefore easier for blood 

 to come to the glomerulus than to get away from it, and that 

 the pressure in the capillaries of the tuft must be higher than in 

 ordinary capillaries, because the resistance beyond them in the 

 comparatively narrow efferent vessel, and especially in the 

 second plexus, is greater than the resistance beyond a single 

 capillary network. And experimental investigation soon showed 

 him that the rate at which urine was formed could be greatly 

 influenced by changes in the blood-pressure. 



On such considerations, Ludwig founded the ' mechanical ' 

 theory of urinary secretion, which, although in a much modified 

 form, still divides with the ' vital ' theory the allegiance of physiolo- 

 gists. It is impossible here to enter in detail into a controversy 

 that has extended over more than half a century and produced an 

 extensive literature. The result of the discussion has been, in 

 our opinion, to establish in its essential principles the ' vital ' 

 theory of Bowman, or at least to show that no purely physico- 

 chemical theory as yet constructed will account for all the facts. 



Ludwig supposed that the urine, qualitatively complete in all 

 its constituents, was simply filtered through the glomeruli, the 



