THE SECRETION OF THE URINE 487 



rectae) running down into it partly from the arterial arches and partly 

 from efferent vessels of the glomeruli nearest the 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 capil- 

 laries, 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. 137). 



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 secretion. 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 con- 

 stituents of urine, including urea, were secreted from the blood by 

 the tubules. To the Malpighian bodies he assigned what he doubt- 

 less 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 Bow- 

 man'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 experi- 

 mental 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 physiologists. 

 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 work 



