400 A MANUAL OF PHYSIOLOGY 



into a close-meshed network of capillaries, from which the blood is 

 collected into interlobular veins running parallel to the interlobular 

 arteries between the pyramids of Ferrein. The straight tubules of 

 the medulla are also surrounded by capillaries given off from straight 

 arteries (arterise rectse) 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 

 rectse) 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. 



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 foun- 

 dation 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 constituents 

 of urine, including 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 com- 

 plete 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 resist- 

 ance beyond a single capillary network. And experimental 



