49 EXCRETION 



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 



Fig. 190. From a Vertical Section of Dog's Kidney to show the Structure of Different 

 Portions of the Renal Tubule (Klein), a, Bowman's capsule enclosing glomerulus, 

 the capillaries of which are arranged in lobules separated by a little connective 

 tissue. The capsule and glomerulus together constitute a Malpighian body or 

 corpuscle; n, neck of capsule; c, c, convoluted tubules, cut in various directions; 

 b, irregular or zigzag tubule; d, e, and/ are straight tubules, which take part in the 

 formation of a medullary ray or pyramid of Ferrein ; d, collecting tubule ; e, e, spiral 

 tubule; /, narrow part of ascending limb of Henle's loop-tubule; 6, c, and e are 

 lined with rodded epithelium. 



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 



