OKGANS OF THE BODY. 535 



regarded as merely consisting in an elimination of matters from the blood 

 which already existed there ? 



From the fact that some of the most important and best known con- 

 stituents of the urine had been met with in this central fluid ( 75), the 

 agent in so many of the exchanges of matter going on in the system, it 

 was for a long time supposed that the secretion of the fluid in question 

 was analogous to the process of filtration, and so essentially dissimilar to 

 the formation of bile in the liver. But though the statements of Zalesky. 

 that urea and uric acid are generated by the kidneys, have been shown to 

 be incorrect, still many circumstances point to caution as regards the 

 acceptance of this old view. Thus, for instance, the acid nature of the 

 urine, the transformation of benzoic into hippuric acid in the kidney itself 

 (Mdssner and Sltepard], and the fact that albumen does not transude 

 under ordinary circumstances. It seems extremely probable, indeed, 

 that the process of excretion of urine partakes both of the nature of 

 secretion and filtration. 



When we consider the structure of the kidney, as described above, 

 the question also naturally presents itself Which of the two vascular 

 apparatuses, the glomerulus or the network investing the uriniferous 

 tubes, presides over the excretion of the fluid 1 ? 



When we remember that the kidney and glomerulus go hand and hand 

 among the vertebrates, we must be inclined to ascribe to this portion of 

 the vascular system the greatest importance, even though the gland cells of 

 the convoluted tubes do possess the power of secretion, and represent 

 something more than a mere passive epithelial lining, which is hardly to 

 be doubted. It is only the straight canals running from the external 

 surface of the medullary rays to the points of the papillae, which present 

 the latter in our opinion. 



If we bear in mind that in man and in the mammalia the vas afferens 

 breaks up into branches in the glomerulus, besides being arranged in con- 

 volutions, and that these branches combine again to form a smaller vas 

 efferens, that a retardation of the blood must be brought about in the 

 convolutions of the glomerulus, owing to the greater area to be traversed 

 by it, will be clear ; and that this sluggishness must be succeeded by 

 rapid circulation in the narrow efferent vessel, giving way again to 

 a second and more clearly marked retardation in the capillary network 

 around the uriniferous tubes, is also plain. This narrowness of the vas 

 efferens produces, then, a greater or less degree of obstruction to the blood 

 in the glomerulus, and, consequently, to an increase of lateral pressure, 

 far exceeding that of the second capillary system ; it favours thus excre- 

 tion. The blood in the capillary network, on the other hand, investing the 

 uriniferous tube, flows certainly under smaller pressure, and appears partly 

 to possess the power of absorption, and to rob the urine as it passes of 

 some of its water again (Ludivir/}. The peculiar disposal, further, of the 

 derivatives of the vas efferens, first around the passages of the medullary 

 ray, and subsequently around the convoluted tubes of the cortex, seems 

 to indicate some physiological purpose beside all this. 



The progress of the urine towards the openings on the papillae takes 

 place without any muscular aid, merely through the vis a tergo produced by 

 the continuous secretion behind pushing forward the columns of fluid in 

 the uriniferous tubes. Besides this, in the ureters the gravitation towards 

 the bladder comes in aided by the contraction of the muscular walls of 

 the ureter (Engelmanri). Owing to the well-known anatomical arrange- 

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