URINARY SYSTEM. 467 



admirably arranged to receive the albumen reabsorbed by the 

 epithelium; and so, too, are the capillaries of the malpighian 

 tufts (the glomeruli) arranged to allow a filtration of tho 

 serum ; in fact, it is owing to this circulatory system, which 

 we have called the renal portal vein, that the solution of this 

 twofold phenomenon may be found, viz. filtration and re- 

 absorption, which constitute the two phases essential to the 

 secretion of urine. Comparative physiology illustrates this 

 twofold phenomenon still more perfectly : among the ophid- 

 ians (snakes, etc.), which secrete a solid urine, a liquid is 

 found at the beginning of the uriniferous tubes, which gradu- 

 ally becomes thickened in its course, until it finally acquires 

 the characteristic semi-solid consistency. 



Thus, to sum up, the secretion of urine is composed of two 

 distinct phases: 1, A phenomenon of simple filtration in the 

 glomerulus; 2, To this purely mechanical phenomenon there 

 succeeds a vital work on the part of the globular elements of 

 the epithelium of the uriniferous tubes. 



This epithelium of the uriniferous tubes, then, simply ab- 

 sorbs, but does not secrete ; formerly it was supposed to have 

 something to do with the formation of urea, but it is now 

 proved that all the urea found in the urine is primarily con- 

 tained in the blood. The origin of urea in the kidney is re- 

 duced to a simple question of experiments, the results of 

 which show that urea pre-exists in the blood, and is not 

 formed in the kidney; that the blood of the renal vein nor- 

 mally contains less urea than that in the renal artery ; that 

 ligation of the ureters produces the sume symptoms as abla- 

 tion of the kidneys. In France, Prevost and Dumas, Segalas 

 and Vauquelin, 1 Cl. Bernard and Barreswil, Picard 2 (These 

 de Strasbourg, 1856), have arrived at these results; yet, in 

 Germany, their researches have been opposed on account of an 

 assumed error in the estimation of urea; Oppler, Perls, Her- 

 mann, Hoppe-Seyler, and Zalesky contend that a large 

 amount of urea is formed in the renal tissue, just as ptyaline 

 is formed in the salivary glands ; a maceration of kidney gives 

 origin to urea in the same way that a maceration f the 

 parotid gland gives rise to animal diastase. Finally, Zalesky 

 pretends that ablation of the kidneys (nephrotomy) and liga- 

 tion of the ureter produce different symptoms ; that, after 



1 Journal de Magendie. Vol. II. p. 354. 



2 J. Picard, u De la Presence de 1' Urine dans le Sang et de sa 

 Diffusion dans 1'Organisme." 



