URINARY SYSTEM. 465 



found to consist of a liquid identical with the serum of the 

 blood: this proves that the serum, filters out in the glom- 

 eruli. 



This is the first phenomenon of the secretion of urine : fil- 

 tration of the serum of the blood. 



We will now learn how the product of filtration in the 

 glomcruli is transformed into urine : this transformation evi- 

 dently occurs in the sinuous course of the uriniferous tubes 

 (tubuli uriniferi), through which the filtered liquid is carried 

 from its original point to the pelvis of the kidney. 



Those authors who see in the filtered liquid simply pure 

 water cannot conceive the formation of urine except by a 

 secretion from the walls of the uriniferous canaliculi, a secre- 

 tion to which is added the substances that the urine should 

 contain. On the other hand, those who, like Wittich and 

 ourselves, 1 see in the filtered product a very diluted urine, 

 believe that the formation of the urine is accomplished by a 

 simple aqueous reabsorption, effected by the medium of the 

 uriniferous tubes, thus giving to the urine the desired con- 

 centration. 



As we have demonstrated that the product of the filtration 

 in the glomeruli is the serum of the blood ; and as a compara- 

 tive study of the composition of both serum and urine shows, 

 in a general manner, that in the composition of the two 

 liquids the serum differs from the urine only by the %>resence 

 of albumen; we are induced to believe that the formation of 

 urine consists in the absorption of this albumen, an absorp- 

 tion that necessarily occurs along the circuit of the uriniferous 

 tubes. 



This explanation of tlie second phase of the work of the 

 kidneys is a necessary consequence of the theory advanced 

 in the earlier portion of this book ; it is true that there is no 

 way by which we can verify the theory ; but we may be 

 allowed to examine whether what we know of the structure 

 of the kidneys is favorable to this view. 



In the first place, the length and the form of the convoluted 

 tubes, a form so closely resembling the intestinal convolu- 

 tions, naturally leads to the theory that we have also in the 

 kidneys a system or apparatus for absorption, and in which 

 the course of the liquid is retarded for the purpose of favor- 



1 V. Wittich, Virchow's " Archiv fur Pathologische Anat- 

 omie." Vol. X. Donders, " Physiologic des Menschen." Leip- 

 zig, 1859, Vol. I. 



30 



