192 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tures water filtered through from the blood, carrying with it not only the in- 

 organic salts, but also the specific elements (urea) of the secretion. There was 

 thus formed at the beginning of the uriniferous tubules a complete but diluted 

 urine, and in the subsequent passage of this liquid along the convoluted tubes 

 it became concentrated by diffusion with the lymph surrounding the outside 

 of the tubules. So far as the latter part of this theory is concerned it has 

 not been supported by actual experiments ; recent histological work (see below) 

 seems to indicate that the epithelial cells of the convoluted tubules have a 

 distinct secretory function, and that they give material to the secretion rather 

 than take from it. 



Bowman's theory of urinary secretion, which has since been vigorously 

 supported and extended by Heidenhain, was based apparently mainly on his- 

 tological grounds. It assumes that in the glomeruli water and inorganic salts 

 are produced, while the urea and related bodies are eliminated through the 

 activity of the epithelial cells in the convoluted tubes. 



Elimination of Urea and Related Bodies. Numerous facts have been 

 discovered which tend to support the latter part of Bowman's theory namely, 

 the participation of the cells of the convoluted tubules in the secretion of the 

 specific nitrogenous elements. In birds the main nitrogenous element of the 

 secretion is uric acid instead of urea, and it is possible, owing to the small solu- 

 bility of the urates, to see them as solid deposits in microscopic sections of the 

 kidney. When the ureters are ligated the deposition of the urates in the kid- 

 ney may become so great as to give the entire organ a whitish appearance. 

 Nevertheless histological examinations of a kidney in this condition shows that 

 the urates are found always in the tubes and never in the Malpighian corpus- 

 cles. From this result we may conclude that the uric acid is eliminated 

 through the epithelial cells of the tubes. Heidenhain has shown by a striking 

 series of experiments that the cells of the tubes possess an active secretory 

 power. In these experiments a solution of indigo-carmine was injected into 

 the circulation of a living animal after its spinal cord had been cut to reduce 

 the blood-pressure and therefore the rapidity of the secretion. After a certain 

 interval the kidneys were removed and the indigo-carmine precipitated in situ 

 in the kidney by injecting alcohol into the blood-vessels. It was found that 

 the pigment granules were deposited in the convoluted tubes, but never in the 

 Malpighian corpuscles. 



Still further proof of definite secretory functions on the part of the cells 

 of the tubules is given by the results of recent histological work upon the 

 changes in the cells during activity. Van der Stricht 1 and Disse 2 both 

 describe definite morphological changes in the epithelial cells of the convoluted 

 tubes and ascending loop of Henle which they connect with the functional 

 activity of the cells. The details of the descriptions differ, but the two authors 

 agree in finding that the material of the secretion collects in the interior of the 



1 Comptes rendus, 1891, and Travail du Laborataire d' Histologie de V Universite de Gand, 1892. 



2 Eeferate und Beitrage zur Anatomic und Entwickelungsgeschichte (anatomische Hefte), Merkel 

 and Bonnet, 1893. 



