THE URINE 319 



spiral and zigzag tubules (Fig. 95) ; the clear cell, on the other hand, 

 possesses more the characteristics of the epithelial lining of ducts. 



Vascular Mechanism. — The vascular arrangements of the 

 kidney are under the control of a rich supply of vaso-constrictor 

 nerves, while dilator nerves are also known to exist. If the 

 general blood-pressure be constant, dilatation of the renal vessels 

 means an increased secretion of urine, while constriction of the 

 vessels means a reduced secretion. An increase in the general 

 blood-pressure produces an increase in the amount of blood in 

 the kidney, and this is rendered evident by the swelling of the 

 organ in the oncometer and an increased production of urine. 

 If the increased general blood-pressure is accompanied by a 

 constriction instead of a dilatation of the small arteries of the 

 kidney, such, for instance, as when the vaso-constrictor nerves 

 are stimulated, then the increased blood-pressure cannot lead to 

 increased secretion, but, on the contrary, the amount of urine 

 becomes less and the kidney shrinks. A fall in general blood- 

 pressure, such as is caused by dividing the spinal cord, brings 

 about a reduction in the flow through the kidney, and the blood- 

 pressure becomes so low that the secretion of urine is entirely 

 suspended. It is thus evident that the vasomotor influence over 

 the kidney is of the greatest importance, and largely regulates the 

 amount of urine manufactured. If the renal vein be obstructed, 

 the pressure of blood in the kidney rises, but no urine is secreted ; 

 evidently, therefore, an increased flow of blood through the 

 kidney is as essential, to secretion as is increased blood-pressure. 



The Theories of Urinary Secretion are two in number. One 

 put forward by Bowman regarded the epithelium of the glomeru- 

 lus as the seat of secretion of the water and inorganic salts of 

 the urine, while in the convoluted tubules the epithelium secreted 

 the urea and other organic substances. The view is essentially 

 secretory, and is opposed to the other formulated by Ludwig, 

 which is essentially mechanical in character. Ludwig regarded 

 the glomerulus as the seat of formation of the entire urinary 

 constituents under the influence of pressure brought about by the 

 peculiar structural arrangements of the part. In the glomerulus, 

 according to this view, a highly diluted fluid was formed which, 

 in its passage to the pelvis of the kidney, was exposed to absorp- 

 tion by the epithelial cells, by which means it became concen- 

 trated. The majority of physiologists accept the Bowman theory. 



If the whole process of urinary secretion were a question of 

 pressure in the glomerulus, then ligature of the renal vein should 

 result immediately, though not continuously, in a greater secretion 

 of urine. As a matter of fact, we know the secretion ceases. 

 This experiment is a very crucial one. The fact of closing the 

 venous outlet must greatly raise the capillary pressure, and 



