FUNCTIONS OF THE KIDNEY 87 



substance are seen the pyramids of Malpighi, whose 

 apices open into the caUces or first branches of the ure- 

 ter. Histology must be consulted for the minute anat- 

 omy of the organ. 



As no distinct secretory nerve fibers have been demon- 

 strated in the kidney, the mechanism of the secretion of 

 urine can be explained only by supposing that a part of 

 the process is simple filtration or osmosis, depending on 

 an abundant blood supply with sufficient pressure, while 

 the remainder is due to the " vital action" of the cells 

 lining the glomeruli and tubules. "Assuming that 

 nearly all the constituents of urine preexist in the blood 

 and are simply taken out of the circulation by the kid- 

 ney, it may be stated that, for the most part, the water 

 and salts are extracted by the cells of the Malpighian 

 bodies, while the urea and related nitrogenous solids are 

 removed by the cells of the convoluted tubes ; so that the 

 specific gravity of the fluid is raised by passing down 

 the tubes." (Jones and Bunce). Diuretics may act, 

 therefore, by increasing the amount of blood flowing 

 through the kidney, by increasing the pressure of the 

 blood, by promoting osmosis or by stimulating cell ac- 

 tivity. 



After urine has been formed in the kidney it is col- 

 lected in the pelvis of the ureter, which contracts to the 

 ureter proper, a long slender tube partly composed of 

 nonstriated muscle fiber, which runs behind the perito- 

 neum to the bladder, and enters the bladder so obliquely 

 that the weight of urine in that organ presses on and 

 closes the ureteral openings and prevents any back flow 

 of urine. It is well to remember that, in the female the 

 neck of the uterus is between the two ureters just be- 

 fore they enter the bladder. 



The bladder is an ovoid muscular sac which receives 



