.248 



NUTRITION 



tion, being, however, more conspicuous. Some diuretics, such as caffeine, 

 seem to act on the renal epithelium. Others act directly on the renal 

 vessels, increasing the blood-pressure locally. 



THE DISCHARGE OF URINE. The urine collects in the hilum of the 

 kidney under a pressure (in the dog) of 60 mm. of mercury. The ducts 

 of Bellini enter through the pyramids very obliquely in such a way that 

 the greater the pressure within the hilum the more tightly are their 

 orifices closed. This arrangement acts as a perfect automatic valve to 

 prevent regurgitation of the urine upward into the collecting-tubes and 

 consequent interference with the secretory process of the tubules in cases 



Fia 134 



of impacted calculus, etc, From the hilum the urine passes in a contin- 

 uous trickle through the pelvis of the kidney into the ureter. This tube, 

 composed of fibrous, muscular, and mucous coats, is about 43 cm. long 

 and 4 or 5 mm. in diameter, and connects the kidney with the urinary 

 bladder. The muscle is of the smooth variety, the fibers running both 

 longitudinally and circularly. It is supplied with sensory and motor 

 nerves. The former are useful perhaps in adapting the vigor of the 

 peristalsis to the resistance the latter has to ovecome, as, e. g., in passing 

 calculi through it. In man three sorts of forces seem to help in the 

 passage of urine through the ureters: the secretory pushing "force from 



