510 EXCRETION 



a portion of the water and salts, are excreted by the physiological 

 activity of the ' rodded ' epithelium of the renal tubules. The rate of 

 secretion of urine rises and falls with the pressure, and still more with 

 the velocity, of the blood in the renal vessels. No secretory nerves for 

 the kidney have been found ; the effects of section or stimulation of 

 nerves on the secretion can all be explained by the changes produced in 

 the renal blood-flow. Some diuretics act by increasing the blood-flow, 

 others directly on the epithelium of the tubules or the glomeruli. 



SECTION III. EXPULSION OF THE URINE. 



Micturition. -The urine, like the bile, is being constantly formed; 

 although secretion varies in its rate from time to time, it never 

 ceases. Trickling along the collecting tubules, the urine reaches 

 the pelvis of the kidney, from which it is propelled along the ureters 

 by peristaltic contractions of their walls, and drops from their valve- 

 like orifices into the bladder. When this becomes distended, rhyth- 

 mical peristaltic contractions are set up in it, and notice is given 

 of its condition by a characteristic sensation, which is perhaps aided 

 by the squeezing of a few drops of urine past the tonically con- 

 tracted circular fibres that form a sphincter round the neck of the 

 bladder, and into the first part of the urethra. The desire to empty 

 the bladder can be resisted for a time, as can the desire to empty 

 the bowel. If it is yielded to, the smooth muscular fibres in the 

 wall of the viscus are thrown into contraction. This is aided by an 

 expulsive effort of the abdominal muscles. The sphincter vesicae 

 is relaxed; and the urine is forced along the urethra, its passage 

 being facilitated by discontinuous contractions of the ejaculator 

 urinae muscle, which also serve to squeeze the last drops of urine 

 from the urethral canal at the completion of the act. 



Regurgitation into the ureters is to a great extent prevented by 

 their compression between the mucous and muscular coats of the 

 bladder, where they run for more than half an inch before opening 

 at the posterior angle of the trigone. But it has been shown that 

 a certain amount of back flow can take place. Small bodies like 

 diatoms suspended in water and pigments dissolved in it have been 

 found in. the pelvis of the kidney, the renal tubules, and even the 

 circulation after being injected into the bladder. 



The pressure in the bladder of a man may be made as high as 

 10 cm. of mercury during the act of micturition; about half this 

 amount is due to the contraction of the vesical walls alone, the 

 rest to the contraction of the abdominal muscles. A pressure of 

 16 to 26 mm. of mercury is required to open the sphincter of a 

 rabbit's bladder in life. 



Although the whole performance seems to us to be completely 

 voluntary, there are facts which show that it is at bottom a reflex 



