226 URINE 



1. The first group act as diuretics by being present in 

 blood in excess. According to Cushny they cause diuresis 

 in two ways. 



(a) By Dilution diuresis. By this is meant the diuresis 

 brought about by dilution oi the colloid content, and 

 therefore by diminution of the osmotic pressure of the 

 blood. Among the substances which act in this manner 

 are ivater and the Threshold Bodies such as chlorides. 



(b) By Osmosis. While all saUne substances cause a 

 certain degree of dilution diuresis, some, and in particular 

 the No-Threshold Bodies such as sulphates, act also by 

 exerting through their osmotic pressure a restraining 

 influence on the process of reabsorption of water. 



2. Foreign Substances. — The principal members of this 

 class are caffeine, digitalis and pituitary extract. Their, 

 main diuretic action is probably an indirect one through 

 their effect upon the circulation. Whether they also 

 stimulate the renal cells is undecided. 



THE EXCRETION OF URINE— MICTURITION 



The passage of urine down the ureters is due partly to 

 gravity, partly to pressure of fluid from the tubules, partly 

 to waves of contraction. The ureters pass through the 

 bladder wall obhquely — by this arrangement the pressure 

 inside the bladder prevents the return of urine into the 

 ureters. 



The orifice of the bladder is guarded by three sphincters. 

 Of these, one, the most proximal, is the sphincter trigoni, 

 situated at the neck of the bladder. This is essentially an 

 involuntary muscle, though there may be some vohtional 

 control over it. The other two sphincters, the compressor 

 urethrae and the bulbo-cavernosus, are voluntary. 



Nerve Supplyf. — The bladder is innervated (1) by sym- 

 pathetic fibres emerging from the 11th and 12th dorsal 

 and 1st and 2nd lumbar segments and reaching it by the 

 inferior mesenteric ganglion and hypogastric nerves, 



