Vl] THE EXCRETORY ORGANS 77 



skin is considerable the quantity of urine secreted is diminished. 

 Just as the v^essels in tlie skin are under the influence of vaso- 

 constrictor nerve fibres which react to external cold, so also the 

 vessels supplying the kidneys are controlled b}^ nerve fibres which 

 regulate the supply of blood and the consequent secretion of 

 urine. Thus if the renal nerves are cut, the renal arteries dilate, 

 and the flow of urine increases. On the other hand, if the renal 

 nerves are stimulated the vessels are constricted, and uruiary 

 secretion is diminished. 



It is believed that the glomeruli act somewhat after the 

 manner of a filter and allow the m ater and salts of the blood 

 plasma to pass through. At the same time the cells of the 

 kidney have the definite property of picking out certain substances 

 from the blood and permitting these to be excreted while other 

 substances (e.g. the proteins in solution in the blood) are not 

 normally allowed to pass through. Further, it has been established 

 that certain substances (e.g. pigments such as indigo carmine, 

 and probably some of the substances present in normal urine) 

 are excreted bj^ the epithelial cells lining the convoluted tubules 

 and not by the glomeruli. 



Certain substances known as diuretics promote urinary 

 secretion within a very short time after they are taken into the 

 system, probably through their action on the nerve fibres supply- 

 ing the renal vessels. Thus it is well known that alcohol and 

 various drugs have such an action, and that tea, coffee, etc. 

 contain diuretic substances. 



The urine is driven along the ureters into the bladder largely 

 as the result of pressure by quantity. The ureters are lined by 

 a stratified epithelium surrounded by unstriated muscle which 

 assists in the passage of the urine into the bladder. The walls of 

 the latter organ also contain an abundant supply of unstriated 

 muscle (lined internally by a stratified epithelium). Around the 

 neck of the bladder is a sphincter which is kept contracted 

 excepting during micturition or the evacuation of urine. During 

 micturition the sphincter relaxes, while the walls of the bladder 

 contract. The abdominal muscles also contract, and in the 

 male the perineal muscles. Normally in animals micturition is 

 a reflex act presided over by a centre in the lumbar part of the 

 spinal cord, the accumulation of urine in the bladder being the 

 necessary stimulus for inducing the reflex. In man, however, and 



