THE SECRETION OF URINE 483 



third of the ureter was believed to be devoid of a local ganglionated 

 nervous mechanism. Such, however, is now known to exist, and it 

 is highly probable that the smooth muscle of the ureter executes 

 these rhythmic peristaltic movements by virtue of a local nervous 

 mechanism. 



Although the ureters are supplied by extrinsic nerves, the exact 

 action of these is somewhat doubtful. It is stated that stimulation 

 of the splanchnic fibres, which reach the ureter through the renal 

 plexus, produce acceleration of the upper end of the ureter, while 

 stimulation of the hypogastric nerves has a similar acceleratory effect 

 upon the lower end of the ureter. 



The ureters enter the bladder obliquely at the upper corners of 

 the trigone of the bladder. This oblique course prevents a regurgita- 

 tion of urine. The orifice of the urethra is closed by the thickened 

 circular fibres at the base of the bladder the internal sphincter 

 and by the voluntary muscle the compressor urethrse outside the 

 bladder. The urine, therefore, gradually accumulates in the bladder, 

 and this gradually relaxes to accommodate its' load. The incoming 

 urine raises the pressure within the bladder up to about 15 to 20 c.m. 

 of H^O. At this point the desire to micturate usually manifests itself, 

 and the urine is voided. If, however, this be not done, the bladder 

 further relaxes, and the desire passes away for the time being. 



The Act of Micturition. During the time that urine is accumu- 

 lating within the bladder the organ performs rhythmic movements. 

 As the organ fills, these gradually increase in force, until some urine 

 is forced past the internal sphincter, and then micturition may reflexly 

 take place. This is the case in the decerebrate or spinal animal, or 

 in the involuntary micturition of children with weak control. 

 Normally, however, the reflex is curbed, and when there is desire to 

 micturate, the passage of urine into the first part of the urethra is 

 aided by the voluntary efforts of the individual. The intra -abdominal 

 pressure is raised by closing the glottis, so holding the diaphragm 

 in the inspiratory position, and by contracting the muscles of 

 the abdominal wall. The passage of a few drops of urine through 

 the internal sphincter stimulates the sensory nerve-endings of the 

 pelvic nerve. As a result of this, the sphincter of the bladder is 

 reflexly inhibited, while the body of the bladder contracts down and 

 voids its contents. 



Micturition is therefore a reflex act, the centre for which is situated 

 in the lumbar spinal cord. This centre is, in the adult, under the 

 control of the will, but in the new-born this is not the case. 

 A baby, for the first few months of its life, passes urine in response 

 to the demands of the lower, and not of the higher, reflex arc. It 

 has to be taught control. In some, the nervous mechanism concerned 

 in this reflex is overexcitable, so that even in adult life, when the 

 cerebral control is cut off, either by sleep or excitement, urine is 

 reflexly voided. 



The efferent nerves concerned in the reflex are chiefly the pelvic 

 nerves. In some animals the hypogastric' nerves are also con- 



