THE PHYSIOLOGY OF MICTURITION 1297 



in their action, according to the animal which is the subject of 

 investigation. In the dog the hypogastric nerves cause a strong con- 

 traction of the muscle fibres at the base of the bladder, especially of 

 the trigonum and of the sphincter trigoni. When these nerves are 

 stimulated simultaneously with the pelvic visceral nerves a great rise 

 of intravesical tension may be induced without any flow of urine taking 

 place. In some cases prolonged stimulation of these nerves in the dog 

 causes apparently an active relaxation of the sphincter of the bladder. 

 On the other hand, in the rabbit and the cat these nerves cause 

 an inhibition of the bladder wall. In other animals they may excite 

 either contraction or relaxation (or both) of the detrusor. They always 

 contain motor fibres to the sphincter of the bladder as well as to the 

 constrictor fibres surrounding the urethra. Where this effect is tonic, 

 micturition must be associated with a central inhibition of their 

 tonic activity. On the other hand, the retention of urine and the 

 distension of the bladder may be aided by a reflex dilatation of the 

 bladder wall and a reflex constriction of the sphincter in each case 

 excited through these nerves. Normally therefore both sets of nerves 

 are called into play. The hypogastrics play an especially active part 

 during the accumulation of urine in the bladder, while the pelvic 

 visceral nerves are necessary for the complete evacuation of the bladder 

 which occurs at micturition. 



THE CENTRAL CONTROL OF THE BLADDER 



The nerve-centre which presides over the tonus and contraction 

 of the bladder is situated in the lumbo-sacral spinal cord. If this 

 centre and its connections be intact, micturition may be carried out 

 normally even after section of the cord in the dorsal region. The 

 centre can be excited reflexly by stimulation of almost any sensory 

 nerve, such as the sciatic or the fifth nerve. In many cases where, 

 in consequence of obstruction to the passage of impulses from the 

 higher parts of the central nervous system, micturition is delayed, 

 this act may be excited by the application of cold or hot sponges 

 to the perineum, and it is well known that almost any irritation of 

 the pelvic organs in children may give rise to reflex involuntary 

 micturition. 



In the adult the processes of retention and evacuation of urine are 

 modified and controlled by voluntary effort. The normal action of 

 the sphincter mechanism may be aided by the contraction of the 

 perinseal muscles which keep the urethra closed. The reflex process 

 of evacuation may be set in motion by voluntary contraction of the 

 abdominal muscles, by which the pressure in the bladder is increased 

 and the normal sphincter action overcome. It is probable too that 

 the individual has a certain degree of voluntary power over the 



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