1212 PHYSIOLOGY 



impeded their action. The relaxation of the sphincter must therefore 

 be brought about by impulses descending the pelvic visceral nerves, 

 which we may regard as motor to the detrusor and inhibitory to the 

 sphincter of the bladder. 



Section of the nerve on one side causes no abnormality in micturition. 

 After three weeks, stimulation of the intact nerve causes contraction of the 

 whole bladder, owing to the outgrowth of preganglionic fibres from the sound 

 trunk to the decentralised ganglia of the opposite side (Elliott). Section of 

 both nerves paralyses micturition, but power of partial evacuation of the 

 bladder may return in a few weeks. If now the hypogastrics be cut, or 

 even the sacral cord extirpated, the bladder is not completely paralysed, but 

 its evacuation becomes unconscious and incomplete. 



ACTION OF THE HYPOGASTRIC NERVES. These nerves, which 

 are derived from the sympathetic system, show marked differences in their 

 action, according to the animal which is the subject of investigation. In 

 the dog the hypogastric nerves cause a strong contraction 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 bla*dder 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 action. The hypo- 

 gastrics 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 



