412 MANUAL OF PHYSIOLOGY. 



lumbar enlargement of the spinal cord, and of two sets of nerve 

 channels passing to and from this centre. The centres may be 

 said to be composed of functionally distinct parts a retaining 

 and evacuating part. The retaining centre causes the sphincter 

 muscle to contract. The evacuating centre can excite the de- 

 trusor to action while the sphincter is relaxed by the inhibition 

 of its exciting centre. One set of nerve channels communicates 

 between these centres and the urinary organs, and the other be- 

 tween the cord centres and the cerebral hemispheres. That which 

 connects the special lumbar centres with the bladder, contains 

 motor (efferent) fibres of two kinds, going to the antagonistic 

 muscles, the sphincter vesicae, and the detrusor urinse respectively, 

 and sensory (afferent) fibres of different kinds; those going from 

 the bladder to the nerve cells in the cord which stimulate them 

 and cause the sphincter to remain tonically contracted, and pass- 

 ing from the mucous membrane of the urinary passages to these 

 ganglionic cells in the cord are two sets; one of which excites 

 the contractions of the detrusor urinse and the other inhibits the 

 tonic action of the retaining centre. 



The action of the ganglionic cells that stimulate the sphincter 

 muscle can, to a certain extent, be either aided or checked by 

 means of cerebral influences, so that two kinds of fibres a 

 stimulating and an inhibitory one must pass from the hemi- 

 spheres to the micturating centre in the cord. 



Those cells which govern the motions of the detrusor seem to 

 be least under voluntary control, and are probably only stimulated 

 to action under normal circumstances by the impulses arising from 

 the urinary passages, and hence are simply reflex centres. 



The effect of certain emotions on the act of micturition seems 

 to show that those ganglion cells in the cord which cause the 

 bladder to contract are connected with the higher centres. Thus, 

 extreme terror (in a dog at least) often causes a forcible expul- 

 sion of urine, and great anxiety or impatience seems in man often 

 to have a checking influence, causing great delay in initiating 

 micturition. 



