THE CEEEBRO-SPINAL NERVOUS SYSTEM. 489 



(b. ) Centre for Micturition, or the Vesico-Spinal centre. The vesico- 

 spinal centre acts in a very similar way to that of the ano-spinal. The 

 centre is also in the lumbar enlargement of the cord. It may be stimu- 

 lated to action by impulses descending from the brain, or reflexly by 

 the presence of urine in the bladder. The action of the brain may be 

 voluntary, or it may be excited to action by the sensation of distention 

 of the bladder by the urine. The sensory fibres concerned are the pos- 

 terior roots of the lower sacral nerves. The action of the centre thus 

 stimulated is double, or it may be supposed that the centre consists of 

 two parts, one which is usually in action and maintains the tone of the 

 sphincter, and the other which causes contraction of the bladder and 

 other muscles. When evacuation of the bladder is to occur, impulses 

 are sent on the one hand to its muscles and to certain other muscles, 

 which cause their contraction, and on the other to the sphincter urethrae 

 which procures its relaxation. The way having been opened by the re- 

 laxation of the sphincter, the urine is expelled by the combined action 

 of the bladder and accessory muscles. The cerebrum may act not only 

 in the way of stimulating the centre to action, but also in the way of in- 

 hibiting its action. The abdominal muscles may be called into action as 

 in defecation. 



(c.) Centre for Emission of Semen, or Genito- Spinal centre. The 

 centre situated in the lumbar enlargement of the spinal cord is stimulated 

 to action by sensory impressions from the glans penis. Efferent im- 

 pulses from the centre excite the successive and co-ordinate contractions 

 of the muscular fibres of the vasa deferentia and vesiculae seminales, and 

 of the accelerator urinse and other muscles of the urethra; and a forcible 

 expulsion of semen takes place, over which the mind has little or no con- 

 trol, and which, in cases of paraplegia, may be unfelt. 



(d.) Centre for tlie Erection of the Penis. This centre is also situ- 

 ated in the lumbar region. It is excited to action by the sensory nerves 

 of the penis. Efferent impulses produce dilatation of the vessels of the 

 penis, which also appears to be in part the result of a reflex contraction 

 of the muscles by which the veins returning the blood from the penis are 

 compressed. 



(e.) Centre for Parturition. The centre for the expulsion of the 

 contents of the uterus in parturition is situated in the lumbar spinal cord 

 rather higher up than the other centres already enumerated. The stimu- 

 lation of the interior of the uterus by its contents may, under certain 

 conditions, excite the centre to send out impulses which produce a con- 

 traction of the uterine walls and expulsion of the contents of the cavity. 

 The centre is independent of the will, since delivery can take place in 

 paraplegic women, and also whilst a patient is under the influence of 

 chloroform. Again, as in the cases of defecation and micturition, the 



