THE NERVOUS SYSTEM, 579 



organs are under the control of certain circumscribed portions of the 

 cord. The special centres are the following (on each side ) : 



(a.) The Defcecation, or Ano- Spinal centre. The mode of action of 

 the ano-spinal centre appears to be this. The mucous membrane of the 

 rectum is stimulated by the presence of fasces or of gas in the bowel. 

 The stimulus passes up by the afferent nerves of the haemorrhoidal and 

 inferior mesenteric plexus to the centre in the cord, situated in the 

 lumbar enlargement, and is reflected through the pudendal plexus to 

 the anal sphincter on the. one hand, and on the other to the muscular 

 tissue in the wall of the lower bowel. In this way is produced a relaxa- 

 tion of the first and a contraction of the second, and expulsion of the 

 contents of the bowel follows. The centre. in the spinal cord is par- 

 tially under the control of the will, so that its action may be either 

 inhibited or augmented. The action may be helped by the abdominal 

 muscles which are under the control of the will, although under a strong 

 stimulus they may also be compelled to contract by reflex action. 



(b.) The 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 stimulated to 

 action by impulses descending from the brain, or reflexly by the pres- 

 ence 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 posterior 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 evacu- 

 ation of the bladder is to occur, impulses are sent to one part of the 

 centre on the one hand, and from it to the bladder and to certain other 

 muscles which cause their contraction, and on the other to the other 

 part of the centre, inhibiting its action on the sphincter urethras which 

 procures its relaxation. The way having been opened by the relaxation of 

 the sphincter, the urine is expelled by the combined action of the blad- 

 der and accessory muscles. The cerebrum may act not only in the way of 

 stimulating the centre to action, but also in the way of inhibiting its 

 action. The abdominal muscles may be called into action as in defse- 

 cation. 



(c.) The 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 impulses 

 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 urinae and other muscles of the urethra; and a forcible expul- 



