THE NERVOUS SYSTEM 



muscles. When evacuation of the bladder occurs sensory impulses pass to 

 that part of the center which discharges impulses to the bladder and to cer- 

 tain accessory muscles which cause their contraction; and impulses pass to 

 that part of the center which inhibits the tonic action on the sphincter ure- 

 thrae 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 bladder and accessory muscles. The cerebrum may exert its influence 

 on the reflex not only by stimulating the center to action, but also by inhibit- 

 ing its action. 



The Genito-spinal Center. The presence of the genito-spinal center is 

 proven by the fact that dogs, and even man, are known to discharge semen 

 when the lumbar cord is severed and all voluntary motion and sensibility are 

 lost. The center situated in the lumbar enlargement of the spinal cord is 

 stimulated to action by sensory impressions from the glans penis. Efferent 

 impulses from the center excite the successive and co-ordinate contractions of 

 the muscular fibers of the vasa deferentia and vesiculae seminales and of the 

 accelerator urinae and other muscles of the urethra; and a forcible expulsion 

 of semen takes place, which, in cases of paraplegia, are not felt. 



The Erected Center. This center is also situated in the lumbar region and 

 is a vascular center, already described in the chapter on Circulation. It 

 is reflexly excited to action by the sensory nerves of the penis, and also in the 

 normal animal by impulses passing down from the cerebrum. Efferent 

 impulses produce dilatation of the vessels of the penis. 



The Parturition Center. The center for the expulsion of the contents of 

 the uterus in parturition is situated in the lumbar spinal cord rather higher 

 up than the other centers already enumerated. The stimulation of the 

 uterus may, under certain conditions, excite the center to send out impulses 

 which produce a contraction of the uterine walls and expulsion of the con- 

 tents of the cavity. The center is independent of the will since delivery 

 takes place in paraplegic women, and also while a patient is under the influ- 

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

 abdominal and thoracic muscles assist; their action being for the most part 

 reflex and involuntary. 



Inhibition of Reflex Actions. Movements such as are produced by stimu- 

 lating the skin of the lower extremities in the human subject, after division 

 or disorganization of a part of the spinal cord, do not always occur when the 

 cerebrum is active and the connection between the cord and the brain is in- 

 tact. The reflex which would occur in the animal with spinal cord only is 

 suppressed or inhibited in the normal animal through the regulative action of 

 the higher cerebral centers. When one is anxiously thinking, even slight 

 stimuli may produce involuntary and reflex movements. So, also, during 

 sleep, such reflex movements may be observed, when the skin is touched or 

 tickled; for example, when one touches the palm of the hand of a sleeping 



