526 THE NERVOUS SYSTEM 



The Ano-Spinal or Dejecation Center. The mode of action of the ano- 

 spinal center appears to be this: The mucous membrane of the rectum is 

 stimulated by the presence of feces or of gas in the large bowel. The stim- 

 ulus passes up by the afferent nerves of the hemorrhoidal and inferior mesen- 

 teric plexuses to the center situated in the lumbar enlargement of the cord, 

 and is reflected through the pudendal plexus to the anal sphincter, and to the 

 muscular tissue in the wall of the lower bowel. In this way there is produced 

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

 of the contents of the bowel follows. The center in the spinal cord is 

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

 inhibited or augmented. The action ma'y be helped by the abdominal 

 muscles, which are voluntary muscles, but are also stimulated to contract by 

 reflex action. 



The Vesico-Spinal or Micturition Center. The vesico-spinal center acts 

 in a very similar way to that of the ano-spinal. The center is also in the 

 lumbar enlargement of the cord. It is stimulated to action reflexly by the 

 presence of urine in the bladder. The action may be voluntary and is excited 

 by the sensation of distention of the bladder by the urine. The sensory fibers 

 concerned are the posterior roots of the lower sacral nerves. The action of the 

 spinal center is double, or it may be supposed that the center consists of two 

 parts, one of 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 occurs impulses pass to that part of the center 

 which discharges impulses to the bladder and to certain accessory muscles 

 which cause their contraction; and impulses pass to that part of the center 

 which inhibits the tonic action on the sphincter urethrae 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 inhibiting 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 coordinate 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, over which the cerebral centers have little or no control, 

 and 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 



