482 THE NERVOUS SYSTEM. 



go out efferent impulses which cause contraction of the muscular 

 fibers of the vasa deferentia, seminal vesicles, and accelerator 

 urinae, the result of which is to produce an ejection of semen. 



Anospinal Center. The act of defecation is governed by the 

 anospinal center and has been already described (p. 266). 



Vesicospinal Center. The act of micturition is under the in- 

 fluence of the vesicospinal center. This act has been already 

 described (p. 430). 



Trophic Centers. It has already been seen that when nerve- 

 fibers are divided they undergo degeneration, and that this is ex- 

 plained by the fact that under these circumstances their connection 

 with certain nerve-cells is severed, and that they are thus deprived 

 of the nutritive influence which such centers exert. Such centers 

 are called trophic centers, and the cells of the anterior cornua of 

 the cord and the ganglia on the posterior roots of the spinal nerves 

 are familiar illustrations. That these are true trophic centers for 

 nerves seems to be beyond dispute, but this is an entirely different 

 question from that which deals with trophic nerves as regulating 

 the nutrition of tissues other than nerves. About the existence 

 of such nerves there is considerable doubt. 



Other Centers. Some writers describe a center for erection of 

 the penis, and locate it in the lumbar enlargement. The afferent 

 nerves from the penis cause this center to send out efferent im- 

 pulses by which the blood-vessels are dilated and the muscles are 

 compressed, thus preventing the return of the venous blood from 

 the penis and bringing about erection. A center for parturition 

 is described as being located in the lumbar region of the cord, 

 above the centers already mentioned ; under the influence of this 

 the muscular tissue of the uterus contracts at the proper time and 

 expels the fetus. Other reflex centers are described, but the 

 tendency to extend the number of such centers seems to be beyond 

 what the actual facts warrant. However, enough has been said 

 to show the great importance of the spinal cord as a nervous 

 center, independently of its function as a conductor of nervous 

 impulses to and from the brain. 



Functions of Spinal Nerves. Stimulation of an anterior 

 root causes contraction in the muscle to which it is distributed, 

 while its division is followed by a loss of motion in the same 

 muscle. In neither instance is sensation affected. If after the 

 division the distal portion of the nerve is stimulated, muscular 

 contraction will follow, while stimulation of the proximal end, 

 that which is in connection with the cord, will produce no effect. 

 The anterior roots are therefore efferent and motor, and are dis- 

 tributed to muscles. 



Stimulation of a posterior root causes a sensation of pain in the 

 part to which the nerve is distributed. Division of the root 

 causes a loss of sensation in that part. If after division the 



