878 



THE CENTRAL NERVOUS SYSTEM 



in the voluntary, they remain within the spinal cord, and terminate on 

 the effector neurons, which are the anterior horn cells. 



The outflow from the spinal cord of involuntary internuncial fibers, 

 which we shall hereafter style connector fibers, occurs along the an- 

 terior spinal roots, but is somewhat irregular in distribution, because 

 it is interrupted in the cervical and lumbar regions, where the nerve 

 plexus for the extremities come in. There are, therefore, three main 

 regions of outflow for the connector fibers a thoracicolumbar, a bulbar, 

 and a sacral; and the fibers (Fig. 229) do not behave in the same manner 

 in all of them. The fibers of the thoracicolumbar region form the so- 

 called sympathetic system, and run by the corresponding white rami 

 communicantes either immediately to the ganglia of the sympathetic 



Post root 

 gang. 



us 



Fig. 228. Diagram to illustrate the different arrangements of the internuncial neurons of the 

 voluntary and involuntary nervous systems. In both systems the afferent fiber terminates (by col- 

 laterals) around a ceU of the gray matter of the cord. In the voluntary system this cell is sit- 

 uated in the posterior horn, and its axon travels to an anterior horn cell. In the involuntary 

 system, on the other hand, it is located in the lateral horn, and its axon leaves the cord by the 

 anterior root and travels by the white ramus into a sympathetic ganglion, where it connects with 

 a nerve cell, whose axon forms the postganglionic fiber. . (From Gaskell.) 



chain, or by the splanchnic nerves to the abdominal ganglia. In the gan- 

 glia are situated the cells of the effector neurons. The fibers of the sacral 

 region connect with effector neurons, forming the pelvic ganglionic group 

 (pelvic nerves, nervi erigentes) ; and those of the bulbar outflow with 

 effector neurons located either peripherally or in the ganglia of the 

 vagus and the seventh, ninth, and eleventh cranial nerves. In the mid- 

 brain there is a fourth group of involuntary connector fibers running to 

 effector neurons found in the ciliary ganglia. 

 The anterior roots of many of the spinal nerves are therefore not 



