894 



CENTRAL NERVOUS SYSTEM 



somatic as well as visceral sources, it is interesting that the functional 

 acts which these reflexes may perform find no representation in the 

 motor areas of the cerebral cortex, and that in only exceptional cases 

 can they be initiated voluntarily. 



The efferent side of the arc is interesting because of its anatomical 

 organization. The motor path connecting the spinal cord with the 

 smooth muscles and glands of the viscera consists of two neurons. The 

 cell body of the first lies in the lateral horn of the gray matter of the 

 cord. It is known as an internuncial neuron and its fiber, which is med- 

 ullated, is called a connector fiber or preganglionic fiber. These fibers 

 terminate in synapse with the cell body of the second neuron or effector 



Fig. 218. Diagram illustrating the different arrangements of the internuncial neurons of the 

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

 laterals) around a cell 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 autonomic 

 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.) 



neuron, which lies in an outlying ganglion. These neurons give off 

 fibers known as postganglionic fibers which extend to the effector which 

 is innervated. The postganglionic fibers are not medullated (except in 

 the case of the path to the sphincter pupillaB from the third nerve). 



Connector fibers are given off from three distinct regions of the cen- 

 tral nervous system. The anatomical arrangements and physiological 

 activities of these regions are distinctive. The bulbar outflow consists of 

 connector fibers lying chiefly in the vagus, but also in the third, seventh, 

 ninth, and eleventh cranial nerves. The sacral outflow consists of fibers, 

 leaving the cord with the second to fourth sacral nerves, which join 

 to form a common nerve trunk (the pelvic nerve or nervus erigens) 



