THE PHENOMENON OF CONDUCTION. 79 



eral tissues in the corresponding spinal nerve in which they form the 

 peripheral sensory nerve fibers. It follows that a section or lesion 

 of the posterior root will result in a degeneration of the branch 

 entering the cord, this branch having been cut off from its nutri- 

 tive relationship with its cells of origin. The degeneration will in- 

 volve the entire length of the branch and its collaterals to their 

 terminations among the dendrites of other spinal or bulbar neurons 

 (see the chapter on the spinal cord). After a lesion of this sort 

 the stump of the posterior root that remains in connection with 

 the posterior root ganglion maintains its normal structure. On the 

 other hand, a section or lesion involving the spinal nerve will be 

 followed by a degeneration of all the fibers, efferent and afferent, 

 lying to the peripheral side of the lesion, since these fibers are cut 

 off from connection with their cells of origin, while the fibers in the 

 central stump of the divided nerve will retain their normal structure. 

 Afferent and Efferent Fibers in the Cranial Nerves. The 

 first and second cranial nerves, the olfactory and the optic, contain 

 only afferent fibers, which arise in the former nerve from the olfac- 

 tory epithelium in the nasal cavity, in the latter from the nerve 

 cells in the retina. The third, fourth, and sixth nerves contain 

 only efferent fibers which arise from the nerve cells constituting 

 their nuclei of origin in the midbrain and pons. The fifth nerve 

 resembles the spinal nerves in that it has two roots, one containing 

 afferent and the other efferent fibers. The efferent fibers, consti- 

 tuting the small root, arise from nerve cells in the pons and mid- 

 brain, the afferent fibers arise from the nerve cells in the Gasserian 

 ganglion. This ganglion, being a sensory ganglion, is constituted 

 like the posterior root ganglia. Its nerve cells give off a single 

 process which divides in T, one branch passing into the brain by way 

 of the large root, while the other passes to the peripheral tissues as a 

 sensory fiber of the fifth nerve. The seventh nerve may also be 

 homologized with a spinal nerve. The facial nerve proper consists 

 of only efferent fibers, which arise from nerve cells constituting 

 its nucleus of origin in the pons. The geniculate ganglion, attached 

 to this nerve shortly after its emergence, is similar in structure to 

 the Gasserian or a posterior root ganglion. Its nerve cells send off 

 processes which divide in T and constitute afferent fibers in the 

 so-called nervus intermedius or nerve of Wrisberg. The eighth 

 nerve consists only of afferent fibers which arise from the nerve 

 cells in the spinal ganglion of the cochlea, cochlear branch, and from 

 there constituting the vestibular or Scarpa's ganglion, the vestibu- 

 lar branch. Both of these ganglia are sensory, resembling the 

 posterior root ganglia in structure. The ninth nerve is also mixed, 

 the efferent fibers arising from the motor nucleus in the medulla, 

 while the sensory fibers arise in the superior and petrosal ganglia 



