86 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 spiral ganglion of the cochlea, cochlear branch, and from 

 those 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 mixed, 

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

 while the sensory fibers arise in the superior and petrosal ganglia 

 found on the nerve at its emergence from the skull. The tenth is a 

 mixed nerve, its efferent fibers arising in motor nuclei in the me- 

 dulla, the afferent fibers in the nerve cells of the ganglia lying upon 

 the trunk of the nerve at its exit from the skull (ganglion jugulare 

 and nodosum). The eleventh and twelfth cranial nerves contain 

 only efferent fibers that arise from motor nuclei in the medulla. 



It will be seen from these brief statements that in all the nerve 

 trunks of the central nervous system — that is, the spinal and the 

 cranial nerves — the cells of origin of the efferent fibers lie within 

 the gray matter of the brain or cord, while the cells of origin of the 

 afferent fibers lie in sensory ganglia outside the central nervous 

 system, — namely, in the posterior root ganglia for the spinai 

 nerves, in the ganglion semilunare (Gasseri), the g. geniculi, the 

 g. spirale, the g. vestibulare, the g. superius and g. petrosum of the 

 glossopharyngeal, and the g. jugulare and g. nodosum of the vagus. 

 These various sensory ganglia attached to the cranial nerves corre- 

 spond essentially in their structure and johysiology with the posterior 

 root ganglia of the spinal nerves. 



Independent Irritability of Nerve Fibers. — Although the 

 nerve fibers under normal conditions are stimulated only at their 

 *nds, the efferent fibers at the central end, the afferent at the 

 peripheral end, yet any nerve fiber may be stimulated by artificiaJ 

 means at any point in its course. Artificial stimuli capable of 

 affecting the nerve fiber — that is, capable of generating in it a nerve 

 impulse which then propagates itself along the fiber — ^may be divided 

 into the following groups : 



1. Chemical stimuli. Various chemical reagents, when applied 



