80 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 physiology 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 

 ends, the efferent fibers at the central end, the afferent at the 

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

 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 

 directly to a nerve trunk, excite the nerve fibers. Such reagents 

 are concentrated solutions of the neutral salts of the alkalies, acids, 

 alkalies, glycerin, etc. This method of stimulation is not, however, 

 of much practical value in experimental work, since it is difficult or 

 impossible to control the reaction. 



2. Mechanical stimuli. A blow or pressure or a mechanical in- 

 jury of any kind applied to a nerve trunk also excites the fibers. 

 This method of stimulating the fibers is also difficult to control 

 and has had, therefore, a limited application in experimental work. 

 The mechanical stimulus is essentially a pressure stimulus, and the 

 difficulty lies in controlling this pressure so that it shall not actually 

 destroy the nerve fiber by rupturing the delicate axis cylinder. 

 Various instruments have been devised by means of which light 

 blows may be given to the nerve, sufficient to arouse an impulse, 

 but insufficient to permanently injure the fibers. The results ob- 

 tained by this method have been very valuable in physiology as con- 

 trols for the experiments made by the usual method of electrical 

 stimulation. It may be mentioned also that under certain condi- 



