260 I'lIYSIOLOGY FOR DENTAL STUDENTS. 



each having its own center. This fact justifies the statement 

 which we have already made that the brain stem is really an up- 

 ward prolongation of the spinal cord, and just as we saw thai 

 each posterior root of the spinal cord is characterized by pos- 

 sessing a ganglion, so also is there a ganglion in the senior n 

 divisions of the cranial nerves. This ganglion, however, is often 

 difficult to find. The nerve cells which compose it unite with 

 the fibers of the sensory root by a T-shaped junction, and the 

 fibers terminate by synapsis around the cells of the sensory 

 nuclei. The ganglion of the fifth nerve is the Gasserian. Those 

 for the eighth are the ganglia found in the cochlea and internal 

 auditory meatus (Scarpa's ganglion). The ganglia of the ninth 

 and tenth nerves are situated along the course of the nerves. 



The approximate position of the various ganglia will be best 

 learned by consultation of the accompanying diagram (Fig. 47). 



In the brain stem there are three sensory or afferent nuclei, a 

 long, combined one for the ninth, tenth and eleventh nerves, ex- 

 tending practically from the upper to the lower limits of the 

 medulla, one for the eighth in the center of the pons, and a 

 very long one for the fifth, extending from near the upper limit 

 of the pons down into the spinal cord. The motor or <ff<r<nt 

 nuclei for the third, fourth, sixth and twelfth nerves are com- 

 posed of cells shaped like those of the anterior horn of the spinal 

 cord. They lie near the middle line and extend throughout 

 the whole length of medulla and pons.' The motor nuclei of the 

 fifth, seventh, ninth, tenth and eleventh lie outside the above. 



It is important that the following functions of fhrst mrrcs 1>< 

 studied by dental students : 



THE THIRD NERVE. The third nerve controls: (1) the mus- 

 cles of accommodation inside the eye; (2) all of those which 

 are attached to the outside of the eyeball, except the muscle 

 which moves it out (external rectus), and the one which rotates 

 it down and out (the superior oblique) ; and (3) the elevator 

 muscle of the eyelids (levator palpebras). When the third nerve 

 is paralyzed, the symptoms are therefore: (1) drooping of the 

 vvelid (ptosis) so that the chin is tilted upward when the pa- 

 iient looks at anything; (2) inability to see clearly unless when 



