CRANIAL NERVES. 



821 



parts it represents 'the accessory or anterior nucleus of the hypo- 

 glossus; farther up, the proper nucleus of the facial; and in the 

 pons there is found the motor root of the trigeminus. 



The gray masses of the posterior horns of the cord, prolonged 

 into the medulla oblongata and cut by the sensory decussation or 

 fillet, form the sensitive nuclei of the cranial nerves. The base of 

 the posterior horn forms the sensory nucleus of the mixed nerves, 

 namely: glosso-pharyngeal, vagus, and spinal accessory. Above 

 these nuclei there is a gray layer which represents the oblongata cen- 

 ter of the internal root of the auditory; higher still arises the sen- 

 sory nucleus of the trigeminus. The head of this horn, under the 

 name of gray nucleus of Eolando, ascends in the pons to form the 

 ascending root of the trigeminus. 



Among the twelve pairs of cranial nerves, ten have their points 

 of origin in cells of the gray matter of the cord. This latter has 

 been prolonged into the medulla oblongata and pons in the form of 

 four motor and sensory columns. Thus these cranial nerves are com- 

 parable to spinal nerves. 



COMPAKABLE TO SPINAL NERVES. The law of double root is as 

 applicable here as to the spinal nerves. Those nerves destined for 

 movement originate in the prolongations of the anterior horns, while 

 those which preside over sensibility take their origin in gray matter 

 of the medulla and pons which has sprung from the posterior horns 

 of the spinal cord. 



Point of Difference. There is this difference, however, between 

 cranial and spinal nerves: In the spinal nerves, the two roots are 

 intimately united just outside of the spinal-cord substance to form 

 a mixed nerve. In the case of the cranial nerves the posterior sen- 

 sory roots and the anterior motor roots remain, for the most part, 

 separated to form nerves that are either exclusively motor or exclu- 

 sively sensory. In other words, the cranial nerves represent the dis- 

 sociated spinal nerves in which the anterior and posterior roots 

 remain habitually isolated to form nerves which are either fine con- 

 ductors of motion or sensation, dependent upon their source. 



In the hypoglossal alone are fulfilled the true characteristics, 

 for in numerous cases it is found to have a ganglion upon its pos- 

 terior root. 



The mesencephalon has been considered to possess parallel fea- 

 tures with the spinal cord, in that it is formed of a series of seg- 

 ments corresponding to the cranial nerves. As the student already 

 knows, each spinal nucleus has peripheral conductors which bring to 



