592 ORIGIN OF THE ENCEPHALIC NERVES. 



The facial nerve (portio dura of the seventh pair,) is con- 

 sidered the principal motor trunk to the fifth pair. It emerges 

 from the groove between the corpus pyramidale and olivare, but 

 may be traced backwards according to Solly, through the pons 

 varolii, to the inner side of, and in contact with, the sensory root 

 of the fifth pair of nerves. It then divides into two roots ; one 

 arising from the motor column* of the spinal cord, and the other 

 from the cerebeller or arciform fibres of the anterior column (see 

 fig. 224.) 



The motor externus or abductor oculi (sixth pair) proceeds 

 from the anterior narrowed part of the corpus pyramidale, and 

 is exclusively a nerve of motion. 



The trigeminus or fifth, is partly a mixed nerve, and arises 

 by two roots, one for sensation and one for motion. The former 

 called the portio major, constituting the great bulk of the nerve, 

 emerges, from between the transverse fibres of the pons varolii, 

 and can be traced down through the medulla oblongata, to the 

 posterior columns of the spinal cord, an inch and a half below 

 the pons. On the portio major, is formed exclusively the gang- 

 lion of Gasser ; upwards of a hundred filaments have been 

 counted in this root. 



The motorial root, portio minor, traverses the pons by a dis- 

 tinct aperture and appears to originate from the antero-lateral or 

 motorial portion of the spinal cord. These motor fibres join the 

 third branch of the fifth pair, beyond the ganglion of Gasser, 

 and supply the muscles of mastication. 



The patheticus or fourth nerve, arises from the valve of Vieus- 

 sens, near the tubercula quadrigemina, where it is placed in close 

 connexion with other nerves going to the eye and its auxiliary 

 parts ; viz., the optic and the motor oculi communis. 

 The motor oculi communis, or third nerve, emerges from the 

 inner side of the crus cerebri, but it takes its rise by two roots ; 

 one of which can be traced to the locus niger of the crus, and 

 is there continuous with the ascending motor columns of the 

 medulla oblongata ; the other passes round the outer margin of 

 the crus, just in front of the pons, and is closely connected with 

 that part of the origin of the optic nerve, nearest the peduncle. 



