THE CRANIAL OR ENCEPHALIC NERVES. 



721 



6. Sixth Pair (Abducentes), or External Oculo-motor Nerves. (Fig. 335, 5.) 



The external oculo-motor originates from the medulla oblongata, imme- 

 diately behind the pons Varolii, by from five to eight converging roots, 

 which appear to issue from between the inferior corpus pyramidale and the 

 lateral fasciculus of the medulla (Figs. 337, 338, &). 



It is directed immediately forward, leaves the pons Yarolii in lying close 

 to the inner side of the superior maxillary nerve, and traverses the sphe- 

 noidal canal, which already lodges the ophthalmic branch of the fifth pair 

 and the common oculo-motor nerve, to pierce the bottom of the orbit. It is 

 entirely expended in the external rectus (or abductor) muscle of the eye, 

 after giving off a small ramuscule to the external portion of the posterior 

 rectus. 



7. Seventh Pair, or Facial Nerves. (Figs. 110, 336, 337, 338.) 



The facial (portio dura) is a nerve exclusively motor at its orgin, but 

 which becomes mixed, during its course, by the addition of several sensitive 

 branches. 



Origin. It emanates from the medulla oblongata, immediately behind 

 the pons Varolii, and appears to originate at the external extremity of the 

 transverse band that margins the posterior border of that protuberance. 

 But if we attempt to trace its origin in the substance of the medulla 

 oblongata, we see the single fasciculus it constitutes, at its point of 

 emergence, descend into the groove of separation between the pons Varolii 

 and the above-mentioned band ; it then traverses nearly the whole thickness 

 of the medulla, passing between the lateral cord or column, and that portion 

 of the restiform body which is continuous with the large root of the fifth 

 pair. Arrived near the bottom of the fourth ventricle, the facial nerve 

 separates into several roots some 

 anterior, others posterior which 

 are soon lost in the cells forming 

 the corresponding nucleus. Among 

 these fibres are some which remain 

 isolated from the preceding, and, 

 passing the median line, enter the 

 facial nucleus of the opposite side 

 (Fig. 337). 



Course. Scarcely has the 

 facial nerve left the medulla 

 oblongata, before it is directed 

 outwards, to pass into the internal 

 auditory meatus, along with the 

 auditory nerve, which lies in con- 

 tact with it behind. It afterwards 

 enters the aqueduct of Fallopius, courses along it, and follows its inflexions, 

 which results in its forming a bend forward at a short distance from the 

 internal opening of the canal, and a curve whose concavity is anterior, on 

 its passage behind the cavity of the tympanum. On leaving the aqueductus 

 Fallopii by the stylo-mastoid foramen, it is hidden beneath the deep face 

 of the parotid gland, and continues to be inflected forward, passing between 

 that gland and the guttural pouch, and reaches the posterior border of the 

 inferior maxilla, where it issues from beneath the anterior margin of the 

 parotid to become superficial, and plo,ce itself on the masse ter muscle. 



Fig. 337. 



Sufi,' 



rey nucfeus 

 of Trn/iez~ 



SECTIQN MADE THROUGH THE SUMMIT OF THE 



MEDULLA OBLONGATA, SHOWING THE DEEP 

 ORIGINS OF THE FIFTH AND SIXTH NERVES, 

 AND PORTIO DURA OF THE SEVENTH. 



