THE FACIAL NERVE. 1251 



The motor part is by far the larger of the two and constitutes both anatom- 

 ically and functionally the more important portion of the nerve. The deep origin 

 of the motor root is from the facial nucleus (Fig. 933), an oval collection of some 

 half dozen groups of large multipolar neurones, which measures about 5 mm. in 

 length, and is situated in the posterior portion of the tegmentum of the pons. It 

 lies within the formatio reticularis medial to the spinal root of the trigeminal nerve 

 and, in its lower part, close to the fibres of the corpus trapezoides ; higher up it is 

 tilted dorsally and separated from these fibres by the superior olive, to the upper 

 and outer side of which it lies. Although the facial nucleus is situated close to the 

 superficial origin of the seventh nerve, the root-fibres instead of taking a direct 

 route to the ventral surface of the brain-stem follow a devious course. The intra- 

 cerebral part of the nerve has been divided for convenience of description into a 

 radicular, an ascending and an emergent portion. 



The radicular portion consists of numerous loose fasciculi of root-fibres which arise from 

 the dorso-lateral aspect of the nucleus of origin and pass backward and slightly inward. The 

 upper fibres stream over the dorso-lateral surface of the nucleus of the abducent nerve and then, 

 with the other fibres of the motor root, bend mesially along the floor of the fourth ventricle. 

 As they near the mid-line they turn sharply upward and assemble to form a solid strand, the 

 ascending portion of the seventh nerve. This upward course continues for about 5 mm. and in 

 this situation the nerve is separated from the floor of the fourth ventricle, beneath which it runs 

 within the funiculus teres, only by the lining ependyma and lies immediately dorsal to the pos- 

 terior longitudinal bundle and mesial to the abducent nucleus. The nerve now bends abruptly 

 outward at a right angle and enters upon the emergent portion of its course, during which it 

 crosses the dorsal aspect of the abducent nucleus and passes backward and ventro-laterally, be- 

 tween its own nucleus of origin and the spinal root of the trigeminal nerve, to gain the exterior 

 of the brain-stem (Fig. 1066). 



The central and cortical connections of the motor part of the facial nerve include paths 

 whereby the nucleus is brought under the influence of the reflex and the cortical centres. 

 (a) While not beyond dispute, it is probable that a limited number of root-fibres are connected 

 with the facial nucleus of the opposite side. () The evidence adduced from clinical observa- 

 tions and pathological findings points to the existence of a special group of cells from which 

 arise the fibres supplying the orbicularis palpebrarum and frontalis muscles. These fibres, 

 sometimes called the superior facial nerve, may retain their functional integrity notwithstanding 

 the occurrence of paralysis of the other muscles supplied by the seventh nerve, (c) The latter, 

 morever, is brought into association with the visual and auditory centres by paths, probably 

 within the posterior longitudinal fasciculus, by which the facial cells respond to the impulses of 

 sight and hearing, as shown by the automatic closure of the eyelids, (d) Connection with the 

 hypoglossal nerve has been assumed in explanation of the coordinated action of the muscles of 

 the lips with those of the tongue, (e) The motor facial nucleus is brought under the influence of 

 the cortical area by the cortico-bulbar fibres which proceed as axones from the motor neurones 

 lying within the lower part of the precentral convolution. These fibres descend in company 

 with the cortico-spinal tracts to appropriate levels and end around the radicular cells of the 

 facial nucleus of the opposite side, a few fibres, however, probably terminating in the nucleus 

 of the same side. 



The superficial origin of the motor root is at the lower border of the pons, to 

 which it may be adherent, in a groove between the inferior olive and the inferior 

 cerebellar peduncle (Fig. 1046). Just above the facial as it escapes, often as several 

 strands of root-fibres, lies the fifth nerve and to its outer side is the auditory, from 

 which it is separated by the sensory root of the seventh. 



Emerging from the surface of the brain-stem, the nerve passes outward, its 

 motor and sensory roots ununited, to the internal auditory meatus, through which 

 it passes above and anterior to the auditory. At the bottom of the meatus the 

 seventh and eighth nerves part company, the facial entering the facial canal, whose 

 course it follows throughout. At first the canal is directed horizontally outward, 

 between the cochlea and the vestibule, until it reaches the mesial tympanic wall. It 

 then bends abruptly backward, passes above the fenestra ovalis and turns down- 

 ward, behind the pyramid, in the posterior wall of the tympanic cavity, to end at the 

 stylo-mastoid foramen. The point where the canal turns backward marks a corre- 

 sponding bend, the genu, of the facial nerve. In this situation is found the geniculate 

 ganglion and here the two roots fuse to form a single trunk. After emerging from 



