NERVUS FACIALIS AND NERVUS INTERMEDIUS WRISBERGI.
The facial nerve arises in the nucleus nervi facialis, which lies within the ventral area of the pontile tegmentum, ventro-lateral to the abducent nucleus. The fibres spring ing from the nucleus first proceed dorsally, pass around the nucleus of the abducent nerve—the facial knee and the colliculus course ventrally and emerge from the brain-stem at the posterior border of the pons, lateral to the olive. The volun tary innervation of the nucleus is effected by fibres, which start from the lower third of the precentral convolution, pass through the internal capsule (knee), then through the cerebral peduncle to the pons, and, finally, to the homo- and the contralateral facial nucleus. The path includes, as in the case of the other motor nerves: a. The central neurone—from cerebral cortex to nucleus; b. The peripheral neurone—nucleus, peripheral nerve, muscle.
The facial nucleus resembles the oculomotor nucleus in including a number of different groups of cells, in which, in the first place, we distinguish two chief groups, an upper and a lower facial nucleus. The upper contains cells, whose axones form collec tively the superior facial branch; the lower, cells whose axones form the inferior facial branch. Moreover, the superior facial nucleus receives its innervation from the motor centres of both hemispheres, a fact of clinical significance. This bilateral innervation
explains why, in central facial paralysis, the muscles supplied by the upper facial division are not involved in the paralysis, since innervation is still provided by the unaffected central neurones of the other hemisphere. In peripheral facial palsy, on the contrary, all the muscles supplied by both the upper and lower facial are paralyzed.
The nervus intermedius Wrisbergi (the nerve of Sapolini, by whom it was regarded as the thirteenth cerebral nerve) is a mixed nerve, which accompanies the facial and continues as the chorda tympani. The motor fibres take origin in a small cell group lying dorso-medial to the facial nucleus. The sensory fibres arise within the gan glion geniculi. The axones arising from the cells of this ganglion divide into two branches.
One of these passes peripherally and forms, after joining the motor fibres, the peripheral nervus intermedius, that continues as the chorda tympani. The other branch passes centrally, enters the brain-stem and ends within the nucleus tractus solitarii as part of the gustatory path (page 189).