808 THE NERVOUS SYSTEM 



The facial nerve is also composed of two parts. Its larger part is purely efferent 

 or motor, while its considerably smaller root or pars intermedia contains for the 

 most part sensory fibres. Both the facial nerve and the abducens have their nuclei 

 within the level of the pons, though the roots of both appear from under its inferior 

 border. 



The nucleus of origin of the motor portion of the facial lies in the ventro-lateral 

 region of the reticular formation, in line with the nucleus ambiguus. The axones 

 given off by the cells of this nucleus collect into a bundle which, instead of passing 

 ventrally and directly to the exterior, courses at first dorso-mesially up to the mesial 

 side of the nucleus of origin of the abducens (ascending root of the facial); then it 

 turns and courses anteriorly for a few millimetres, parallel with the nucleus of the 

 abducens and immediately beneath the floor of the fourth ventricle (genu facialis) ; 

 then it again turns abruptly and pursues a ventro-lateral direction to its point of 

 exit at the inferior border of the pons, near the entrance of the vestibular nerve. Its 

 exit usually involves a few of the pons fibres. In transverse sections through the 

 middle of the nucleus of the abducens the genu of the facial appears as a compact, 

 transversely cut bundle at the dorso-medial side of this nucleus. 



The pars intermedia (nervus intermedius) of the facial is chiefly afferent or sen- 

 sory. It is said to contain efferent secretory (salivary) fibres which arise from cells 

 scattered in the reticular formation about the dorsal periphery of the facial nucleus. 

 Though in company with the large motor portion during the exit of the latter, yet, 

 quite close to the point of attachment, the pars intermedia may frequently be dis- 

 tinguished as a separate root. Shortly, however, it becomes intimately united with 

 the main trunk. Its afferent fibres arise from the cells of the geniculate ganglion of 

 the facial, and from numerous cells scattered within the trunk of the nerve. These 

 cells correspond to those of the spinal ganglia, each giving off a single process which 

 bifurcates into a peripheral and a central branch. The central branch finds its 

 nucleus of termination in what may be called the mixed lateral sensory area of the 

 brain-stem the region bounded by the nucleus of termination of the vestibular 

 nerve, the superior extremity of the nucleus of the solitary tract, and the nucleus of 

 termination of the trigeminus. 



The nucleus of origin of the (motor) facial nerve receives impulses from the lower 

 portion of the anterior central gyrus of the cerebral cortex by way of the pyramid, 

 which gives off fibres both to the nucleus of the same side and by way of arcuate 

 fibres through the raphe to the nucleus of the opposite side. It also receives fibres 

 from the root of the trigeminus, the great sensory nerve of the face, and from the 

 central connections of the auditory nerve. 



The abducens is a small, purely motor nerve, which supplies the external rectus 

 muscle of the eye. Its nucleus of origin lies close to the mid-line in the medial em- 

 inence of the floor of the fourth ventricle, and in line with that of the hypoglossus. 

 Its root-fibres pursue a ventral course, inclining a little outwards and curving in- 

 feriorly to emerge from under the inferior border of the pons. They pass lateral to 

 the pyramid, and often between some of its fasciculi. The nucleus receives cortical 

 or voluntary impulses by way of the pyramid, chiefly of the opposite side. Its con- 

 nection with the auditory apparatus and the medial longitudinal fasciculus has 

 already been noted. It probably receives afferent impulses through the fibres of 

 the trigeminus. 



The trigeminus is considerably larger than any of the nerves inferior to it, and 

 has the most extensive central connections of any of the cranial nerves. It is a 

 mixed nerve, but, unlike the facial, its sensory roct is the larger (portio major) and its 

 small motor root (portio minor) is separate from the main trunk soon after its emer- 

 gence. The two roots pass together through the brachium pontis in line with the 

 facial nerve, and find their nuclei in the grey substance underlying the floor of the 

 fourth ventricle. 



The large sensory portion serves as the nerve of general sensibility for the face 

 from the vertex of the scalp downwards, and thus it corresponds to the afferent 

 fibres not only for its own motor root, but of all the nerves giving motor supply 

 to structures underlying its domain. Its fibres arise from its large, trilobed, scni iluniir 

 (Gasserian) ganglion, situated outside the brain. This corresponds to the dorsal root 

 ganglion of a spinal nerve, and its cells give off the characteristic T-fibres with periph- 

 eral and central branches. The central or afferent branches upon entering the brain- 

 stem bifurcate into ascending and descending divisions, just as the entering dorsal 



