FIFTH CEANIAL NEEVE. 195 



The motor nucleus which lies in the grey matter at the 

 lateral angle of the fourth ventricle, at the level of origin of 

 the nerve root from the pons, may be damaged by gross 

 lesions at this level, or involved together with other bulbar 

 nuclei, in degenerative disease. The nucleus is connected with 

 the motor cortex of the opposite frontal region by fibres which lie 

 in the genu of the opposite internal capsule. (Fig. 12, p. 179.) 



The descending root of the fifth, which arises in the grey 

 matter around the aqueduct of Sylvius, is now held to be 

 motor rather than sensory and probably joins the motor 

 division of the nerve. The rare condition known as facial 

 hemiatrophy is thought to be due to disease of this root. 



The roots of the nerve, on their way to the surface, intersect 

 the fibres of the middle cerebellar peduncle and appear on the 

 lateral surface of the pons nearer to its upper than its lower 

 border. The position of the issuing root is made use of as an 

 arbitrary point of division between the pons and the middle 

 peduncle. A tumour in this locality may occasion paralysis 

 of the fifth nerve with deafness, from implication of the eighth 

 nerve, and cerebellar symptoms from interference with the 

 cerebellar peduncle. 



In the membranes at the base of the brain lie the nerve 

 roots, the Gasserian ganglion and the three terminal divisions. 

 The ganglion bears the same relation to the sensory fibres as 

 a posterior root ganglion does to a sensory spinal nerve. The 

 small motor root does not enter the Gasserian ganglion but 

 passes beneath it to join the mandibular nerve. The position 

 of the ganglion in a hollow near the apex of the petrous 

 bone, renders it liable to implication by tumours or other 

 diseases of the middle or posterior fossa, or of the petrous 

 bone. Gummatous or tuberculous disease of the membranes 

 may involve the issuing root, the ganglion, or the terminal 

 divisions between the ganglion and their foramina of exit. 



The ophthalmic division of the fifth traverses the cavernous 

 sinus and the sphenoidal fissure, and is most likely to suffer 

 in conjunction with the motor nerves of the eyeball. It may 



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