Facial Nerve 65 



panying artery, free haemorrhage may occur if resection is being re- 

 sorted to. 



The mental nerve, emerging through the foramen in the body of 

 the jaw in a line with the root of the second bicuspid, may be found 

 by an incision through the integument (the exact position of the fora- 

 men having been first determined by means of a sharp probe) and 

 through fibres of the depressor anguli oris and labii inferioris. Another 

 way of reaching the nerve is by freely incising the mucous membrane 

 as it is reflected from the lower jaw to the back of the lip, and working 

 carefully down with a director in the region below the second bicuspid. 



When the fifth nerve is paralysed, and the patient puts a cup to his 

 lips, ' he feels only half of it ; it seems to him exactly as though it were 

 broken.' (Fagge.) 



No. 6. The abducens (because it supplies the external rectus) arises 

 in the floor of the fourth ventricle, and emerges between the anterior 

 pyramid and pons. It passes through the inner part of the cavernous 

 sinus, lying on the outer side of the carotid artery, and, entering the 

 orbit by the sphenoidal fissure, ends on the inner surface of the ex- 

 ternal rectus. At its origin this nerve does not decussate with its 

 fellow across the median line, as the third does, because the two 

 external recti have no experience of working in concert. When the 

 nerve is paralysed the eye-ball is drawn inwards, and the patient sees 

 a double image, because the yellow spots cannot both be placed 

 in focus unless the head be turned and tilted to make up for the 

 error (see p. 80). As he cannot abduct the eye-ball, he turns his 

 head. And in this, as in every other case of ocular paralysis which is 

 the result of cerebral lesion, the patient apologetically turns his head 

 towards the side of the cerebral lesion he keeps looking towards the 

 cerebral damage, as it were. 



No. 7. The facial nerve arises, together with the sixth, from a 

 nucleus in the floor of the fourth ventricle, and leaves the medulla just 

 below the pons, through the groove between the olive and restiform. 

 It enters the internal auditory meatus with, but above, the auditory 

 nerve, and, passing into the aqueductus Fallopii, leaves the petrous 

 bone by the stylo-mastoid foramen. It then passes through the 

 parotid gland and divides into the temporo-facial and cervico-facial 

 trunks, which, by secondary divisions, form the pes anserinus. 



Some of the fibres of origin of the two facial nerves decussate across 

 the middle line, and thus it happens that with a lateral pontine lesion 

 there may be paralysis upon the corresponding or upon the opposite 

 side of the face ; hemiplegia, moreover, may exist with facial paralysis 

 of the same or of the opposite side. The decussation takes place in 

 the pons, at the level of the apparent origin of the fifth pair. A lesion 

 anterior to this causes facial paralysis on the same side as the hemi- 

 plegia, that is, upon the opposite side to that of the lesion. But if 

 the lesion be behind this level ' crossed paralysis ' is produced that 



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