THE EIGHTH OR ACOUSTIC NERVE 1003 



unable to recognize the difference between bitters and sweets, acids and salines, from involvement 



of the chorda tympani. The mouth is dry, because the salivary glands are not secreting; the 



sense of hearing is affected from paralysis of the Stapedius, but there is no hemiplegia. When the 



cause of the paralysis is from fracture of the base of the skull, the acoustic nerve and the petrosal y > 



nerves, which are connected with the intumescentia ganglioformis, are also involved. When i 



the injury to the nerve is after its exit from the stylomastoid foramen, all the muscles of expression 



except the Levator palpebrae, together with the posterior belly of the Digastric and Stylohyoid, 



are paralyzed. There is smoothness of the forehead, and the patient is unable to frown; the A .^ 



eyelids cannot be closed, and the lower lid droops, so that the punctum is no longer in contact I *^ ,4^ 



with the globe, and the tears run down the cheek; there is smoothness of the cheek and loss of 



the nasolabial furrow; the nostril of the paralyzed side cannot be dilated; the mouth is drawn 



to the sound side, and there is inability to whistle; food collects between the cheek and gum 



from paralysis of the Buccinator. 



The facial nerve is at fault in cases of so-called histrionic spasm, which consists in an almost 

 constant and uncontrollable twitching of the muscles of the face. This twitching is sometimes 

 so severe as to cause great discomfort and annoyance to the patient and to interfere with sleep, 

 and for its relief the facial nerve has been stretched. The operation is performed by making an 

 incision behind the ear from the root of the mastoid process to the angle of the mandible. The 

 parotid is turned forward, and the dissection carried along the anterior border of the Sterno- 

 mastoid muscle and mastoid process until the upper border of the posterior belly of the Digas- 

 tric is found. The nerve is parallel to this on about a level with the middle of the mastoid process. 

 When found, the nerve may be stretched by passing a blunt hook beneath it and pulling it for- 

 ward and outward. Too great force must not be used, for fear of permanent injury to the nerve. 

 In facial paralysis of extracerebral origin it may be advisable to expose the nerve, cut it across, and 

 anastomose the distal end of the paralyzed nerve to the accessory nerve, or, better, to the hypo- 

 glossal nerve (facioaccessory anastomosis or faciohypoglossal anastomosis). The idea was first 

 proposed by Ballance, and has been put in practice by Ballance and Stewart, Keen, Gushing, 

 Paure. Kennedy, and others. 



THE EIGHTH OR ACOUSTIC NERVE (N. ACUSTICUS) (Fig. 742). Lr 4 ! 



The eighth or acoustic or auditory nerve comprises two distinct sets of fibres 

 which, although both are devoted to the transmission of afferent impulses, differ 

 in their peripheral distribution and in their central connections. The two divi- 

 sions appear blended in the interval between the medulla oblongata and the in- 

 ternal auditory meatus, running obliquely laterofrontad in company with the 

 facial nerve and internal auditory artery. At the internal auditory meatus the 

 two divisions of the nerve are separable, the vestibular division above, the cochlear 

 below. 



The cochlear nerve (radix cochlearis) is the true nerve of hearing, lacking 

 general sensibility, however, and therefore a nerve of special sense. The fibres 

 of this division arise from the cells of the spiral ganglion of the cochlea as axones 

 of bipolar cells whose dendrites or peripheral processes terminate about the 

 (auditory) hair cells of the organ of Corti. The central connections of the cochlear 

 division are described on page 883. 



The vestibular nerve (radix vestibularis) conducts impulses of equilibratory 

 sense from the semicircular canals, utricle, and saccule to the vestibular nuclei. 

 The ganglion of origin of this nerve differs from ordinary sensor ganglia in that 

 its cells are of bipolar structure, having retained this embryonic characteristic 

 of the ganglion cells throughout life. The central processes of the cells of the 

 vestibular ganglion (or ganglion of Scarpa) enter the medulla oblongata with the 

 trunk of the cochlear nerve in the postpontile groove, laterad of the facial nerve, 

 to establish central connections already described on page 883. The peripheral 

 processes constitute the two main branches of the nerve viz., (a) the utriculo- 

 impullar and (6) the sacculoampullar. 



The upper or utriculoampullar branch divides into: 



(a) The utricular branch, passing through the superior macula cribrosa of the 

 vestibule to end in the macula acustica of the utricle. 



