784 



THE NEEYOUS SYSTEM. 



across the zygomatic bone, they supply the orbicularis oculi and zygomatic muscle, 

 and communicate with the zygomatico-facial branch of the maxillary nerve. 

 . The lower zygomatic branches are of considerable size. Passing forwards over 

 the masseter muscle in company with the parotid duct, they supply the orbicularis 

 oculi, the zygomaticus, buccinator, and the muscles of the nose and upper lip. 

 The infra-orbital plexus is formed by the union of these nerves with the infra-orbital 

 branch of the maxillary nerve below the lower eyelid. Smaller communica- 

 tions occur with the infra-trochlear and nasal nerves on the side of the nose. 



The cervico-facial division of the facial nerve supplies three series of secondary 

 branches. 



1. Rami Buccales. The buccal branch (or branches) extends forwards to the 

 angle of the mouth to supply the muscles converging to the mouth, including the 

 buccinator. It communicates with the buccinator branch of the mandibular nerve 

 in front of the anterior border of the masseter muscle. 



2. Eamus Marginalis Mandibulse. The marginal branch of the mandible (O.T. supra- 

 mandibular) passes along the mandible to the interval between the lower lip and 

 chin, and supplies the triangularis oris, quadratus labii inferioris, and orbicularis 

 oris. It communicates with the mental branch of the inferior alveolar nerve. 



3. Ramus Colli. The cervical branch (O.T. infra-mandibular) emerges from the 

 parotid gland near its lower end, and sweeps forwards below the angle of the 

 mandible to the front of the neck. It supplies the platysma, and forms loops of 

 communication with the nervus cutaneus colli. 



NERVUS ACUSTICUS. 



The eighth or acoustic nerve (O.T. auditory) arises from the brain by two 

 roots, medial and lateral. The medial, vestibular root emerges between the olive 

 and the restiform body. The lateral, cochlear root, continuous through the cochlear 

 nucleus with the strise medullares of the fourth ventricle, winds round the lateral 

 side of the restiform body (for the deep connexions, see p. 604). The two roots 

 unite with one another to form the trunk of the nerve, which is attached to the 



Principal 

 vestibular nucleus 



Dorsal cochlear nucleus 

 Restiform body 



Ampulla of lateral semicircula 



Corpus trapezoideum 



Ampulla of superior semicircular duct 



Acoustic nerve 

 Olive Internal acoustic meatus 



Superior division 



Inferior division 



Cochlear nerve 



Ductus cochlearis 



, , ' Saccus end< 

 Ampulla of lymph atici 



posterior 



Saccule semicircular 

 duct 



FIG. 658. SCHEME OF THE ORIGIN AND DISTRIBUTION OF THE ACOUSTIC NERVE. 



brain on the lateral side of the facial nerve and nervus intermedius, at the lower 

 border of the pons (Fig. 643, p. 768). 



The nerve passes laterally through the internal acoustic meatus, lying below 

 the facial nerve and nervus intermedius (Fig. 647, p. 771). In the meatus the 

 trunk separates into two divisions, an upper consisting of vestibular fibres only 

 and a lower which consists mainly of cochlear fibres but contains also some 

 vestibular fibres. The divisions subdivide, and their branches pass through the 

 lamina cribrosa, to supply the several parts of the labyrinth. 



The superior division in the internal acoustic meatus usually receives fibres 



