THE VESTIBULAR NERVE 



949 



enters that cavity close to the posterior border of the membrana tympani. It crosses the cavity, 

 running on the medial surface of the tympanic membrane at the junction of its upper and middle 

 thirds, covered by the mucous membrane lining the tympanic cavity, and passes to the medial 

 side of the manubrium of the malleus above the tendon of the tensor tympani. It leaves the 

 tympanic cavity and passes to the base of the skuU through a small foramen (the iter chordae 

 anterius) at the medial end of the petro-tympanic (Glaserian) fissure. At the base of the skull 

 it incUnes downward and forward on the medial side of the spine of the sphenoid, which it 

 frequently grooves, and, on the medial side of the pterygoideus externus, it joins the posterior 

 border of the lingual nerve at an acute angle. Some of its fibres (motor chiefly) leave the lingual 

 nerve and pass to the sub-maxillary ganglion, and others (sensory) continue forward to the 

 tongue, where, in company with fibres of the lingual nerve, they terminate in the epithelium 

 covering the anterior two-thirds of the tongue. Some probably serve to convey sensations of 

 taste, most of them are fibres of general sensibility. Before it joins the lingual nerve the chorda 

 tj'^mpani receives a communicating twig from the otic gangUon (figs. 738, 741). 



THE VESTIBULAR NERVE 



The vestibular nerve is purely sensory. With the peripheral processes of its 

 cells of origin terminating in the neuro-epithelium of the semicircular canals and 



Fig. 742. — The Left Membranous Labyrinth op a Human Fcetus-of lO Weeks (30 mm.), 

 Lateral Aspect. Vestibular ganglion and nerve, red; cochlear nerve, yellow. (Streeter, 

 American Journal of Anatomy.) 



^ral. 



the vestibule, and their central processes conveying impulses which are dis- 

 tributed to the gray substance of the cerebellum and spinal cord, the nerve com- 

 prises a most important part of the apparatus for the equilibration of the body. 

 It has been customary to describe the vestibular [radix vestibularis] and the coch- 

 lear [radix cochlearis] nerves combined as the acoustic (auditory) or eighth cranial 

 nerve. While the two are blended in a common sheath from near the medulla to 

 the bottom of the internal auditor}^ meatus, they are likewise partly enclosed in 

 the same sheath with the facial and glosso-palatine nerves and the internal audi- 

 tory artery which accompany them in this meatus. At the bottom of the meatus 



