THE EIGHTH NERVE, OR AUDITORY 617 



When the facial nerve is divided or in any other way paralyzed, the loss of 

 function in the muscles which it supplies interferes with the perfect exercise 

 of the organs of the special senses. Thus, in paralysis of the facial nerve the 

 orbicularis palpebrarum being powerless, the eye remains open through the 

 unbalanced action of the levator palpebra. The conjunctiva is thus contin- 

 ually exposed to the air and dust and is liable to repeated inflammation, 

 which may end in thickening and opacity of the cornea. 



The sense of taste may be weakened or wholly lost in paralysis of the 

 facial nerve, which involves the chorda tympani. This result, which has 

 been observed in many instances of disease of the facial nerve in man, appears 

 explicable on the supposition that the chorda tympani is the nerve of taste 



pars interned---^] 



r. auric, 



petros. 5 up. maj. 



L motor "ZU. 



FIG. 413. Dissection of the Sensory and Motor Divisions of the P'acial in a 2o-cm. Embryo 



' (Pig). (Streeter.) 



to the anterior two-thirds of the tongue, its fibers being distributed with the 

 so-called gustatory or lingual branch of the fifth. Streeter has just published 

 a study of the development of the seventh and eighth nerves in which he 

 traces the chorda tympani through the pars intermedia, as shown in figure 

 413, thus settling this oft-disputed question. 



The Eighth Nerve, or Auditory. The eighth nerve consists of two 

 divisions, anatomically distinct and functionally independent. These are 

 the vestibular and the cochlear divisions of the auditory nerve. 



The cochlear division arises in the spiral ganglion and passes to the 

 medulla to establish immediate connections with the ventral cochlear nucleus 

 and the tuberculum acusticum. The central relations of these nuclei are 

 established by the striae acusticae, the trapezoideus, and the lateral fillet with 

 the internal corpus geniculatum and the inferior corpus quadrigeminum of 

 the opposite side, as told by figure 414. These latter nuclei send tracts to 

 the auditory center in the superior temporal gyrus. 



The vestibular division arises in the vestibular ganglion, which is entirely 

 distinct from the cochlear ganglion, and enters the medulla, passing to the 

 lateral or chief auditory nucleus. From this point the relations are not fully 



