1256 HUMAN ANATOMY. 



When all branches of the facial are paralyzed the symptoms are characteristicc 

 Only one side of the forehead wrinkles ; the tears fail to enter the canaliculi, and 

 flow over the cheek ; the eye cannot be closed ; foreign bodies on its surface are not 

 removed by the lid, and conjunctivitis from irritation results. The af!ected half of 

 the face is expressionless, and the corner of the mouth on that side remains partly 

 open and hangs down, so that the saliva tends to run out. The mouth is drawn to 

 the opposite side ; the upper lid cannot be elevated ; whistling is impossible be- 

 cause the orbicularis cannot now pucker the lips ; food lodges in the affected side 

 of the mouth, because the buccinator muscle is paralyzed, and, for the same reason, 

 the mucous membrane often gets caught between the teeth. 



In those cases of facial paralysis in which the lesion of the nerve is posterior 

 to the stylo-mastoid foramen, attempts have been made recently to restore function 

 to the peripheral portion by dividing the trunk posterior to the parotid gland, 

 and anastomosing the peripheral end to a neighboring cranial nerve, as the spinal 

 accessory or the hypoglossal. The results have not been entirely satisfactory. 



The line of the main trunk of the nerve is from the slight depression between the 

 back of the ear and the mastoid process, forward and slightly downward. It passes 

 through the deeper portion of the parotid gland. 



THE AUDITORY NERVE. 



The eighth or auditory nerve (n. acusticus) is not only, as its name implies, the 

 nerve by which sound impulses are transmitted to the brain, but also the nerve of 

 equilibration. It consists of two portions, the cochlear, the true nerve of hearing, and 

 the vestibular, which is concerned with equilibration. 



Traced from the brain toward the ear, the auditory nerve arises at its super- 

 ficial origin by two roots, a mesial (radix vestibularis) and a lateral (radix coch- 

 learis), which embrace the inferior cerebellar peduncle, the mesial passing to the 

 inner and the lateral to the outer side of the peduncle. The nerve thus formed by 

 the union of these two roots, leaves the surface of the brain-stem at the posterior 

 border of the pons, where it is adherent to the middle cerebellar peduncle. To its 

 inner side and closely associated with it are the motor and sensory roots of the facial 

 nerve (Fig. 1046), which lie within a groove on the mesial surface of the auditory 

 and with it enter and traverse the internal auditory canal. Within the latter, the 

 auditory nerve separates into two divisions, of which the superior and larger is the 

 vestibular nerve (n. vestibuli) and the inferior and smaller is the cochlear 

 nerve (n. cochleae). Although in a general way these divisions continue the 

 corresponding roots, this agreement, as to the source of their fibres, is not complete, 

 since, as will be more fully noted, strands of vestibular fibres are incorporated with 

 the cochlear nerve. 



On reaching the bottom of the internal auditory canal, the facial nerve leaves 

 the meatus and enters the facial canal, while the fibres of the auditory nerve dis- 

 appear through apertures in the lamina cribrosa (Fig. 201) to gain the several 

 parts of the membranous labyrinth of the internal ear. During their journey through 

 the meatus, the vestibular and facial trunks are connected (fila anastomica) by a 

 branch which passes from the pars intermedia to the vestibular nerve, and by one 

 from the latter to the geniculate ganglion. These apparent communications between 

 the seventh and eighth nerves are, in fact, only aberrant strands of facial fibres that 

 return to the seventh after temporary association with the auditory. 



The vestibular nerve divides into three terminal branches which pass through 

 apertures in the cribriform plate above the falciform crest and supply: (i) the tdriclCy 

 (2) the superior ^.nA (3) the external semicircular canal. Not all the fibres of the 

 vestibular root, however, are included in these branches ; of the three branches given 

 of? by the cochlear nerve two, (4) those to the saccule and (5) to \}l\& posterior semi- 

 circular canal, are vestibular fibres incorporated with the cochlear, although seem- 

 ingly derived from the cochlear nerve. The remaining branch of the cochlear nerve 

 contains the cochlear fibres proper, which traverse the numerous foramina of the 

 tractus spiralis foraminosus and the central canal of the modiolus to supply the organ 

 of Corti within the membranous cochlea. 



