450 NERVOUS SYSTEM. 



SECT. IV. OF THE NERVES OF THE ORGAN OF HEARING. 



The Nerves which pass through the petrous bone, and are 

 either wholly or partially spent upon the organ of hearing, come 

 from three sources. 1. The Auditory Nerve ; 2. The Portio 

 Dura ; 3. The Trigeminus, or Fifth Pair. 



The Meatus Auditorius Internus conducts the first two, and 

 has its bottom divided by a ridge into two fossae, of which the 

 upper one is the smaller. This bottom, it has been observed, 

 corresponds with the base of the modiolus, and is cribriform. 

 One foramen, larger than any of the others, and in the superior 

 fossa, transmits the portio dura or facial nerve: all the others 

 are occupied by the filaments of the auditory nerve. 



1. The Auditory Nerve divides at the bottom of the meatus 

 into fasciculi of filaments ; one of which penetrates into the ves- 

 tibulum through the foramina behind that for the portio dura, 

 and is distributed upon the sacculus ellipticus, and upon the am- 

 pulla of the superior and of the exterior membranous canal; 

 other filaments get to the sacculus sphericus; and a third fasci- 

 culus of filaments is distributed to the ampulla of the posterior 

 membranous canal. These several filaments are said to pre- 

 serve, when they fir'st penetrate into the bony labyrinth, a fibrous 

 appearance, and are interlaced; they also penetrate the parietes 

 of the membranous labyrinth, and have their extremities bathed 

 in its fluid, in which place they are converted into soft pulp, re- 

 sembling mucus, or the retina. 



Another very considerable fasciculus of filaments penetrates 

 into the canals of the modiolus, and enters through them into 

 the cavity of the cochlea, along the Zona Ossea, and between 

 its tables; they terminate also by a soft pulp on the internal face 

 of the lining membrane of the cochlea. One of these filaments, 

 conspicuous for its size, goes through the central canal of the 

 modiolus and terminates in the infundibulum.* 



2. The Facial Nerve,, or Portio Dura, is only eonnnected to 



* For a knowledge of the minute distribution of the auditory nerve, the pro. 

 fession is signally indebted to the distinguished Scarpa,. in his Disquisitiones de 

 Auditu et Olfactu. 



