540 



ORGAN OF HEARING. 



ditorlus s. acusticus. Fr. Nerfauditifou acous- 

 tique. Germ. Der Gehornerv. The internal 

 auditory meatus of the temporal bone appears 

 to end in a cul-de-sac ; but, examined more 

 closely, the bottom is found divided into two 

 unequal-sized depressions, an upper and a 

 lower, by a crest, which extends backwards 

 from the anterior and outer wall of the meatus. 

 The upper depression, which is the smaller, is 

 subdivided into two, an anterior and a posterior, 

 by a small vertical pillar. The anterior leads 

 into the aqueduct of Fallopius, and gives pas- 

 sage to the facial nerve. The posterior is almost 

 funnel-shaped, and presents two or three pretty 

 large apertures, and several smaller and less 

 distinct ones. These are the mouths of small 

 canals which lead into the vestibule, and their 

 terminating orifices produce that sieve-like ap- 

 pearance in the pyramidal elevation between 

 the hemi-elliptical and hemispherical depres- 

 sions, and which extends towards the ampullary 

 dilatations in the superior horn of the vestibule. 

 The lower and larger depression presents two 

 subdivisions. The anterior and larger corres- 

 ponds to the base of the axis of the cochlea ; 

 it presents a spiral groove or tract tract us spi- 

 raiisjoraminulentus, answering to the turns of 

 the cochlea, and perforated like a sieve by nu- 

 merous apertures, which diminish in size towards 

 the centre, where there is one opening larger 

 than the rest. The posterior subdivision of the 

 lower depression is a small superficial fossa, 

 perforated by two or three larger, and a great 

 number of smaller apertures, which open into 

 the hemispherical depression of the vestibule, 

 producing the sieve-like spot already mentioned 

 as existing there. Below this superficial fossa 

 there is a pretty large hole, leading into a 

 canal in the posterior wall of the vestibule, 

 which opens by several small orifices, forming 

 the sieve-like spot within the mouth of the 

 ampullary dilatation of the lower extremity of 

 the posterior vertical semicircular canal. 



The different minute apertures we have de- 

 scribed give passage to the fibrils of the audi- 

 tory nerve. 



The internal auditory meatus is lined by 

 dura mater. 



The facial nerve enters the internal auditory 

 meatus along with the auditory nerve. At the 

 bottom of the meatus there is a communication 

 between the two nerves, which was first pointed 

 out by Mr. Swan. Separating from the audi- 

 tory nerve, the facial leaves the internal meatus, 

 by entering the aqueduct of Fallopius. 



From its origin to about where it enters the 

 internal auditory meatus, the auditory nerve 

 presents most distinctly the delicate-walled 

 tubular structure of brain. Within the mea- 

 tus it assumes the ordinary thick-walled cylin- 

 drical tubular structure of nerves ; a circum- 

 stance overlooked by Ehrenberg, when he ad- 

 duced, as a peculiarity of the special nerves of 

 sense, that they presented throughout their 

 course the so-called varicose tubular structure. 



The auditory nerve divides into two branches 

 an anterior, or cochlear ; and a posterior, or 

 vestibular branch, which externally remain 



united together as far as the bottom of the 

 meatus. The former is whiter, and has its fila- 

 ments more compactly bound together than the 

 latter. Examined under the microscope, the 

 cylindrical tubules of the cochlear nerve ap- 

 peared to me to be larger than those of the 

 vestibular, and to contain, or at least to give out, 

 a greater quantity of nervous medulla. 



The anterior branch, or cochlear nerve, ner- 

 vus cochlea, is something like a flat tape rolled 

 on itself longways. It proceeds forwards to 

 that depression at the bottom of the internal 

 auditory meatus, already described as corres- 

 ponding to the base of the axis. Here it resolves 

 itself into a number of fine filaments, which 

 enter the apertures in the spiral tract of holes. 

 Traversing the small bony canals leading from 

 those apertures into the substance of the axis, 

 they enter the bony spiral lamina according as 

 their turn comes, by bending nearly at a right 

 angle, and spread out upon it. The first fila- 

 ments given off are the largest, the rest gradually 

 diminish in size. 



The first turn of the spiral lamina is supplied 

 by those which enter the first turn of the spiral 

 tract of holes ; the second turn receives the 

 filaments which traverse the bony canals, into 

 which the fine apertures of the second turn of 

 the spiral tract lead ; and the last half turn is 

 supplied by that large bundle of filaments ter- 

 minating the nerve, and which, entering the 

 axis by the large opening in the centre of the 

 spiral tract, emerges at its summit. 



The vestibular nerve, nervus vestibuli, which 

 presents a small gangliform enlargement, di- 

 vides into three branches. The uppermost, 

 which is the largest, lies in the depression be- 

 hind the entrance to the aqueduct of Fallopius. 

 Its filaments having penetrated the vestibule 

 by the small apertures of the canals already 

 mentioned in the pyramidal elevation, arrange 

 themselves into three fasciculi; of which one is 

 distributed to the common sinus, and the other 

 two to the ampullae belonging to the superior 

 vertical and to the horizontal semicircular tube. 

 The fibrils of the next branch enter the ves- 

 tibule by the apertures at the bottom of the 

 hemispherical depression, and terminate in the 

 saccule. 



The third, or lowest branch, which is the 

 smallest, enters that canal described in the pos- 

 terior wall of the vestibule, and which opens 

 by a sieve-like spot within the ampullary dila- 

 tation of the posterior semicircular canal. Its 

 fibrils are distributed to the ampulla of the 

 posterior semicircular tube. 



Such is the description of the divisions of 

 the auditory nerve as given by most authors, 

 and as it has appeared to me in the examina- 

 tions I have made. Krause and Breschet, how- 

 ever, describe the mode of division differently. 

 The former says the nerve of the saccule comes 

 off from the cochlear nerve ; the latter, that the 

 cochlear nerve (which he calls the posterior 

 fasciculus of the auditory nerve) gives off both 

 the saccular nerve and the filaments to the pos- 

 terior ampulla. I have not at present an oppor- 

 tunity to repeat my examination of the parts, 



