THE AUDITORY APPARATUS. 95I 



vestibular scala, and Lowenherg's, or the collateral scala ,- so that, m reality, there 

 are four cochlean scalae (Fig. 512). 



We do not, therefore, find in the cochlea — as in the other regions of the 

 labyrinth — a system of membranous cavities included in osseous cavities. 



The structure of the membranes that limit the auditive scala is not perfectly 

 known, and is still disputed by anatomists ; but connective, epithelial, and nerve 

 elements appear to form their base. 



With regard to the organ of Corti, it is a very curious and interesting portion 

 of the auditory scala, being formed of a series of solid and elastic arches resting 

 by their extremities on the membrane — the basilar — that separates the auditory 

 from the tympanic scala, their convexity being towards the superior, or membrane 

 of Corti. These arches number about three thousand in Man, and are composed 

 of two portions or articles — an external and an internal, united by a thickening 

 in the vicinity of the membrane of Corti. To these elastic arches are added 

 conical or fusiform ciliated cells, the function of which is to increase and transmit 

 to the terminations of the auditory nerve, the slightest vibrations of the organ 

 of Corti. (For fmther details, see works on Histology.) 



Fluids of the Labyrinth. 



These liquids are of two kinds — one is contained in the membranous labyrinth, 

 the other in the osseous labyrinth. 



The fluid of the membranous labyrinth — or endo-lymph of Breschet — is con- 

 tained in the sacs and tubes constituting the membranous vestibule and semi- 

 circular canals. It is limpid and fluid like water. The fluid of the osseous 

 labyrinth, or peri-lymph of Breschet, fills the two scalfe of the cochlea, and 

 bathes the external surface of the vestibule and membranous semicircular canals, 

 which it separates from the corresponding walls of the osseous labyrinth. 



Distribution and Termination of the Auditory Nerve in the 

 Membranous Labyrinth. 



This nerve (the i^ortio mollis of the seventh pair) divides, as we have said, 

 into two branches — a cochlear and a vestibular. 



The cochlear branch, the largest, reaches the base of the cochlea, where it 

 breaks up into a large number of fasciculi, one portion of which expands over 

 the first turn of the lamina spiralis, the other on the second, and a third on the 

 third ; the latter ramifications penetrate to the auditory scala, and terminate on 

 the organ of Corti. This anatomist has seen, on the primary fibres of these 

 ramifications, a gangUonic cell {ganglion spiral), at the point where they leave 

 the lamina spiralis ossea. The primary fibres finally lose their myelin sheath, 

 and, reduced to their axile filament, they terminate in the cells accompanying the 

 arches of Corti. 



The vestibular branch divides into three portions, their terminal filaments 

 passing through the openings of the perforated spots (foramina 7iervina)^ and 

 ramify in the wall of the sacculus, utriculus, and the ampulla; at the extremities 

 of the three semicircular canals. 



Article II. — Middle Ear, or Case of the Tympanum. 



Excavated in the substance of the petrous portion of the temporal bone, on 

 the limit of the petrous and mastoid sections — but chiefly in the latter — the middle 



