AUDITORY SENSATIONS 515 



hearing. The internal ear consists essentially of a membranous sac form. 

 originally by an involution of the epithelium covering the surface of 

 embryo. In the course of development the sac, which is filled with 1 

 endolymph, becomes much modified in shape, forming from before back 

 wards the scala media of the cochlea, the saccule, the utricle, and the three 

 semicircular canals. At certain parts of its inner surface thickenings of the 

 epithelium occur, which become connected with the terminations of the 

 eighth nerve. The membranous labyrinth lies inside a bony case, the osseous 



FIG. 249. The membranous labyrinth. 

 CM, canalis or scala media of the 

 cochlea ; s, saccule ; u, utricle ; so, semi- FIG. 250. Vertical section through the 



circular canals. cochlea. 



labyrinth, from which it is separated by the perilymph. The osseous laby- 

 rinth is formed from before backwards by the cochlea, the vestibule, and the 

 semicircular canals. 



The utricle, the saccule, and the semicircular canals are concerned with 

 the functions of equilibration. At present we have only to deal with the 

 structure of the cochlea and the end-organs contained in the scala media. The 

 cochlea is a spiral tube of bone 20 to 30 mm. long, divided by the scala media 

 into two parts, viz. scala vestibuli and scala tympani, which are continuous at 

 the apex of the spiral (helicotrema). The essential part of the organ of hear- 

 ing is contained in the scala media. The sound waves falling on the ear and 

 striking the membrana tympani are transmitted with diminished amplitude 

 but increased force by the chain of ossicles to the fenestra ovalis ; through 

 these they are communicated to the perilymph which fills the vestibule. The 

 vibrations travel from the vestibule to the scala vestibuli. Every rise of 

 pressure in this canal will cause an actual movement of fluid and will push 

 the scala media towards the scala tympani. The increased pressure thus 

 communicated to the scala tympani causes a bulging of the membrane 

 closing the foramen rotundum. Movement inwards of the stapes causes 

 therefore a movement outwards of this membrane and vice versa, and the 

 wave of pressure in passing from one aperture to the other must be 

 communicated to the scala media with all the sensitive structures which it 

 contains. 



The scala media is triangular in cross-section, - having its apex at the 

 spiral lamina and its base at the outer wall of the cochlea. It is separated by 

 the membrane of Reissner from the scala vestibuli and by the basilar mem- 

 brane from the scala tympani. The basilar membrane is composed of a 

 number of elastic fibres, which pass in a radial direction from the spiral 



