AUDITORY SENSATIONS 



573 



the tympanic membrane. It acts therefore as an antagonist to the 

 tensor tympani. It is not yet known what exact part this muscle plays 

 in audition. 



THE END-ORGANS OF HEARING 



The movements of the stapes are communicated to a fluid, the 

 perilymph, and by this to the endolymph, which immediately bathes 

 the end-organ of hearing. The internal ear consists essentially of a 

 membranous sac, formed originally by an involution of the epithelium 

 covering the surface of the embryo. In the course of development the 

 sac, which is filled with the endolymph, becomes much modified in 

 shape, forming from before backwards the scala media of the cochlea, 

 the saccule, the utricle, and the three semicircular canals. At certain 



.C 



FIG. 247. The membranous labyrinth. 

 CM, canalis or scala media of the 

 cochlea; s, saccule; u, utricle; sc, semi- 

 circular canals. 



FIG. 248. Vertical section through the 

 cochlea. 



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 labyrinth, 

 from which it is separated by the perilymph. The osseous labyrinth 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 (helico- 

 trema). The essential part of the organ of hearing 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 



