STRUCTURE OF THE EAR 303 



the membrane for high and low notes. When the mem- 

 brane is stimulated the muscle undergoes reflex contrac- 

 tion. The view is also held that the tensor tympani 

 protects the drum from over-stretching by allowing it to 

 slacken when no sounds fall upon it. 



The function of the stapedius is not known with certainty. 



The Internal Ear 



Embedded in the temporal bone is a system of canals, 

 the bony labyrinth, part of which forms a spiral tube, the 

 cochlea. Within the bony labyrinth is an inner system, 

 the membranous- labyrinth, composed of the saccule, 

 utricle, semicircular canals, and a part within the cochlea 

 known as the scala media. Both labyrinths are filled with 

 fluid, that filhng the bony labyrinth being called perilymph, 

 that filhng the membranous labyrinth, endolymph. The 

 cochlea is the only part of the labyrinth with which we 

 are now concerned. About 25 mm. in length, it is wound 

 around a central pillar, the modiolus. From the modiolus 

 a ledge projects into the canal of the cochlea throughout 

 its course. This ledge is therefore known as the spiral 

 lamina. From the outer edge of the spiral lamina two 

 membranes stretch across the canal of the cochlea, dividing 

 this into three parallel compartments, the scala vestibuli 

 uppermost, the scala tympani lowest, and the scala media 

 between. The scala vestibuli is separated from the scala 

 media by the thin membrane of Reissner, and the scala 

 media from the scala tympani by the basilar membrane 

 and the structures situated upon it. At the bhnd end of 

 the canal of the cochlea the basilar membrane is deficient, 

 and scala vestibuh and scala tympani communicate. Both 

 the scala vestibuli and scala tympani form part of the 

 bony labyrinth and contain perilymph, and both are in 

 connection through membranes with the middle ear, the 

 former at the fenestra ovahs, the latter at the fenestra 

 rotunda. The scala media, on the other hand, is, as stated 



