HEARING. 



157 



nicate the vibration of the tympanum to the internal ear (Fig. 43). 

 The handle of the malleus is attached to the tympanum, so that 

 this bone moves with each vibration. This motion is communi- 

 cated to the incus, which passes it on to the stapes. The stapes 

 forms a sort of piston in the foramen ovalis, and is therefore capa- 

 ble of transmitting to the fluid in the cavity of the labyrinth the 

 impulses which it receives. 



Describe the arrangement of the internal ear. 



The internal ear, or labyrinth, is situated in the dense petrous 

 portion of the temporal bone, and consists of three essential parts : 

 the vestibule (Fig. 44), and opening 

 from it the semicircular canals and the 

 cochlea. There is another opening, 

 the sequeductus vestibuli, whose use 

 is unknown, and still others for the en- 

 trance of the auditory nerve-filaments. 

 Within the bony structure is a mem- 

 brane of fibrous and epithelial tissue, 

 the membranous labyrinth, which fol- 

 lows the bony structure and contains 

 a colorless fluid, the endolymph, and a 

 fluid surrounds this membranous laby- 

 rinth, the perilymph. 



What is the function of the semicir- 

 cular canals ? 



These canals are arched cylindrical 

 spaces in the solid bone which open at 

 each end of the arch in the vestibule. 

 They are three in number, and two 

 are nearly vertical and one is horizontal. These canals are arranged 

 in such a manner that the planes of the two vertical canals are at 

 right angles, one being fore and aft, and the other transverse (Fig. 

 44). Their use does not seem to be directly connected with the 

 auditory function of the part, but to be connected more with the 

 sense of equilibrium. The movement of the fluids in the canals, 

 arranged in the directions of the three dimensions, may serve to 

 produce sensations which lead to the formation of accurate judg- 

 ment of changes in the position of the body. 



What is the cochlea ? 



It is a part of the labyrinth which derives its name from its 



External View of a Cast of the Left 

 Labyrinth (Henle) : /, fenestra 

 cochleae, or round window ; a, 

 fenestra vestibuli, or oval win- 

 dow ; b, ampulla of superior semi- 

 circular canal ; rf, common shaft 

 of union of these two canals ; e, 

 ampulla of the horizontal semi- 

 circular canal ; g, tractus spiralis 

 foraminosus. 



