xi EAR 189 



The membranous labyrinth does not fit tightly into 

 the cavity of the auditory capsule in which it is con- 

 tained, a space being left between it and the surrounding 

 bone and cartilage (Fig. 10) . This space is filled by loose 

 connective-tissue and a fluid called perilymph, by which 

 the membranous labyrinth is surrounded and protected 

 from shocks. As you learnt in studying the skull, the 

 outer wall of the auditory capsule is perforated by a 

 small aperture, the fenestra ovalis (Fig. 10, fen. ov), in 

 which is fixed the stapes (stp), a small nodule of 

 cartilage connected with a bony rod or columella (col), 

 the cartilaginous hammer-shaped outer end of which, or 

 extra-columella, is fixed to the inner side of the tympanic 

 membrane (tymp. memb). The columella lies in the 

 tympanic cavity (tymp. cav), which is bounded externally 

 by the tympanic membrane, internally by the auditory 

 capsule, and at the sides chiefly by muscles and connec- 

 tive-tissue ; while below it communicates with the 

 pharynx by the Eustachian tube (eus. /). 



When sound-waves impinge on the tympanic mem- 

 brane, the vibrations to which they give rise are trans- 

 mitted by the columella to the stapes, and so to 

 the perilymph. Thence they are communicated to the 

 endolymph and act as stimuli to the auditory cells of 

 the acoustic spots, and the impulses being carried to the 

 brain by the auditory nerve, give rise to the sensation of 

 sound. Whether or not all the acoustic spots are truly 

 auditory in function is not known : it seems that the 

 semicircular canals are really concerned with the sense of 

 direction and velocity and with the maintenance of 

 equilibrium. 



The sense of sound can be destroyed by injury to the 

 membranous labyrinth, by cutting the auditory nerve, by 

 destroying the brain, or to a great extent at least by 

 injury to the tympanic membrane or columella. 



