XI EAR 189 



The membranous labyrinth does not fit tightly into the 



cavity of the auditory capsule in which it is contained ; but 



a space is left between it and the surrounding bone and 



cartilage (Fig. 10). This space is filled by a fluid called 



perilymph, by which the mem'branous 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, Xhs. 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 (Figs. 9 and 10, 



col), the cartilaginous hammer-shaped outer end of which, 



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



membrane (Fig. 9, 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 connective 



tissue ; while below it communicates with the mouth by 



the Eustachian tube (eus. t). 



When sound-waves impinge on the tympanic membrane, 

 the vibrations to which they give rise are transmitted 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 certain that the semicircular canals are 

 really organs for 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. 



Notice that the general plan of all the sensory organs, 



