xi EAR 189 



The membranous labyrinth does not fit tightly into the 

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

 space being left between it and the surrounding bone and 

 cartilage (Fig. 10). This space is filled by loose connec- 

 tive-tissue and a fluid called perilymph, by which tile 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, 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 extern- 

 ally 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 pharynx 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, 



