698 Comparative Morphology of Chordates 



they are under voluntary control may be able to render himself tempo- 

 rarily deaf by contracting (one or both of?) the muscles. 



From the fenestra vestibuli the vibrations are passed on to the 

 perilymph. The relation of the fenestra to the cochlear passages (Fig. 

 518) is such that the vibrations most readily progress up the spiral 

 scala vestibuli and, at the apex of the cochlea, pass into the scala 

 tympani and continue downward along it to arrive finally at the 

 fenestra cochleae (rotunda), to whose membrane the vibrations are 

 imparted. The perilymph being an incompressible fluid, the membrane 

 of the fenestra cochleae must move outward synchronously with each 

 inward movement of the stapedial plate. 



As the vibrations pass through the perilymph of the scala vestibuli, 

 they are freely transmitted to the endolymph of the membranous 

 cochlea, the two fluids being separated only by a membrane of extreme 

 thinness (Fig. 516). The vibrations of the cochlear endolymph are 

 somehow caused to stimulate the "hair cells" of the organ of Corti. 

 Just how is more a matter of conjecture than of precise knowledge. 

 Whether the agencies immediately concerned are the pillar cells 

 (least likely), or the transverse fibers of the basilar membrane, or 

 vibrations of the membrana tectoria in close relation to the "hairs" 

 projecting from the sensory cells into the endolymph, is a matter for 

 further investigation. But the progressive change in the dimensions of 

 the histologic elements of the organ of Corti along its spiral length 

 makes it seem certain that the organ is locally adapted for response to 

 vibrations of a particular frequency ranging from lowest frequency at 

 the apex to highest at the base of the cochlea. A musical chord made 

 by striking three piano keys would presumably affect three quite 

 separate regions of the organ. It is a mechanism which serves for dis- 

 criminative hearing. 



The sensory range of the cochlea is not the same in all mammals. 

 Dogs respond to vibrations whose frequency is above the range of the 

 human ear — i.e., "supersonic" for man. The larynx of the bat can 

 produce supersonic vibrations, but the bats hear them. While flying 

 in darkness, bats emit these high-pitched sounds (i.e., "sounds" to 

 them) which enable them to avoid collision with one another. Reflec- 

 tion of the vibrations by surfaces of external objects (echoes) enables 

 the bats to avoid hitting the objects. The pinnae of bats are relatively 

 large. 



The sensory structures of the semicircular canals serve for equili- 

 bration, not for hearing. The planes of the three canals are at angles of 

 approximately 90 degrees to one another. The vertical canals are at 

 angles of about 45 degrees to the sagittal plane of the animal. Any 



