574 THE HUMAN BODY. 



the lamina spiralis at the apex more than compensating for 

 the less size of the bony tube there) and is stretched tight 

 across, but loosely in the other direction. A membrane so 

 stretched behaves as a set of separate strings placed side by 

 side, somewhat as those of a harp but much closer together; 

 and each string would vibrate at its own period without in- 

 fluencing much those on each side of it. Probably, then, 

 each transverse band vibrates to simple tones of its own 

 period, and excites the hair-cells which lie on it, and through 

 them the nerve-fibres. Perhaps the rods of Corti, being stiff, 

 and carrying the reticular membrane, rub that against the 

 upper ends of the hair-cells which project into its apertures 

 and so help in a subsiduary way, each pair of rods being 

 especially moved when the band of basilar membrane carrying 

 it is set in vibration. The tectorial membrane is probably a 

 "damper;" it is soft and inelastic, and suppresses the vibra- 

 tions as soon as the moving force ceases. 



Function of the Vestibule and Semicircular Canals. 

 Many noises are merely spoiled music; they are due to tones so 

 combined as not to give rise to periodic vibrations; these are 

 probably heard by the cochlea. If a single violent air-wave 

 ever cause a sound sensation (which is doubtful, since any vio- 

 lent push of an elastic substance, such as the air, will cause it 

 to make several rebounds before coming to rest) we perhaps 

 hear it by the vestibule ; the otoliths, there in contact with 

 the auditory hairs, are imbedded in a tenacious gummy mass 

 quite distinct from the proper endolymph, and are not 

 adapted for executing regular vibrations, but they might 

 yield to a single powerful impulse and transmit it to the hair- 

 cells, and through them stimulate the nerves. There is reason 

 to believe that the semicircular canals have nothing to do 

 with hearing; their supposed function is described in Chapter 

 XXXVI. 



Auditory Perceptions. Sounds, as a general rule, do not 

 seem to us to originate within the auditory apparatus; we 

 refer them to an external source, and to a certain extent can 

 judge the distance and direction of this. As already men- 

 tioned, the extrinsic reference of sounds which reach the laby- 

 rinth through the general skull-bones instead of through the 

 tympanic chain is imperfect or absent. The recognition of 

 the distance of a sounding body is possible onlv when the 

 sound is well known, and then not very accurately; from its 



