794 HEARING [CH. LYI. 



hair-like processes. There are several rows of these on the outer 

 and one row on the inner side. Between them are certain sup- 

 porting cells called cells of Deiters (fig. 495, x). This structure rests 

 upon the basilar membrane ; it is roofed in by a fenestrated mem- 

 brane or lamina reticularis into the fenestrse of which the tops of 

 the various rods and cells are received. When viewed from above, 

 the organ of Corti shows a remarkable resemblance to the key- 

 board of a piano. The top of the organ is roofed by the membrana 

 tectoria (fig. 494, t) which extends from the end of the limbus 

 (Us, fig. 494), a connective-tissue structure on the spiral lamina. 

 The spiral ganglion from which the cochlear nerve-fibres originate is 

 situated in the spiral lamina. The peripheral axons of its bipolar 

 cells arborise around the hair-cells of the organ of Corti : the central 

 axons pass down the modiolus, and thence to the pons (see p. 676). 



Physiology of Hearing. 



Sounds are caused by vibrations ; when a piano-string is struck, 

 it is thrown into a series of rapid regular vibrations ; the more 

 rapidly the vibrations occur the higher is the pitch of the musical 

 note; the greater the amplitude of the vibration, the louder or 

 more intense is the tone; if the vibrations are regular and simple 

 (pendular), the tone is pure ; if they are regular but compound, the 

 tone is impure, and its quality or timbre is dependent on the rate 

 and amplitude of the simple vibrations of which the compound 

 vibrations are composed. The vibrations are transmitted as waves, 

 and ultimately affect the hair-cells at the extremities of the 

 auditory nerve in the cochlea. The semicircular canals are not 

 concerned in the sense of hearing ; their function in connection with 

 equilibration is described in Chapter LI. The external and 

 middle ears are conducting; the internal ear is conducting and 

 receptive. In the external ear the vibrations travel through air ; in 

 the middle ear through solid structures membranes and bones ; and 

 in the internal ear through fluid, first through the perilymph on the 

 far side of the fenestra ovalis ; and then the vibrations pass through 

 the basilar membrane and membrane of Eeissner, and set the endo- 

 lymph of the canal of the cochlea in motion. 



This is the normal way in which the vibrations pass, but the endolymph may be 

 affected in other ways, for instance through the other bones of the head ; one can, 

 for example, hear the ticking of one's watch when it is placed between the teeth, 

 even when the ears are stopped. From this fact is derived a valuable practical 

 method of distinguishing in a deaf person what part of the organ of hearing is at 

 fault. The patient may not be able to hear a watch or a tuning-fork when it is held 

 close to the ear ; but if he can hear it when it is placed between his teeth, or on his 

 forehead, the malady is localised in either the external or middle ear ; if he can hear 

 it in neither situation, it is a much more serious case, for then the internal ear or the 

 nervous mechanism of hearing is at fault. In disease of the middle ear the hearing 

 of low tones is especially affected ; high tones appear to be transmissible by bone- 

 conduction more readily than low. 



