722 HEARING. [CH, LIII, 



of the auditory nerve. These are derived from the cochlear 

 division already mentioned. This passes up the axis of the 

 cochlea, and in its course gives off fibres to the lamina spiralis. 

 These fibres are thick at their origin, but thin out peripherally, 

 and containing bipolar ganglion cells form the ganglion spirale. 

 Beyond the ganglion at the edge of the lamina the fibres pass up 

 and become connected with the organ of Corti, arborising around 

 the hair-cells. 



Physiology of Hearing. 



Sounds are caused by vibrations ; when a bell or 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, that is, it is shriller. The vibrations are trans- 

 mitted as waves through the air, 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 XLIX. 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 Reissner, and set the endolymph 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 syhat 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 connection with the external ear there is not much more to 

 be said ; the pinna in many animals is large and acts as a kind of 

 natural ear-trumpet to collect the vibrations of the air ; in man 

 this function is to a very great extent lost, and though there are 

 muscles present to move it into appropriate postures, they are not 



