METHOD OF TESTING SOUND-CONDUCTION. 807 



inwards to the scala tympani, and can be bulged outwards towards the tympanic 

 cavity by the impulse communicated to it by the movement of the perilymph 

 (fig. 577, r). The flexion waves must correspond in number and intensity to the 

 vibrations of the auditory ossicles, and must also excite the free terminations of 

 the auditory nerve, which float free in the endolymph. 



As the endolymph of the saccule and utricle lying in the vestibule receives the 

 first impulse, and as these communicate anteriorly with the cochlea, and posteriorly 

 with the semicircular canals, consequently the motion of the 

 perilymph must be propagated through these canals. To 

 reach the cochlea, the movement passes from the saccule 

 (lying in the fovea hemispherica) along the scala vestibuli to 

 the helicotrema, where it passes into the scala tympani, where 

 it reaches the membrane of the fenestra rotunda, and causes it 

 to bulge outwards. From the utricle (lying in the fovea hemi- 

 elliptica), in a similar manner the movement is propagated Externalappearance of 

 through the semicircular canals. Politzer observed that the the labyrinth, fenes- 

 endolymph in the superior semicircular canal rose when he ' tra ovalis, cochlea to 

 caused contraction of the tensor tympani by stimulating the tne left, and (/) the 

 trigeminus, just as the base of the stapes must be forced talf and (s) posterior 

 against the perilymph with every vibration of the membrana semicircular canal 

 tympani. (left). 



[Practical. It is well to view the organ of hearing as consisting of two 

 mechanisms : 



1. The sound-conducting apparatus. 



2. The sound-perceiving apparatus. 



The former includes the outer ear, with its auricle and external meatus; the 

 middle ear and the parts which bound it, or open into it. The latter consists of 

 the inner ear with the expansion of the auditory nerve in the labyrinth, the nerve 

 itself, and the sound-perceiving and interpreting centre or centres in the brain 

 (p. 703).] 



[Testing the Sound-conduction. In any case of deafness, it is essential to 

 estimate the degree of deafness by the methods stated at p. 798, and it is well to 

 do so both for such sounds as those of a watch and conversation. We have next to 

 determine whether the sound-conducting or the sound-perceiving apparatus is affected. 

 If a person is deaf to sounds transmitted through the air, on applying a sounding 

 tuning-fork to the middle line of the head or teeth, if it be heard distinctly, 

 then the sound-perceiving apparatus is intact, and we have to look for the cause of 

 deafness in the outer or middle ear. In a healthy person, the sound of the tuning- 

 fork is heard of equal intensity in both ears. In this case the sound is conducted 

 directly to the labyrinth by the cranial bones. In cases of disease of the sound- 

 conducting mechanism, the sound of the tuning-fork is heard loudest in the deafer 

 ear. Ed. Weber pointed out that, if one ear be stopped and a vibrating tuning- 

 fork placed on the head, the sound is referred to the plugged ear, where it is heard 

 loudest. It is assumed that when the ear is plugged, the sound-waves transmitted 

 by the cranial bones are prevented from escaping (Mach). If, on the contrary, the 

 sound be heard loudest in the good ear, then in all probability there is some affection 

 of the sound-perceiving apparatus or labyrinth, although there are exceptions to 

 this statement, especially in elderly people. Another plan is to connect two tele- 

 phones with an induction machine, provided with a vibrating Neef 's hammer. The 

 sounds of the vibrations of the latter are reproduced in the telephones, and if they 

 be placed to the ears, then the healthy ears hear only one sound, which is referred 

 to the middle line, and usually to the back of the bead. In diseased conditions 

 this is altered it is referred to one side or the other.] 



