508 



HEARING. 



direct from them, or sometimes the otolithcs 

 are so placed with reference to the expansion 

 of the vestibular nerve, as to be able to com- 

 press it against the cranium. 



But in man and those animals in whom, in 

 addition to a more complicated labyrinth, there 

 is also an external auditory passage and tym- 

 panum, it would appear that the sonorous 

 vibrations are conducted in two ways ; first, 

 through the meatus externus and tympanum 

 to the vestibule and semicircular canals; and, 

 secondly, through the bones of the head di- 

 rectly to the auditory nerve. Sounds pro- 

 ceeding from external bodies, as Weber ob- 

 serves, are conveyed in the former way ; but 

 the oscillations of one's own voice, although 

 they in part find their way by the external pas- 

 sage, are chiefly conducted by the cranial bones; 

 and, as Professor \Vheatstone has remarked, 

 those sounds are best heard which are articu- 

 lated most in the mouth, and with that cavity 

 least open, as e, ou, te, kew. Closing both ears 

 by firmly pressing the hands upon them, one's 

 own voice is not heard less distinctly, but on 

 the contrary more loud and clear than when 

 botli ears are left open; and if only one tar be 

 closed, the voice is heard more distinctly and 

 louder in that ear than in the open one.* 



The observations and experiments of Weber 

 render it very probable that the cochlea is that 

 part of the labyrinth which is more particularly 

 suited to appreciate sounds communicated 

 through the solid case of the head, or, to use 

 his words, that sounds propagated through the 

 bones of the head are heard specially by the 

 cochlea, but that sounds conducted through the 

 external meatus are perceived by the membra- 

 nous vestibule and semicircular canals more 

 easily than by the cochlea. The following con- 

 siderations favour these views. 



It is an admitted fact in acoustics that 

 sounds are most perfectly conducted by sub- 

 stances of uniform elasticity, and that when 

 propagated from air or water to a solid, or 

 from a solid to air or water, they are conducted 

 much less completely. Now, inasmuch as the 

 cochlea may be regarded as part and parcel of 

 the cranial bones, the sounds which are pro- 

 pagated by these bones would reach the nervous 

 expansion in that portion of the labyrinth by the 

 most direct route; whereas, to aft'ect the re- 

 maining parts of the labyrinth, the sound must 

 be conducted from the bone through the peri- 

 lymph to the membranous vestibule and semi- 

 circular canals. Moreover, when it is con- 

 sidered that the cochlear nerves are disposed 

 in a radiated manner in the lamina spiralis, 

 it will appear evident that the oscillations pro- 



iiagated to the petrous portion of the temporal 

 x>ne must exert a direct influence on the coch- 

 lear portion of the auditory nerve. 



One or two experiments with the tuning-fork 

 show not only that the cranial bones do con- 

 duct, but also that sounds, inaudible or im- 

 perfectly audible through the meatus externus, 

 may be distinctly heard when the sounding 



* E. H. Weber DC A u.iitu iu AuuoUi. Auatoui. 

 et Physiolog. Lips. 1U34. 



body is brought into contact with a bone of the 

 cranium or face. When the tuning-fork is put 

 into vibration by striking it against any solid 

 body, if held near the external ear its vibrations 

 are heard distinctly, but let the handle be 

 applied to the teeth or to the superior maxilla, 

 and the sound appears much louder; or if the 

 fork be held near the ear until the sound has 

 almost died away, and then its handle be ap- 

 plied to the superior maxilla or the teeth, the 

 sound seems greatly to revive and continues 

 for a considerable period, the handle being 

 kept in contact with the bone. 



When the conducting stein of the sounding 

 tuning fork is placed on any part of the head, 

 if both ears be closed by being covered with 

 the hands, a considerable augmentation of the 

 sound will be observed.* If the sounding- 

 fork be kept in contact with the head for a 

 short time, both ears remaining open, and then 

 one ear be closed, the sound will be greatly 

 augmented in the closed ear, and will appear 

 to be heard exclusively by it. This experiment 

 is more striking if the stem of the tuning-fork 

 be applied to the mastoid process on one side : 

 when both ears remain open, the sound seems 

 to be heard chiefly by the ear in the vicinity 

 of which the stem is placed, but when the 

 opposite ear is closed, it appears as if the 

 sound were transferred from the open to the 

 closed ear; and if the ear be alternately 

 opened and closed, the sound will alternately 

 appear to be transferred from the one to the 

 other. Similar phenomena may be observed 

 if both ears are closed on the first application 

 of the tuning-fork. The sound is at first heard 

 in the adjacent ear, and either remains in it 

 or is transferred to the opposite one, according 

 as the former remains closed or is opened. 

 Mr. \Vheatstone adds that if the meatus and 

 concha of one ear be filled with water, the 

 sound from the tuning-fork will be referred to 

 the cavity containing the water in the same way 

 as when it contained air and was closed by the 

 hand. 



These phenomena afford obvious examples 

 of the communication of sound through the 

 bones of the head. The augmentation of the 

 sound in the closed ear appears to result, as 

 Mr. Wheatstone explains,-)- from the recipro- 

 cation of the vibrations by the air contained 

 within the closed cavity, and this explanation 

 is confirmed by the fact that when the meatus 

 is closed by a fibrous substance, such as wool, 

 no increase is obtained. 



The following rationale may be offered 

 of what occurs when the sound from the 

 tuning-fork is communicated to a closed 

 ear, in accordance with the views of Weber 

 respecting the function of the cochlea. The 

 vibrations of the fork are propagated by the 

 bones of the head to the cochlea, the fluid of 

 which being thus thrown into vibration causes 

 the membrane of the fenestra rotunda to vibrate, 



* These experiments were first suggested by 

 Professor Wheatstone. See his experiments on 

 audition in the Journal of the Royal Institution 

 for July 1827. 



t Loc. cit. 



