266 OF SENSATION, AND THE ORGANS OF THE SENSES. 



ratus of the higher classes is most admirably adapted for this purpose. The 

 fenestra ovalis is not, however, the only cjiannel of communication between 

 the tympanum and the labyrinth; for there is, in most animals, a second aper- 

 ture, the fenestra rotunda, leading into the cochlea, and simply covered with 

 a membrane. It is generally supposed that, the labyrinth being filled with a 

 nearly incompressible fluid, this second aperture is necessary to allow of the 

 free vibration of that fluid, the membrane of the fenestra rotunda being made 

 to bulge out, as that of the fenestra ovalis is pushed in. It may, however, be 

 easily shown by experiment, as well as by reference to comparative anatomy, 

 that no such contrivance is necessary ; for sonorous undulations may be excited 

 in a non-elastic fluid, completely enclosed within solid walls at every part, 

 except where these are replaced by the membrane through which the vibra- 

 tions are propagated; and this is precisely the condition, not only of the 

 Invertebrated animals, but even of Frogs ; in which last a tympanic apparatus 

 exists, without a second orifice into the labyrinth. Moreover it is certain, that 

 the vibrations of the air in the cavity of the tympanum, must of themselves 

 act upon the membrane of the fenestra rotunda; and this is perhaps the most 

 direct manner in which the fluid in the cochlea will be affected ; although it 

 will ultimately be thrown into much more powerful action, by the transmission 

 of vibrations from the vestibule. For it has been satisfactorily determined by 

 experiment (xn.), that vibrations are transmitted with very much greater 

 intensity to water, when a tense membrane, and a chain of insulated solid 

 bodies, capable of free movement, are successively the conducting media, than 

 when the media of communication between the vibrating air and the water 

 are the same tense membrane, air, and a second membrane : or, to apply this 

 fact to the organ of hearing, the same vibrations of the air act upon the fluid 

 of the labyrinth with much greater intensity, through the medium of the chain 

 of auditory bones and the fenestra ovalis, than through the medium of the air 

 of the tympanum and the membrane closing the fenestra rotunda. The 

 fenestra rotunda is not to be considered as having any peculiar relation with 

 the cochlea; since, in the Turtle tribe, the former exists without the latter. 



359. In regard to the functions of particular parts of the labyrinth, no cer- 

 tainty can be said to exist. From the experimental results already stated, it 

 appears likely that, the greater the extension of the cavity into the dense sub- 

 stance of the bone, the greater will be the resonance communicated to the 

 fluid, and thence transmitted to the nerves exposed to its influence. It is 

 commonly supposed that the Semicircular Canals have for their peculiar 

 function, the reception of the impressions by which we distinguish the direc- 

 tion of sounds ; and it is certainly a powerful argument in support of this view, 

 that, in almost every instance in which these parts exist at all, they hold the 

 same relative position to each other as in Man, their three planes being nearly 

 at right angles to one another. The idea, however, must be regarded as a 

 mere speculation, the value of which cannot be decided without an increased 

 knowledge of the laws, according to which sonorous vibrations are transmitted. 

 Regarding the special function of the Cochlea, there is precisely the same 

 uncertainty. This part of the organ is peculiar in one respect, that the 

 expansion of the auditory nerve is here spread out (upon the lamina spiralis) 

 in closer proximity with the bone itself, than it is in any other part of the 

 labyrinth ; so that the vibrations of the bone will be more directly communi- 

 cated to the nerve. It is not easy to see, however, what can be the peculiar 

 object of this disposition, in regard to the function of hearing. By M. Duges 

 it is surmised, that by the cochlea we are especially enabled to estimate the 

 pitch of sounds, particularly of the voice ; and he adduces, in support of this 

 idea, the fact, that the development of the cochlea follows a very similar 

 proportion with the compass of the voice. This is much the greatest in the 



