432 ON SENSATION, AND THE ORGANS OF THE SENSES. 



this canal serves for the conduction of the speaker's voice to his ears ; but this 

 is certainly not the case in any considerable degree ; for, when the Eustachian 

 tubes are obstructed by disease, the patient hears his own voice well, though 

 other sounds are indistinct ; and it is easily shown, that its transmission is 

 chiefly accomplished in other ways. The common idea is, that it serves the 

 same purpose with the hole in an ordinary drum ; the effect of which is gene- 

 rally supposed to be, the removal of the impediment to the vibrations of the 

 membrane, that would be offered by the complete inclosure of the air within. 

 It does not appear, however, that any such impediment is really offered ; and 

 the effect of the hole in the drum seems rather to be the communication, to 

 the ear of the auditor, of the sonorous vibrations of the contained air ; which 

 are thus transmitted directly through the atmosphere, instead of being weak- 

 ened by transmission through the walls of the instrument. Hence there is no 

 real analogy in the two cases. The principal object of the Eustachian tube 

 (which is always found where there is a tympanic cavity), seems to be, the 

 maintenance of the equilibrium between the air within the tympanum and the 

 external air ; so as to prevent inordinate tension of the membrana tympani, 

 which would be produced by too great or too little pressure on either side, 

 and the effect of which would be imperfection of hearing. It also has the 

 office of conveying away mucus secreted in the cavity of the tympanum, by 

 means of cilia vibrating on its lining membrane ; and the deafness, conse- 

 quent on occlusion of this tube, is in part explicable by the accumulation, 

 which will then take place in the tympanum. 



565. From what has been stated, it is evident that sonorous undulations 

 taking place in the air, will be propagated to the fluid contained in the laby- 

 rinth, through the tympanum, the chain of bones, and the membrane of the 

 fenestra ovalis to which the stapes is attached, without any loss, but rather 

 an increase, of intensity. Why water should be chosen as the medium through 

 which the impression is to be made upon the nerve, it is impossible for us to 

 say with anything like certainty, in our present state of ignorance as to the 

 physical character of that impression. But, the problem being, to communi- 

 cate to water the sonorous undulations of air, the experimental results already 

 detailed satisfactorily prove that, whilst this may be accomplished, in a de- 

 gree sufficient for the wants of the inferior animals, by the simple interposition 

 of a tense membrane between the air and the fluid, the tympanic apparatus 

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

 tra ovalis is not, however, the only channel of communication between the 

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

 the fenestra rotunda, leading into the cochlea, and simply covered with a mem- 

 brane. 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 ex- 

 cited in a non-elastic fluid, completely inclosed within solid walls at every 

 part, except where these are replaced by the membrane through which the 

 vibrations 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 lias been satisfactorily determined by 



