SENSE OF HEARING. 901 



tion. These facts enable us to understand the influence of the tensor tympani 

 muscle, in modifying the tension of the membrane, and thus causing it to vibrate 

 in reciprocation to sounds having a great variety of fundamental notes. More- 

 over, the fact that some persons are deaf to grave sounds, whilst they readily 

 hear the more acute, is thus accounted for. The tensor tympani, like the iris, 

 is probably excited to operation by a reflex action ; and it is by no means im- 

 probable that one of its functions may be, to prevent the internal ear from 

 being too violently affected by loud sounds, by putting the membrana tympani 

 into such a state of tension as not readily to reciprocate them. 



903. The uses of the Tympanic Cavity are very obvious. One of its pur- 

 poses is to render the vibrations of the membrane quite free ; and the other, to 

 isolate the chain of bones, in such a manner as to prevent their vibrations from 

 being weakened by diffusion through the surrounding solid parts. As to the 

 objects of the Eustachian tube, however, opinions have been much divided. 

 From the experiments of M tiller it appears that it does not increase the intensity 

 of sound, but that it prevents a certain degree of dulness which would attend it 

 if the cavity of the tympanum were completely closed ; of this dulness we are 

 conscious, when any tumefaction of the fauces causes an occlusion of the ex- 

 tremity of the tube. It has been supposed that, among other uses, 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 accom- 

 plished 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 generally supposed to 

 be the removal of an 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 weakened 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 ten- 

 sion 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 the vibratile cilia which clothe its lining mem- 

 brane ; and the deafness, consequent on occlusion of this tube, is in part ex- 

 plicable by the accumulation which will then take place in the cavity. 



904. 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 labyrinth 

 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 communicate to water 

 the sonorous undulations of air, the experimental results already detailed satis- 

 factorily prove that whilst this may be accomplished, in a degree sufficient for 

 the wants of the inferior animals, by the simple interposition of a tense mem- 

 brane between the air and the fluid the tympanic apparatus of the higher 

 classes is most admirably adapted for this purpose. The fenestra ovalis is not, 

 however, the only channel of communication between the tympanum and the 



