The sonorous waves penetrate directly into the auditory canal, or after they have encountered the outer portion of the ear, the sinuosities of which they follow, and the ear itself vibrates to the shock of sounds, and these vibrations are transmitted gradually to the organ. Savart, who has demonstrated this phenomenon by experiments, observes that the very irregular surface of the ear always presents some portion at an angle most favourable to the sonorous waves, whatever may be their direction; in fact, the force with which they act upon its walls is in direct proportion to their approach to the perpendicular.
If, for example, the pavilion of the right ear be covered with some substance which obliterates all the inequalities and transforms it into a plane surface, a sound cannot be heard on that side so well as on the left, when produced at an equal distance from both ears. It is to be presumed, also, that the pavilion, which increases all sounds equally, does not vibrate in unison with any one sound, nor has it, owing to the irregularities of its surface, any one peculiar to itself. And finally, the form of the pavilion, and its inclination in relation to the head, seem to have a certain influence on the acuteness of hearing.
Besides the vibrations which enter directly into the auditory canal, and those which come from the pavilion, this canal receives also those of the bones of the cranium and transmits them to the tympanum. These last and those of the pavilion reach the tympanum sooner than the first, for the reason already stated, that sounds move more rapidly in fluids and solids than in the atmosphere. It receives there-. fore two orders of vibrations, but in passing to this membrane the vibrations of the air are transformed into those of a solid body; from which we may conclude with Savart and Muller, that the function of the tympanum is to serve as a medium between the air and the small bones of the ear, by changing, as we have just seen, the atmospheric vibrations.
Sounds, augmented by the external ear, and concentrated upon the tympanum, are transmitted to the little bones, and again augmented during this transit, by a more close concentration upon the base of the stapes.
We have seen that the contraction of the muscle of the hammer causes tension of the tympanum. This membrane then passes from a state of repose to a variable degree of tension, upon the effects of which physiologists are not agreed. According to Bichat, it is more tense in proportion to the feebleness of the sounds, and to the greater necessity of action of the organ in order to perceive them. According to Muller and Savart, the tension protects the organ of hearing against too violent sounds by lessening the conducting power of the tympanum. According to Longet, the muscle of the malleus has no other function than to obviate variations in the tension, and especially to prevent the entire relaxation of the membrane; in a word, it is the key of the tympanum.
The sonorous waves traverse the chain of little bones, and are transmitted by it to the fluid in the labyrinth, thus changing their medium without losing their intensity. If the little bones were articulated in such a manner as to form a rigid straight line, instead of an elastic broken one, the distance between the tympanum and the fenestra ovalis being susceptible to variation, the result would be that in certain cases the pressure on the tympanum and on the fenestra ovalis would be too great, which the elasticity of the chain and its articulations prevent. The tympanum can only exert a limited pressure on the fenestra ovalis, and when it is at its greatest distance from it, the stapes is held in its place, in front of this opening, by its muscle. This is the theory of Savart, which was adopted and developed by M. Longet.
The walls of the tympanum inclose air. which propagates the vibrations from the tympanum, and transmits them by means of the membrane of the fenestra rotunda to the fluid of the labyrinth. These vibrations lose their intensity on becoming aerial, and this fact has led to the idea that possibly they may differ in their timbre from those transmitted by the little bones.
However this may be, the principal use of the air in the cavity of the tympanum is not to transmit the vibrations of that membrane; but to balance the pressure of the atmosphere on its external surface, and thus to render it completely independent between two equal pressures. This is effected by means of the Eustachian tube, which conducts the air into the middle ear. The temporary obstruction of this canal induces buzzings in the ear, causing temporary deafness, which is intensified by its entire obliteration. The canal also serves as an outlet for mucus and other fluids, which may be secreted in the cavity of the tympanum.
The sonorous waves enter the vestibule by the fenestra ovalis; this opening is closed by the base of the stapes, and receives the vibrations from the chain of little bones. The membrane of the fenestra rotunda transmits to the scala tympani of the cochlea the aerial vibrations of the cavity of the tympanum. This membrane, as Scarpa has remarked, is a secondary tympanum.
On reaching the labyrinth the vibrations are propagated to the fluid which bathes it, and thus reach the membranous labyrinth, and the scala vestibuli of the cochlea, where finally they encounter the extremities of the ramifications of the auditory nerve.
Besides the sonorous aerial waves, the ear perceives, as has already been stated, those which have been caused by an impression on the bones of the skull. Thus, when a sonorous body is held between the teeth, or against the walls of the cranium, the sound is perceived by the auditory apparatus. It is in this way that, in spite of the loss of the tympanum and the small bones of the ear, some persons can still perceive sounds of external origin. But it is indispensable that the membranes of the fenestra ovalis and fenestra rotunda, which close the openings of the labyrinth into the cavity of the tympanum, should still remain perfect, and that the fluid of the labyrinth should still bathe its cavities. But, as may be easily conceived, hearing under these circumstances is very limited, since it can only take place when the sonorous body is in contact with the bones of the head.