152 ON THE SENSES. 



tioD. It is apparent that this will be thrown into vibration, ai il that its vibra- 

 tions will exactly accord in intensity and number with those (f the air- waves, 

 and mediately also with those of the body by which the latter are set in motion. 

 Each wave of condensation, the several particles of which are pressing forward 

 in the direction of the wave, will communicate to the particles of the tympanum 

 the same movement, and therefore increase its inward curvature, and at the same 

 time its tension ; and each wave of rarefaction — the direction of the pai'ticles 

 being now opposed to the general movement of the wave and away from the 

 tympanum — draws the latter, in a certain measure, after it, and the vaulted mem- 

 brane is correspondingly flattened towards the external air passages. It has 

 been a subject of controversy, in a physiological point of view, whether the 

 movement impressed on the tympanum liy the waves of sound, be, like that of 

 the string before described, a normal, vibratory movement, or an undulatory 

 one like that produced by alternate impulsion and retrogression in any fluid, as, 

 for instance, the air. Without entering further into this question, the discussion 

 of which would involve an analytical inquiry into the doctrine of undulations, 

 we shall only remark that, in our own opinion, the tympanum of the ear can, 

 under no circumstances, execute any other movement than vibrations of the 

 former order. If we deliver a smart blow on the head of a real drum it is ev 

 ident that the elastic membrane will be driven inward in a convex or dome- 

 shaped curve, since here, as in the string, the middle particles are most movable ; 

 those lying nearer to the rim are less so, while those immediately at the rim, 

 over which the skin is stretched, are immovable, like the points at which the 

 string is attached. By thus bending inwards, the tension of the membrane is 

 necessarily increased, wherefore the inward bending can only proceed to the 

 point at "which the elastic force of the increasing tension finds itself in equi- 

 librium with the force which has been applied from without ; when the impulse 

 of the latter gives way the elastic force drives the membrane back, as it does 

 the thread, not to its original plane, but with such velocity that it passes that 

 line and is now curved outwardly, until again arrested by the increasing ten- 

 sion and again driven inwardly. Thus the membrane with which the drum 

 is covered continues to oscillate inwards and outwards with diminishing curva- 

 tures, until the whole movement subsides to rest. Now, the membranous tym- 

 panum of the ear is placed under precisely similar conditions, and must there- 

 fore execute the same sort of vibrations ; its particles are not at all movable in 

 an equal degree, so that each, as is the case with the particles of the air, can, in 

 like degree, follow the impulse of the air-wave, the movement of each being ex- 

 clusively dependent on the direction and force of the impelling particles of that 

 wave ; on the contrary, in consequence of the adhesion of the.membrane through- 

 out its who'e circuit to the walls of the auditory canal, the particles next to the 

 adherent edge are, as in the artificial drum-head, immovable ; those at the centre 

 the most movable j the intermediate, movable in a progressive ratio from the 

 periphery to the centre. The magnitude of the movement of each particle depends, 

 therefore, not merely on the impulsive force of the air- waves, but also on its po- 

 sition and consequent mobility. If we represent to ourselves an air-wave of 

 condensation as striking perpendicularly upon the tympanum and simultaneously 

 exerting an equal force upon every particle of the membrane, it is evident, from 

 the diiferent degree of resistance opposed by the several particles — a resistance 

 which increases according to the distance of each particle from the centre — that 

 the result of the impulse will be a convex flexion of the membrane towards the 

 interior cavity. Upon the same principle the succeeding wave of rarefaction 

 will be followed by a similar flexion in the opposite direction. If no wave of 

 rarefaction followed the wave of condensation, the tympanum, when the impulse 

 of the latter was withdrawn, would still spring back through its own elasticity, 

 ind undergo, like the drum skin, after one blow, a series of alternating vibrations 

 iround its line of equilibrium, the velocity of which would depend alone on the 



