574 



HEARING. 



the malleus regulates the memlirana tympani, 

 increases its tension, and thus limits the extent 

 of the excursions of its vibrations. The con- 

 traction of the stapedius muscle causes the 

 base of the stapes to compress the membrane 

 of the fcnestra ovalis to a greater or less extent, 

 so that the degree of tension of that membrane 

 depends on the condition of this muscle. 

 Compression exerted upon the membrane of 

 the fenestra ovalis extends to the perilymph 

 and through it is propagated to the membrane 

 of the fenestra rotunda, and in this way the 

 same apparatus which regulates the tension of 

 the membrane of the fenestra ovalis performs 

 that office for that of the fenestra rotunda, and 

 Savart has devised a little apparatus which 

 very prettily illustrates the manner in which 

 this may take place. In a disc of wood 

 ( b, >,g. 265) 



Fig. 265. 



of a sufficient 

 thickness.hehol- 

 lows out two ca- 

 vities, o and r, 

 which commu- 

 nicate at their 

 bottoms with 

 each other by a 

 narrow canal (c) 

 hollowed in the wood, but not open on its sur- 

 face ; a thin membrane is extended over each 

 of the cavities. Thus, the air contained in 

 these cavities may pass easily from one to the 

 other, and may always maintain the same 

 degree of elastic tension in both. If, then, 

 a vibrating glass be brought near the mem- 

 brane r covered with a layer of sand, it will 

 be found to enter freely into vibration, as 

 evinced by the active movements of the grains 

 of sand. If now pressure be made with the 

 finger on o, r will become convex in propor- 

 tion as o is rendered concave by the pressure, 

 and when in this state, the movements of the 

 sand will be much less considerable than be- 

 fore, presenting an effect precisely similar to 

 that produced on the tympanic membrane by 

 an increase of tension. Thus, the extent of 

 the excursions of the vibrations of the mem- 

 brane r is limited by the pressure exerted upon 

 o, and as the membranes of the two fenestra; 

 are related to each other in an analogous man- 

 ner, we may argue that pressure upon the larger 

 one, that of the fenestra ovalis, will occasion 

 tension of the smaller, that of the fenestra 

 rotunda, thereby limiting the extent of the 

 excursions of its vibrations. 



Moreover it appears, upon reference to the 

 anatomy of these parts, (seejig. 252, p. 550,) that 

 the only muscles which have been satisfactorily 

 demonstrated are tensors of the tympanum ; and 

 that at whatever extremity of the chain of ossicles 

 muscular effort be first exerted, a correspond- 

 ing effect will be produced at the other ; that 

 when the stapedius muscle acts, the malleus is 

 thrown into a position favourable to the tension 

 of the membrana tympani, and, on the other 

 hand, the contraction of the internus mallei 

 depresses the stapes, and consequently in- 

 trcases the tension of the membranes of the two 

 feuestra;. The cessation of muscular action 



restores all three membranes to their original 

 laxity, nor does it appear that they admit of 

 any further decree of relaxation through the 

 influence of any vital process. The incus 

 forms a bond of connexion between the two 

 other bones, and its motions depend entirely 

 upon theirs in consequence of its articulation 

 with both, while from the fixedness of its con- 

 nexion with the masloid cells, as well as its 

 intermediate position, and its not having any 

 muscles inserted into it, it is obvious that its 

 motions must be much more limited than those 

 of the other bones. Its use seems to be to 

 complete the chain in such a way, that by 

 reason of its double articulation with the mal- 

 leus on the one hand and the stapes on the 

 other, the tension of the tympanic membranes 

 may be regulated without any sudden or vio- 

 lent motion, which could scarcely be avoided 

 were the conductor between the membranes of 

 the tympanum and fenestra ovalis one piece of 

 bone. 



But whence the necessity of at all adding 

 to the ear this complex apparatus of tympanum 

 and tympanic membrane, and why might not 

 the sonorous impressions have been made di- 

 rectly upon the membranes which close the 

 openings to the labyrinth ? Upon this point 

 Savart has offered a conjecture which seems to 

 afford the most probable explanation as to the 

 true object of these parts. If the membranes 

 of the fenestra, he says, had been in imme- 

 diate contact with the atmosphere, their elastic 

 state would have been constantly undergoing 

 changes, under the influence of the vicissitudes 

 of temperature of the air, a circumstance which 

 would, in all probability, impair the power of 

 the organ in detecting differences of sounds. 

 He presumes therefore that the membrana tym- 

 pani prevents this contact of the atmosphere 

 with the membranes of the labyrinth, and that 

 the cavity of the tympanum and the mastoid 

 cells form a kind of receptacle in which the 

 air, which finds its way into the tympanum 

 through the Eustachian tube, acquires the con- 

 stant temperature of the body, and establishes 

 in front of the openings of the labyrinth a sort 

 of atmosphere proper to themselves, the tem- 

 perature of which does not vary. 



This same acute observer remarks that the 

 size of the membrana tympani in all proba- 

 bility, in the different species of animals, 

 exerts much influence upon the number of 

 sounds which they can perceive, and at the 

 same time upon the limits at which those 

 sounds begin or cease to be audible. Were 

 the tympanic membrane in man of greater size 

 than it is, there is no doubt that instead of 

 beginning to hear sounds which result from 

 about thirty vibrations in a second, we should 

 be able to hear only sounds of a higher pitch. 

 Moreover it may be reasonably presumed that 

 animals who have the membrana tympani 

 much larger than that of man, hear much 

 graver sounds than those which result from 

 thirty vibrations in a second; and, on the other 

 hand, there must be other animals who hear 

 very acute sourds only. 



Even in the human species, we observe in 



