SENSE OF HEARING. 417 



part of the membrane, making it incline inwards (Fig. 110) : 

 this convexity or tension is produced, either by variations in 

 pressure in the air in the drum, or by the action of a muscle 

 (internal muscle of the malleus). If from any cause the air 

 of the drum becomes rarefied, the exterior air presses upon 

 the membrane, forcing it deeper into the 

 cavity of the tympanum, and, conse- 

 quently, stretching it while increasing its 

 convexity (in the direction indicated by the 

 arrows in Fig. 110). The internal musde 

 of the malleus (tensor tympani) produces 

 the same effect : it draws inwards the handle 

 of the bone, and consequently the mem- 

 brane, increasing the convexity and the ten- 

 sion of the latter. 1 This is the only mus'cle 

 whose action or existence has been satisfac- 

 torily demonstrated ; either the other so- 

 called muscles of the internal chamber of 

 the ear have no existence (anterior and ex- 

 ternal muscles of the malleus), or their 

 action is not yet perfectly understood Fig. no. Membrane 

 (muscle of the stapes, stapedius), at all ^ ( j he . t y in anuin 

 events they do not produce the effect of re- 

 laxing the membrane, which, by means of its elasticity, re- 

 turns of itself to the position of repose directly its tensor 

 muscle ceases to contract. 



The purpose of this temporary tension of the membrane is 

 now easily understood. Bichat supposed that the tension of 

 the membrane must be increased in order to increase the 

 power of the sound ; this hypothesis is, however, contrary to 

 the laws of physics, and Savart has shown that the tension 



1 Some persons possess the power of contracting the nmsculus 

 tensor tyrapani at will, and thus stretching the membrane of the 

 tympanum. This tension is manifested by a slight cracking sound, 

 which is produced in the ear at each contraction of the muscle: the 

 movements made by the membrane under the influence of these 

 voluntary contractions may also be observed by the aid of the 

 speculum. Nearly all those physiologists whose attention has 

 been directed to this circumstance, and who have attempted to 

 produce this contraction, have succeeded with the greatest ease: 

 Berard, Miiller, and Wollaston are especially mentioned as having 

 done so. (Bormafont, op. cit., p. 270.) 



* a a, Membrane of the tympanum. 6, The malleus, c, The incus. <?, The 

 stapes. 



27 



