192 THE HUMAN BODY. 



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. Accord- 

 ing to Miiller and Savart, the tension protects the organ 

 of hearing against too violent sounds by lessening the con- 

 ducting 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 dis- 

 tance 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. 



