the Mode in which it administers to the Perception of Sound. 125 



When the membrana tympani moves outwards, as it will do 

 when a rarefied wave is incident upon it, it carries along with it 

 the handle of the malleus, and the tendency will be to pull out 

 the base of the stirrup-bone, a tendency which, no doubt, will 

 be in some degree yielded to*. And we may thus see how the 

 incidence of a rarefied wave may give rise to motion of the fluid 

 in the labyrinth, and consequently to such an excitation of the 

 auditory nerve as will occasion the perception of sound. 



It has been already observed that when the membrana tym- 

 pani has moved outwards, it has no property by which it can 

 restore itself to its original position. 



This function is performed by another and most important 

 part of the auditory apparatus — to wit, the muscles of the ear, 

 which are thus described by Mr. Wharton Jones. 



" Some anatomists admit four muscles — three attached to the 

 malleus, and one to the stapes. Of the three attached to the 

 malleus, two are described as having for their action the relaxa- 

 tion of the membrana tympani ; but these so-called laxatores 



tympani are merely ligaments Two muscles only can be 



strictly demonstrated, and these two are both tensors of the 

 tympanum." (Cyclop. Anat. and Physiology, vol. ii. p. 547.) 



Of these two muscles, the principal (tensor tympani) is at- 

 tached to the anterior surface of the handle of the malleus ; and 

 by its action et the handle of the malleus is drawn inwards and 

 forwards, whilst the head is moved in the opposite direction. . . 

 The result of this movement of the bone is that the membrana 

 tympani .... is also drawn inwards and stretched. - " In addi- 

 tion to which, C( the base of the stapes is forced against the ves- 

 tibular fenestra, in consequence of the movement communicated 

 by the head of the malleus to the incus, which tends to press 

 inwards the long extremity of the latter." (Ibid. p. 549.) 



The second and smaller of the muscles (stapedius) is "in- 

 serted into the posterior and upper part of the head of the 

 stapes." 



" The first effect of the action of this muscle will be to press 

 the posterior part of the base of the stapes against the vestibular 

 fenestra. At the same time the long branch of the incus will 

 be drawn backwards and inwards, and the head of the malleus 

 being by this movement of the incus pressed forwards and out- 

 wards its handle will be carried inwards, and the membrana 

 tympani thus put upon the stretch." (Ibid. p. 549.) 



It thus appears that it is the effect of both muscles : — 



* The action which takes place along the chain of bones is exactly that 

 which occurs along the bell-wires when a chamber-bell is rung. Of the 

 degree in which the stapes will yield to the tendency to pull it out more will 

 be said hereafter. 



