HEARING. 



259 



inwards at its extremity to articulate with the third ossicle, 

 the stapes. The stapes, as its name implies, is shaped exactly 

 like a stirrup; it lies horizontally, 

 its head articulating with the incus, 

 and two branches extending inwards 

 to its base, which is fixed by means 

 of a ligament which surrounds it in 

 the fenestra ovalis. Thus, it will be 

 perceived, the tympanic ossicles are 



suspended between the tip of the Fig. 128. TYMPANIC Ossi- 

 processus gracilis of the malleus in 

 front, and the extremity of the hori- 

 zontal process of the incus behind, 

 and are capable of a swinging motion 

 round the line joining those points, 

 of such a description that the de- 

 scending process of the incus swings 

 outwards or inwards with the handle 

 of the malleus, and communicates 

 its movement to the stapes. 



192. This is precisely the principal 

 movement which takes place, and it 

 is accomplished by two muscles. 

 The tensor tympani muscle arises 

 from the cartilaginous wall of the Eustachian tube, and 

 entering the tympanum, is confined by a ledge of bone 

 to the inner wall, till it is opposite the position of the 

 malleus; it then becomes tendinous, and stretches across 

 the cavity, to be attached near the base of the handle 

 of that bone, pulling it inwards, and so increasing the con- 

 cavity of the membrana tympani, and putting that membrane 

 on the stretch. An antagonistic muscle, the laxator tympani, 

 less distinct, arises likewise from the Eustachian tube, and 

 is attached to the malleus above the level of the processus 

 gracilis; thus pulling the head of the bone inwards, and 

 swinging the handle outwards. Now, as has been mentioned, 

 the internal ear is filled with liquid, which is incompressible; 

 and it is surrounded with unyielding walls, save only at the 

 two fenestrse; when, therefore, the tensor tympani pulls the 

 handle of the malleus inwards, so as to make the membrana 



CLES OF JKIGHT CAR. a, 



Processus gracilis of mal- 

 leus; b, posterior process 

 of incus; the line ab is 

 the axis of rotation. 

 When c, the handle of 

 the malleus, with fibres 

 of the membrana tym- 

 pani radiating from its 

 extremity, is pulled in- 

 wards by e, the tendon 

 of the tensor tjonpani, 

 the incus likewise ro- 

 tates arid pushes d, the 

 stapes, in at the fenestra 

 ovalis; /, tendon of sta- 

 pedius. 



