156 ON THE SENSES. 



the labyrinth, is as follows : If we imagine a human head vertically divided in 

 the middle from front to rear, and from the right half all the parts, both solid 

 and soft, to be removed until we arrive at the cavity of the tympanum and thus 

 bring to view from within the apparatus of the right ear with the membrane of 

 the tympanum, the appearances presented to us, when we look a little obliquely 



from above, will be those represented in the ac- 

 ^^' ' companying figure. T r indicates the tympa- 



num, H the malleus, A the incus, S t the stapes. 

 The handle of the malleus is firmly united to 

 the membrane of the tympanum, being inserted 

 between the two soft layers of which the latter 

 , consists and extending from the upper border 

 to the centre, so that, like the rod a c in the 

 schematistic figure, it must necessarily follow 

 the movements of the membrane. The neck 

 and the heavy spherical head of the malleus 

 project freely over the border of the membrane 

 into the cavity of the tympanum ; the long pro- 

 jection, on the other hand, runs, at the height of 

 the upper border of the tympanum, forward, 

 and is here tightly fastened, by one of the short, 

 elastic ligaments which spring from its point, 

 to the wall of the cavity. The neck of the malleus articulates with the summit 

 of the incus, the two surfaces being reciprocally adapted to one another, and 

 their edges secured by a capsular ligament which permits but slight displace- 

 ments. Through this arrangement the short, thick process of the incus is in 

 such a position as to form a right line with the long projection of the malleus, 

 being directed transversely backward as the former forward, and is fastened to 

 the hinder wall of the cavity by a sort of capsule which allows of its turning 

 on its longitudinal axis. The long projection of the malleus and the short pro- 

 cess of the incus answer therefore together to the transverse axis marked d ein 

 the first delineation, around which turns the little rod marked a h. The longer 

 process of the incus runs parallel to the handle (manuhrium) of the malleus, 

 from which it is separated by a small interval, its course through the cavity be- 

 ing rather more inward and backward. As the membrane of the tympanum lies 

 obliquely this process is consequently placed obliquely ; but its lower end, which 

 corresponds to the rod a f of the first delineation, bends upwards, so that the 

 foot-plate of the stapes, which is united to it, looks in a certain degree in the 

 same direction. This foot-plate is introduced, like the piston of a syringe, but 

 more loosely, into an oval aperture (fenestra ovalis) of the bony wall of the so- 

 called labyrinth; around its edge there extends to the margin of the aperture a 

 small membranous border which renders the aperture water-tight, while, through 

 its elasticity, it permits the stapes to make slight excursions up and down in the 

 receptacle. The opposed opening, (fenestra rotunda,) whose necessity was shown 

 above, exists in the labyrinth in the shape of a small round window, closed with 

 an elastic membrane. 



It will not be difficult for the reader, with the aid of our previous representa- 

 tion of the general scheme, to conceive in what manner this apparatus, under its 

 actual conditions, must operate when the membrane of the tympanum is made 

 to oscillate by the impression of the air- waves. If the membrane of the tym- 

 panum be driven inwards, both malleus and incus move as if they were but one 

 piece, though, at first, it is true, only the handle of the malleus is directly moved ; 

 but the incus follows the impulse, the adjustment admitting of no counter action. 

 Both turn upon the common axis, which consists of the long projection of the 

 malleus, and the short process of the incus, so that the neck and head of the 

 malleus, and thus the part of the apparatus lying above the axis, describe an arc 



