n 5 8 



THE EAR. 



atmospheric pressure. As the handle of the malleus is attached to the 

 membrana tympani, it must also move inwards ; but as this bone 

 rotates round its axis, the head of the malleus is carried outwards, 

 and it carries the upper part of the body of the incus along with it. 

 The incus also rotates round an axis passing through the lower part 

 of the body, so that the long process passes inwards, driving the stapes 

 before it. During the negative phase of the sound-wave — that is to 

 say, when the pressure is less than atmospheric pressure — the reverse 

 action takes place, with this difference, that the malleus can unlock 

 itself from the incus and swing some distance outwards without carry- 

 ing the incus along with it. 



In the transmission of the effect of a sound-wave from the membrana 

 tympani to the labyrinth, there is a change in the amount of force, and 

 a corresponding change in the amplitude of the movement. We may 

 regard the mechanism of transmission as serving the purpose of con- 

 veying periodical variations of 

 pressure, occurring in the air, 

 into a minute cavity or space full 

 of fluid, in which the terminal 

 organ of hearing exists. It is 

 extremely difficult to transmit 

 sound-waves from air into a fluid, 

 especially if the cavity containing 

 the latter be of very small dimen- 

 sions. This result is attained 

 with a minimum loss of energy 

 by the chain of bones. The chain 

 may be regarded as a lever, in 

 which the manubrium of the 

 malleus forms the long arm, the 

 axis of rotation passing through 

 the malleus and incus forms the 

 fulcrum, and the long process of 



Fig. 421-To show the relations of the malleus the mcug carry i ng t he stapes, 

 and incus to one another. — Helmholtz. „ ,, ' . J ° i > 



m, Malleus; *, incus; p, its long process ; forms the short arm. I he pres- 

 t, tooth of incus fitting against ridge of sure of the sound-wave, applied 

 malleus; a-b, axis of rotation of the by the membrana tympani to the 



bones ; x, y, z, line -joining short pro- J , . „ ,, J K-, 



cess of incus with end of handle of malleus, manubrium of the malleus, may 



be regarded as the force, while 

 the resistance is at the base of the stapes, by which the pressures 

 are communicated to the labyrinth. The mechanism is therefore 

 a lever of the second order. Careful measurements have shown 

 that the length of the long arm is to the length of the short arm as 

 1-5:1. The resulting force at' the stapes is therefore correspondingly 

 increased, while the amplitude of the movement at the stapes is 

 correspondingly less than at the tip of the manubrium. Thus the 

 pressures are conveyed with great efficiency from the membrana 

 tympani to the labyrinth, and the amplitude of the oscillation is 

 diminished so as to be adapted to the small capacity of the labyrinth. 

 Further, it is to be observed that as the drum-head is nearly twenty 

 times larger than the fenestra ovalis, the energy of the movements 

 of the membrana tympani is concentrated on an area twenty times 

 smaller ; hence the pressure of a sound-wave, acting on the membrana, 



