MECHANISM OF MIDDLE EAR 255 



direction as the mcmubrhnn. This movement is directly trans- 

 mitted to the stapes. The chain of bones, therefore, acts as a bent 

 lever whose fulcrum is at a,* the power arm being represented by 

 the dotted line and the load arm by the line i^ — a. According to 

 Helmholtz, the distance p^ — a is 9-5 mm., and i — a is 6-3 mm. The 

 movement at i therefore will be only two-thirds of the move- 

 ment at p, but will have 1| times the intensity. We have now to 

 consider the transmission of this power to the foot of the stapes. If 

 X — y represents the range of the tip of the incus, c — d the distance 

 from c, the hinge (loAver annular ligament) to d the centre of the 

 foot of the stapes, and c — i the distance from the tip of the long 

 process of the incus to the lower end of the stapes, then the range of 



motion of the centre of the foot of the stapes will be -, r: 



{c-i) 



which, according to the scale of a drawing given by Helmholtz, 



would give a leverage of about 2 — 1 . 



So that, on the whole system of bones there is a leverage of 

 about 3 — 1. This theoretical value is, however, reduced by the 

 friction of the levers and by the damping effect of the air filling 

 the internal ear. It has been estimated that half the force is thus 

 dissipated. 



Three further points about the chain of ossicles claim our 

 attention. Firstly, by the position of the axis, A — B, the mass of 

 the heads of the hammer- and anvil-bones are above the line, 

 while the lever arms are below the line on which the bones rotate. 

 This keeps these two bones suspended in equilibrium. Secondly, 

 there are tooth-like processes on the surface of the hammer which 

 engage with the body of the anvil, enabling each to move the other 

 in the to-and-fro movements of the drum. In the case of unusually 

 sudden alterations in air-pressure, e.g. a blow on the ear, these 

 processes slip over each other and prevent damage to the internal 

 ear. Thirdly, by the way in which they are hung on opposing 

 ligaments, and controlled by opposing muscles, they form a kind 

 of balance wheel which is verv sensitive to the transmission of 

 power in small vibrations. Its efficiency in this respect is derived 

 from the fact that the elastic forces balance one another in the 

 mechanical centre of the system, and so practically the whole 

 power applied to the drum is transmitted to the foot of the 

 stapes. 



B. The pressure in the internal ear is reinforced not only by 

 the system of lever transmission but by the relative sizes of the 

 membranes at either end of the chain of ossicles. The area of 



* a = junction of dotted line witii the axis .4 — B ; i = tip of lony process of 

 incus ; p = point of application of power, i.e. tip of manubrium mallei (Fig. 04). 



