572 PHYSIOLOGY 



long process of the incus moves inwards. The stapes, or stirrup bone, is 

 fixed in the fenestra ovalis of the internal ear, in the inner surface of 

 the tympanum, by the annular ligament. It is placed almost at right 

 angles to the long process of the incus, and therefore is pressed into 

 the foramen ovale when this process moves inwards. The breaking up 

 of the connection between the tympanic membrane and the foramen 

 ovale into three bones, connected by joints, must tend to prevent any 

 propagation of sound by direct continuity of substance. The vibra- 

 tions of the membrane result in actual movements of the whole bones, 

 which represent a chain of levers reproducing exactly the movements 

 of the membrane. In the transmission of a sound wave from the 

 membrana tympani to the labyrinth there is a change in the amount of 

 force as well as in the amplitude of the movement. The three bones 

 can be regarded as forming a lever with two arms, one of which is the 

 manubrium of the malleus, and the other the long process of the 

 incus. The length of the former is to that of the latter as 3 to 2, so 

 that the movements transmitted from the tympanic membrane to the 

 base of the stapes are diminished in the proportion of 3 to 2 and have 

 their force increased in the proportion of 2 to 3. Moreover, as the 

 drum of the ear has an area which is about twenty times that of the 

 foramen ovale, the energy of its movements is concentrated on an area 

 twenty times smaller. Hence the pressure of a sound wave acting on 

 the tympanic membrane is increased thirty-fold (^ x 20) when it acts 

 on the base of the stapes. 



The computation of the actual energy, involved in the movement 

 of these structures by sound waves, which are just perceptible to the 

 ear, yields striking results as to the extreme sensitiveness and efficiency 

 of this apparatus. Lord Rayleigh has estimated that the amplitude 

 of the movement of an aerial particle involved in the propagation 

 of sound at the limits of audibility is less than one ten-millionth 

 of a centimetre. By other methods it has been calculated 

 that the ear is affected by vibrations of molecules of the air not 

 greater than -0004 mm., which is equal to O'l of the wave length of 

 green light. These results show that the amounts of energy, required 

 to influence the eye and the ear respectively, are of the same order of 

 magnitude. 



Two muscles are found in the tympanum, viz. the tensor tympani 

 and the stapedius muscles. When the tensor tympani contracts it 

 draws the handle of the malleus inwards and so increases the tension of 

 the tympanic membrane. Direct observation has shown that a con- 

 traction of this muscle occurs whenever sounds fall on the membrane, 

 and that this reflex contraction is bilateral even when the stimula- 

 tion of the ear is unilateral. The stapedius muscle tilts the base of the 

 stapes and at the same time draws it slightly outwards, so relaxing 



