514 PHYSIOLOGY 



the incus and of the head of the malleus move outwards together, while the 

 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 vibrations 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, yield 

 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 Ol 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 mem- 

 brane. Direct observation has shown that a contraction of this muscle 

 occurs whenever sounds fall on the membrane, and that this reflex contraction 

 is bilateral even when the stimulation 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 the tympanic membrane. It acts therefore as an 

 antagonist to the tensor tympani. It is not yet known what exact part 

 this muscle plays in audition. 



THE END-ORGANS OF HEARING 



The movements of the stapes are communicated to a fluid, the perilymph, 

 and by this to the endolymph, which immediately bathes the end- organ of 



