MIDDLE EAE 605 



above and sprinkling with starch grains the ossicles as they lie within the 

 movements of the different parts can be readily followed under a low power 

 microscope. When the drum is set into vibration by sound waves it is readily 

 seen that the whole chain of ossicles vibrates so as to convey the vibrations 

 to the plunger in the oval window. Many experimenters have noted the 

 remarkable way in which the apparatus responds to vibrations varying very 

 greatly in rate. Tones of low and high pitch appear to be recorded with 

 equal impartiality and fidelity. Experiment therefore confirms our sensa- 

 tions which show that the ear responds to vibrations varying from 40 to 

 40,000 per second. It is stated that the natural period of the ossicles and 

 drum, owing to their small size, is very much more rapid even than I^BO 

 second, and it is because of this that the system is able to respond faithfully 

 to the vibrations of longer period used in audition. 



Direct observation therefore shows that the ossicles form levers which 

 together conduct the vibrations from the drum to the plunger of the oval 

 window. It is necessary to consider the effect of this lever system 

 on the amplitude and force of the vibration. Motion is applied to the manu- 

 brium of the malleus and is communicated to the long process of the incus. 

 The former is one and one- half times the length of the latter and 

 therefore the stapes moves with two-thirds the amplitude of the drum. 

 If the levers moved without friction this would be accompanied by an 

 increase in the force of the vibrations of one and one-half times. But 

 owing to the air which surrounds the levers and thus damps their vibration 

 and to the energy required to set them in vibration on account of their mass, 

 it is probable that the force of the vibrations which reaches the oval window 

 is not more than half that incident on the manubrium. The drum on the 

 other hand has an area which is about twenty times that of the oval window, 

 and the energy incident on the drum and communicated to the 

 manubrium is that much greater than if the sound waves were incident on 

 the oval window direct. But owing to the energy absorbed by the levers the 

 magnification is probably not greater than ten times, that is one- third of the 

 calculated amount. Two other features of the chain of ossicles should be 

 mentioned. In the first place it will be observed that the axis, about which 

 the malleus and incus rotate, passes through the bones, so that the big mass 

 formed by the articular surfaces is above and the levers below the axis of 

 rotation, and the ossicles are approximately balanced. Secondly the 

 articulation between the malleus and incus is saddle-shaped and there is a 

 spur on the malleus which engages with the body of the incus, so that, when the 

 tensor tympani muscle relaxes and the malleus travels outwards, the spur 

 disengages and the incus is therefore not forced to follow. When on the other 

 hand the stapedius muscle is in tonic contraction, the spur is in engagement 

 and vibrations are therefore communicated from one bone to the other. If 

 however the force applied is excessive, owing for example to a box on the ear, 

 then the two bones separate slightly like the limbs of a compass, and the spur 

 passes the body of the incus without communicating the blow to it. In 

 this way rupture of the annular seal between the plunger of the stapes and the 

 oval window is prevented. 



