HEARING. 187 



bone, the INCUS. This in its turn is in connection, by a process running down and 

 in, with the third bone, the STAPES, the inner end of which is continuous with the 

 membrane of the fenestra ovalis. Fig. 54. shows the shape and relationship of 

 the three bones. The malleus is rather loosely attached by a ligament to the 

 upper part of the outer wall of the chamber and besides the manubrium it has a 

 second process, a slender one, nearly at right angles to the manubrium, running 

 forward to the anterior wall and firmly bound there. The bone with which the 

 body of the malleus is joined, the incus, has two processes, one of which, already 

 mentioned, extends down and in to make connection with the stapes, while the 

 other, directed backwards, is firmly bound to the posterior wall of the chamber. 

 The stapes lies almost horizontally between the tip of the process of the incus and 

 the fenestra ovalis. The whole chain of bones acts together as a lever, one arm of 

 which is the manubrium of the malleus, the other the downward process of the 

 incus, together with the stapes, the fulcrum being somewhere in the lower part of 

 the malleus-incus joint (Fig. 55). When the membrane of the tympanum swings 

 in, the tip of the manubrium is pressed inwards. This would tend to move 

 malleus and incus bodily inwards except that they are firmly bound in an antero- 

 posterior direction by the attachment of the processes to the anterior and posterior 

 walls which we have already mentioned. These act as an axis around which the 

 two bones swing. When the tip of the manubrium is moved in, the head of the 

 malleus and the body of the incus swing out. Outward movement of the body 

 of the incus means movement in of the process which is attached to the stapes, 

 and this thrusts the stapes ahead of it. It will be seen in the diagram that the 

 manubrium (the power arm) is about half as long again as the process in the incus 

 (the load arm). The lever therefore reduces the excursion of the waves which 

 move it, but at the same time it increases their intensity. Movements com- 

 municated to the fluids of the internal ear need only be very tiny indeed to 

 stimulate the sensitive cells, so that this cutting down of the magnitude of the 

 sound waves is not a disadvantage. At the same time, even these tiny colums 

 of fluid have an appreciable inertia and on this account the increase in power of 

 the wave is important. A further increase in power is got from the concentration 

 of the force of the wave received by the whole surface of the tympanic membrane 

 into the much smaller area of the fenestra ovalis. 



As we have said, a movement inwards of the membrane causes the head of the 

 malleus to move out, locking with the body of the incus and drawing it along. 

 If the membrane is moved out to any unusual extent, however, as happens for 

 instance when the surrounding pressure falls, the inward movement of the head 

 of the malleus which results is not followed by the incus. As Helmholtz pointed 

 out, the point between them is a cog or ratchet joint and locks firmly for movements 

 in one direction, but slides for large movements in the other. This is probably 

 important as a protection to the other parts of the middle, and to the internal, ear. 



There are two tiny muscles in the middle ear. One of these, the TENSOR 

 TYMPANI, has its origin in a groove above the Eustachian tube, that is, from the 

 inner wall, and is inserted into the neck of the malleus below the axis around which 

 the bone rotates. When this contracts it must draw the manubrium of the 



