THE MOVEMENTS OF THE MALLEUS. 1155 



s ; that is to say, it must cause a relatively great displacement of the centre of 

 the fibre. 



Again, if t = the tension of the fibre, p = the pressure 011 each unit of its 

 length, and r = the radius of curvature, then 



t=pr 



and the forces which act upon both ends of the fibre must be equal to the 

 pressure which acts upon the diameter of the semicircle through a width equal 

 to that of the fibre. 1 Therefore 



Hence the greater the radius of curvature, the greater will be the 

 alterations in tension of the fibre, caused by changes' in the pressure 

 of the air. Further, as the radial fibres are those which are attached 

 to the malleus, it is evident that the variations in the tension of the 

 fibres cause movements of the bones when sound-waves strike the drum- 

 head. Thus a very small change of pressure in the air causes a con- 

 siderable change in the tension of the fibres ; and further, in accordance 

 with the laws regulating the action of the lever, as the force which the 

 fibres exert upon the handle of the malleus increases, the amplitude of 

 movement of that bone diminishes. In this way the special form of the 

 drum-head secures a maximum of efficiency for tones of the feeblest 

 intensity. 



The movements of the malleus. The movements of the malleus 

 are of two, possibly of three kinds. An accurate knowledge of the 

 anatomy of the ligaments and articulations is necessary for their 

 comprehension. 



In the first place, as the malleus is firmly attached to the membrana 

 tympani along the manubrium, it is obvious that the movements of the 

 membrane would tend to carry the bone inwards as a whole. This move- 

 ment, however, is to a great extent prevented by the ligaments attached 

 to the head and neck of the malleus, and in the living subject it is 

 exceedingly minute. A second and also comparatively unimportant 

 movement of the malleus is that of rotation on the axis passing 

 through the length of the bone from above downwards. This move- 

 ment is produced by the tensor tympani, the tendon of which is not 

 inserted into the bone straight, but somewhat at an acute angle or on 

 one side. Hence, when the muscle contracts, it not only pulls the 

 handle of the malleus inwards, but also rotates it round an axis passing 

 from the head of the bone downwards to the tip of the manubrium. 

 This movement, however, is very insignificant. 



By far the most important movement of the malleus is one of 

 rotation round a horizontal axis, which passes through the anterior 

 ligament and head of the malleus, the body of the incus, and the short 

 process of the incus (see Fig. 421 p. 1158). When the manubrium is 

 pushed inwards, the part of the malleus above this axis must move 

 outwards. The ligaments, especially the external and superior, prevent 

 extensive movements of the malleus. 



The movements of the incus. This bone is attached to neigh- 

 bouring structures at three places. Two of these attachments are true 

 joints, namely, the malleo-incudal and the stapedial ; the third is where 

 the short process of the incus is connected with the posterior wall of the 

 tympanic cavity. The malleo-incudal joint is peculiar both as to 



1 Helmholtz, op. cit. 



