THE SENSE OF HEARING. 367 



conical in shape and is thicker than the long process. It has a horizontal posi- 

 tion, and is attached by a thick ligament to the posterior wall of the tympanum. 



The stapes (Fig'. 187) articulates with the end of the long process of the 

 incus ; its plane is horizontal and about at right angles to that process. It 

 measures 3 to 4 millimeters in length and about 2J millimeters in breadth. 

 The base of the stapes is somewhat oval in shape, the superior margin being 

 convex and the inferior being slightly concave. It is set in the fenestra ovalis, 

 an aperture measuring about 3 millimeters by 1| millimeters, and is held in 

 place by a narrow membrane made up of radial fibres of connective tissue. 

 When in position, the inner face of the base of the stirrup is covered with 

 lymphatic endothelium and is washed by the perilymph of the internal ear; 

 the outer face, like the other tympanic bones and the wall of the cavity, is 

 covered by thin mucous membrane. 



Movement of the Ossicles. — The malleus-incus articulation is so arranged 

 that with outward movements of the manubrium the head of the malleus 

 glides freely in the joint ; but the lower margins of the articulating surfaces 

 project in such a way that the prominences lock together when the manubrium 

 moves inward. Thus, in inward movements of the tympanic membrane and 

 its attached manubrium, the malleus and the incus move together like one 

 rigid piece of bone, the motions of the manubrium and the long process of the 

 incus being parallel. Of the malleus-incus articulation Helraholtz 1 says: 

 u In its action it may be compared with the joints of the well-known Breguet 

 watch-keys, which have rows of interlocking teeth, offering scarcely any resist- 

 ance to revolution in one direction, but allowing no revolution whatever in the 

 other." In the outward movements the locking teeth or projections are prob- 

 ably still kept in apposition, under ordinary circumstances, through the clastic 

 reaction of the ligament and the stapedial attachment of the incus. Should, 

 however, the tympanic membrane be forced unduly outward, as by increase of 

 pressure within the tympanum or by rarefaction of air in the auditory meatus, 

 the incus only follows the malleus for a certain distance, the latter completing 

 its motion by gliding in the joint. There is thus no danger of the stapes being 

 torn out of the oval window. The hammer and the anvil, suspended by their 

 ligaments, move freely about an axis one end of which is found at the origin of 

 the anterior part of the anterior ligament of the malleus, and the other end in 

 the origin of the ligament which is continuous with theshort process of the incus 

 ( Fig. 185). In inward motions of the tympanic membrane the ossicles move like 

 a single bone around the axis of suspension j and as the distance measured from 

 the axis of rotation to the tip of the manubrium, where the power is applied, is 

 about one and one-half times the distance to the end of the long process of the 

 incus, where the effect is produced, the motions transmitted to the st;ipes can have 

 but two-thirds the amplitude of the movements of the tip of the manubrium, but 

 have one and one-half times their force. It will be noticed thai a large pro- 

 portion of the mass of both anvil and hammer is found above their axis of rota- 

 tion ; this upper portion acts as a counterpoise to the parts below which arc directly 

 1 Sensations of Tone, trans, by Kllis, ISS'i, p. l.'W. 



