THE AUDITORY OSSICLES AND THEIR MUSCLES. 891 



(a) is only partially fixed in its position by a ligament that secures its short 

 process to the wall of the tympanic cavity, in front of the entrance to the mastoid 

 cells (K). It is materially supported by the rather loose articulation with the 

 head of the malleus (h) , the saddle-shaped articulating surface of which is inserted 

 into a depression in the incus. Special attention must be directed to the ratchet- 

 like lower border of the incus (Fig. 316, S). When the handle of the malleus 

 moves inward, this arrangement causes the long process of the incus (1), which 

 is parallel to the manubrium of the malleus, and is attached to the stapes (S) 

 almost at right angles through the sesamoid bone of Sylvius (s) , to move inward 

 at the same time. If, however, the tympanic membrane, together with the handle 

 of the malleus, is moved outward, as by condensation of the air in the tympanic 

 cavity, the long process of the incus does not make the same movement, as the 

 malleus alone moves away from the ratchetlike edge of the incus. There is, 

 consequently, no pull on the stapes, and, therefore, no disturbing agitation of the 

 endolymph. As Ed. Weber has well shown, the malleus and the incus represent 

 a rectangular lever, whose movement occurs about a common axis (Fig. 316, and 

 Fig. 320 Ax, Ax) . In the movement inward, the incus follows the malleus as if the 

 two were one piece. The common axis (Fig. 316) is not, however, the axial liga- 

 ment of the malleus, but it is formed anteriorly by the processus Folianus (IF), 

 which is directed forward, and posteriorly by the short process of the incus (K), 

 which is directed backward. The rotation of the two ossicles about this axis 

 takes place in a plane perpendicular to 

 the plane of the tympanic membrane. 

 During the rotation, the parts above 

 this axis (head of the malleus and upper 

 part of the incus) move in a direction 

 opposite to that in which move those 

 lying beneath it (manubrium of the 

 malleus and long process of the incus) , 

 as is indicated in Fig. 320 by the 

 direction of the arrow. The movement 

 of the manubrium must always follow 

 that of the tympanic membrane, and 



the reverse, while the excursion of the \\ O.S 



stapes is necessarily the same as that 

 of the long process of the incus. Atten- 

 tion must be called to One more im- FIG. 319- The Auditory Ossicles (right): C.w, head; 

 nnrfant -nrn'rif Ac +1^ Irmrr i~>i--oce nf C, neck; Pbr, short process; Prl, long process; A/, 



portant point. As the long process Of manubrium of the malleus; O',body; G.articulat- 



the incus IS only two-thirds as long ing surface; k short and v long process of the 



as the manubrium (Figs. 316, 317, 320) incus; O.S, lenticular bone; C.s, head; a ante- 



the excursion of the apex of the former, rior and p P sterior hmb ' p < base of the s ^^ s - 



and with it that of the stapes, must 



be correspondingly less than that of the apex of the manubrium. On the other 

 hand, the force of the movement, corresponding to the diminution of the excursion, 

 will be increased. 



Movements of the tympanic membrane inward thus cause less ex- 

 tensive, but more powerful, movements of the base of the stapes against 

 the fluid of the labyrinth, which v. Helmholtz and Politzer estimated 

 to be about 0.07 mm. in amplitude. 



The way in which the vibrations of the tympanic membrane are 

 transmitted to the endolymph through the chain of ossicles is exactly 

 analogous to the method of movement of these parts, as already ex- 

 plained. For the study of this movement, long delicate glass threads 

 have been attached to the various portions of the ossicles, and the 

 movements, when sounds were conveyed to the auditory apparatus, 

 have been thus recorded on smoked paper. Bright particles have also 

 been pasted on the individual parts, whose oscillating movements appear 

 as lines of light, which have been followed and measured with the aid of 

 the microscope. All experiments have proved that the transmission 

 of the sound-vibrations takes place through the mechanism of the 

 rectangular lever formed by the ossicles, as has been described. 



