MOVEMENTS OF THE AUDITORY OSSICLES. 803 



of the membrana tympani. During the rotation, of course the parts above this 

 axis (head of the malleus and upper part of the body of the incus) take a direction 

 opposite to the parts lying below it (the handle of the malleus and the long process 

 of the incus), as is indicated in fig. 585 by the direction of the arrows. The 

 movement of the handle of the malleus must follow that of the membrana tympani, 

 and vice versd, while the movement of the stapes is connected with the movement 

 of the long process of the incus. As the long process of the incus is only two- 

 thirds of the length of the handle of the malleus (figs. 577, 581, 585), of course the 

 excursion of the tip of the former, and with it of the stapes, must be correspond- 

 ingly less than the movement of the tip of the handle of the malleus : while, on the 

 other hand, the force of the movement of the tip of the handle of the malleus, 

 corresponding to the diminution of the excursion, will be increased. 



Mode of Vibration. Thus, the movement of the membrana tympani inwards 

 causes a less extensive but a more powerful movement of the foot of the stapes 

 against the perilymph of the labyrinth. V. Helmholtz and Politzer calculated the 

 extent of the movement to be 0'07 mm. The mode in which the vibrations of the 

 membrana tympani are conveyed to the lymph of the labyrinth, through the chain 

 of ossicles, is quite analogous to the mechanism of these parts already described. 

 Long delicate glass threads have been fixed to these ossicles, and their movements 

 were thus graphically recorded on a smoked surface (Politzer, Hensen). Or, strongly 

 refractive particles are fixed to the ossicles, while the beam of light reflected from 

 them can be examined by means of a microscope {Buck, v. Helmholtz). All the 

 experiments showed that the transference of the sound-waves is accomplished by 

 means of the mechanism * of the angular lever, composed of the auditory ossicles 

 already described. As the vibrations of the membrana tympani are conveyed to 

 the handle of the malleus, they are weakened to about one-fourth of their original 

 strength (Politzer). [The membrana tympani is many times (30) larger than the 

 fenestra ovalis, and the relation in size might be represented by a funnel. The 

 arm of the malleal end of the lever where the power acts is 9J mm. long, while the 

 short or stapedial arm is 6 J mm., so that the latter moves less than the former, 

 but what is lost in extent is gained in force.] 



[Methods. Politzer attached small, very light levers to each of the ossicles, and inscribed 

 their movements on a revolving cylinder. An organ-pipe was sounded, and when the levers 

 were of the same length, the malleus made the greatest excursion and the stapes the least. 

 Buck attached starch grains to the ossicles, illuminated them, and observed the movements of 

 the refractive starch granules by means of a microscope provided with a micrometer.] 



[The ossicles move en masse, and not in the way of propagating molecular 

 vibrations.] As the excursions of the ossicles during sonorous vibrations are, how- 

 ever, only nominal, there is practically no change 

 in the position of the joints with each vibration. 

 The latter will only occur when extensive move- 

 ments take place by means of the muscles. 



The muscles of the auditory ossicles alter the 

 position and tension of the membrana tympani, as 

 well as the pressure of the lymph of the labyrinth. 

 The tensor tympani, which lies in an osseous groove 

 above the Eustachian tube, has its tendon deflected 

 round an osseous projection [processus cochleari- 

 formis], which lies external to it, almost at right 

 angles to the groove above it, and is inserted imme- "*^S^^ Fig. 586. 

 diately above the axes of rotation of the malleus Tensor tympani the Eustachian 

 (fig. 580, M). When the muscle contracts in the tube (left). 



direction of the arrow, t, then the handle of the malleus (n) pulls the membrana 

 tympani (M) inwards and tightens it (fig. 585). This also causes a movement 



