HEARING. 



799 



arm of this lever is about 9J mm., the short, stapedial, 6^ mm. in length ; 

 hence, the movements of the stapes are less than those of the tympanum ; 

 but the loss in amplitude is made up by a gain of force, which is in itself 

 an obvious advantage. 



Thus every movement of the tympanic membrane is transmitted through 

 this chain of ossicles to the membrane of the fenestra ovalis, and so to the 

 perilymph of the labyrinth ; the vibrations of the tympanic membrane are 

 conveyed with increased intensity, though with diminished amplitude, to the 

 latter. That the bones thus move en masse has been proved by recording 

 their movements in the usual graphic method. A very light style attached 

 to the incus or stapes is made to write on a travelling surface; when the 

 membrana tympani is thrown into vibrations by a sound the curves described 

 by the style indicate that the chain of bones moves with every vibration of 

 the tympanum. On the other hand, the comparatively loose attachments of 

 the several bones is an obstacle to the molecular transmission of sonorous 

 vibrations through them. Moreover, sonorous vibrations can only be trans- 

 mitted to or pass along such bodies as either are very long compared to the 

 length of the sound-waves, or, as in the case of membranes and strings, have 

 one dimension very much smaller than the others. Now the bones in ques- 

 tion are not especially thin in any one dimension, but are in all their dimen- 

 sions exceedingly small compared with the length of the vibrations of even 

 the shrillest sounds we are capable of hearing; hence, they must be useless 

 for the molecular propagation of vibrations. 



FIG. 195. Diagram of the Outer Wall of the Tympanum (Right Ear) as Seen from the Mesial 

 Side, showing Insertion of Tensor Tympani. Magnified twice. (After Schwalbe.) m.t, mem- 

 brana tympani; m.b, handle of M, the malleus; 7, the incus; E.t, Eustachian tube; T.T, tensor 

 tympani, the tendon of which is attached to the handle of the malleus; Ig.a, the anterior, and 

 Ig.s, the superior, ligament of the malleus ; ch.t, the chorda tympani nerve passing through the 

 tympanic cavity. 



FIG. 196. The Stapes in Position. Much magnified. (Schwalbe.) 1, the end of the shaft of the 

 incus ; 2, its expansion or os orbiculare ; 2', the articular cartilage of the same ; 3, the capitulum 

 of the stapes ; 3', its articular cartilage ; 4, the hoops of the stapes ; 5, the foot-plate of the stapes ; 

 5', its articular cartilage; 6, the membrane of the fenestra ovalis; ST, the tendon of the stape- 

 dius muscle attached to the capitulum of the stapes.] 



709. The tensor tympani muscle even in a quiescent state is of use in 

 preventing the membrana tympani being pushed out far. (Fig. 195.) When 

 it contracts it renders the membrana tympani more tense, and hence has 

 been supposed to act as a damper, lessening the amount of vibration of the 

 membrane in the case of too powerful sounds ; it is said to be readily thrown 



