988 CONDUCTION THROUGH TYMPANUM. [BOOK in. 



perilymyh is attended with considerable difficulty ; and the 

 parts of the ear which we have spoken of above as constituting 

 the middle and outer ear, serve as an acoustic apparatus for 

 facilitating this transference and thus bringing the aerial waves 

 to act on the auditory epithelium, the action of the apparatus 

 being somewhat as follows. 



Waves of sound falling on the side of the head reach the tym- 

 panic membrane by the external meatus, and throw that mem- 

 brane into vibrations. These vibrations are transmitted through 

 the chain of ossicles to the membrane of the fenestra ovalis and 

 so to the perilymph lying on its far side; sweeping over the 

 perilymph in its continuous cavity the waves eventually break 

 upon the fenestra rotunda, having on their way affected the 

 auditory epithelium. We have first to inquire how this subsid- 

 iary acoustic apparatus performs its work. 



The conduction of sound through the Tympanum. 



613. The chain of ossicles, jointed together, attached to 

 the tympanic membrane at one end, and to the fenestra ovalis 

 at the other, and secured by ligaments, may be regarded as a 

 lever. Observations and experiments shew that the end of the 

 short process of the incus serves as- the fulcrum, the power 

 being applied at the umbo in which the handle of the malleus 

 ends, and the effect being brought to bear on the end of the 

 long process of the incus attached to the stapes. In thus acting 



lg.inc 



FIG. 174. THE LIGAMENTS or THE OSSICLES. (After Hensen.) 



The figure represents a nearly horizontal section of the tympanum, carried 

 through the heads of the malleus and incus. 



M. malleus. 7. incus, t. articular tooth of incus. Ig.a. anterior and Ig.e. ex- 

 ternal ligament of the malleus, lg.inc. ligament of the incus. 

 The line ax represents the axis of rotation of the two ossicles. 



as a lever the heads of the malleus and incus rotate round a 

 horizontal line drawn through them in the direction of the line 

 ax in Fig. 174. Such a lever may be represented by the line 

 xx' in Fig. 175. Careful measurements shew that the whole 

 length of the line from F the fulcrum to P where the power is 

 applied, is about 9-5 mm., while the length from F to TF, where 

 the effect is brought to bear, is about 6*3 mm. Hence when 



