690 



THE EAE 



stapes. This latter bone is deeply placed within the recess of the pelvis 

 ovalis, and continues the bony chain to the fenestra vestibuli, where 

 the foot plate of the stapes is in relation, by its inner surface, with the 

 vestibular perilymphatic space of the internal ear. 



The course of the chain of ossicles is such that they form a lever; 

 the long process of the incus being shorter than the manubrium mallei, 

 the vibrations of the tympanic membrane in response to sound waves 

 are transmitted to the internal ear diminished in amplitude but ex- 



aggerated in intensity. 

 sf 



The com bi n ed activity of 

 the tympanic membrane 

 and the auditory ossicles 

 is said to reduce the am- 

 plitude as much as sev- 

 enty-six times, and to 

 increase the force of the 

 vibrations thirty times. 



Two muscles and sev- 

 eral ligaments are con- 

 nected with the ossicles. 

 The tensor tympani 

 muscle is mostly con- 

 tained within a canal 

 which is parallel to and 

 lies just above the audi- 

 tory tube, and from its 

 bony wall the muscular 

 fibers arise. The wall of 

 the canal forms a conical 

 projection known as the 

 processus cochleariformis, 

 which projects well into 



the cavity of the tympanum, being directed toward the neck of the mal- 

 leus. Leaving its canal at the apex of this conical process the tendon 

 of the muscle bends sharply over the margin of the processus cochleari- 

 formis and passes directly to its insertion into the neck and the adjoin- 

 ing part of the manubrium of the malleus. Hence the naked tendon of 

 the muscle lies within the tympanic cavity. 



The stapedius muscle is similarly contained within the cavity of 

 the pyramid, from whose bony wall its fibers take origin. Passing for- 



FIG. 576. THE CAVITY OF THE TYMPANUM, VIEWED 

 FROM ABOVE. 



i, the body of the incus; I, ligamentous fold of the 

 mucosa; l.a.m., anterior ligament of the malleus; 

 Le.m., external ligament of the malleus; Li., pos- 

 terior ligament of the incus; M, mastoid cell; m, 

 head of the malleus; ra.ra., mucous membrane; n, 

 chorda tympani nerve; pr. o., orbicular process of the 

 incus articulating with the stapes in the depth of 

 the cavity; R, beneath this space is the flaccid por- 

 tion of the tympanic membrane; s.l.m., cut end of 

 the superior ligament of the malleus; sp, spina tym- 

 panica anterior; st, st f , tendon of the stapedius 

 muscle; tt, tendon of the tensor tympani muscle. 

 X 4. (After Schafer.) 



