THE MIDDLE EAR 



615 



with its former course, to articulate, by means of a rounded end or 

 orbicular process, with the head of the stapes. This latter bone is 

 deeply placed within the recess of the pelvis ovalis, and continues 

 the bony chain to the fenestra ovalis, where the foot plate of the 

 stapes is in relation, by its inner surface, with the vestibular peri- 

 lymphatic 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 manu- 

 brium mallei, the vibrations of the tympanic membrane in response 

 to sound waves are 

 transmitted to the in- 

 ternal ear diminished 

 in amplitude but exag- 

 gerated in intensity. 



Two muscles and 

 several ligaments are 

 connected with the os- 

 sicles. 



The tensor tympani 

 muscle is mostly con- 

 tained within a canal 

 which is parallel to and 

 lies just above the 

 Eustachian tube, and 

 from its bony wall the 

 muscular fibres arise. 

 The wall of the canal 

 forms a conical pro- 

 jection known as the 

 processus cochlearifor- 

 mis, which projects 

 well into the cavity of the tympanum, being directed toward the 

 neck of the malleus. Leaving its canal at the apex of this conical 

 process the tendon of the muscle bends sharply over the margin of 

 the processus cochleariformis and passes directly to its insertion 

 into the neck and the adjoining part of the manubrium of the mal- 

 leus. Hence the naked tendon of the muscle lies within the tym- 

 panic cavity. 



The stapedius muscle is similarly contained within the cavity 

 of the pyramid, from whose bony wall its fibres take origin. Pass- 

 ing forward, the muscle makes its exit at the apex of the pyramid, 



FIG. 448. THE CAVITY OF THE TYMPANUM, VIEWED 

 FROM ABOVE. 



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

 mucosa ; I. a. m, anterior ligament of the malleus ; I. e. w, 

 external ligament of the malleus ; I. i., posterior liga- 

 ment of the incus ; Jf, mastoid cell ; m, head of the mal- 

 leus ; m.rn, 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 portion of the tympanic membrane ; 

 s. I. m, cut end of the superior ligament of the malleus ; 

 sp, spina tympanica anterior ; s, st f , tendon of the stape- 

 dius muscle ; ft, tendon of the tensor tympani muscle, 

 x 4. (After Schafer.) 



