410 Til K ORGANS OF SPECIAL SENtiE 



The posterior wall of the tympanum presents above 

 one large and several small apertures leading to the 

 mastoid cells. 



The anterior extremity opens into two canals separ- 

 ated by a process of bone, the processes cochleariformis. 

 The upper of these canals is the smaller and transmits 

 the tensor tympani; the lower contains the Eustachian 

 tube, an osseocartilaginous passage \\ inches long, 

 leading to the pharynx. Both of these canals run in 

 a direction downward, forward, and inward. 



The osseous part of the Eustachian tube is ^ inch 

 long, and to its lower end is attached the triangular 

 piece of fibrocartilage forming the remainder of the 

 tube. The edges of the cartilage are now in contact, 

 but are joined by fibrous tissue. The tube is wide 

 at its lower extremity, and opens at the upper and 

 lateral part of the pharynx, above the hard palate and 

 on a line with the lower turbinated bone. It is lined 

 by epithelium continuous with that of the pharynx. 



The ossicles are three minute movable bones, named 

 the malleus, incus, and stapes. The first is attached 

 to the membrana tympani; the second is between the 

 other two; the last named is attached to the fenestra 

 ovalis of the cochlea. 



THE MUSCLES OF THE MIDDLE EAR. The tensor 

 tympani runs in the canal previously mentioned. 

 Arising from the under surface of the petrous portion, 

 the cartilage of the Eustachian tube, and the margins 

 of its own canal, its tendon is reflected over ' the 

 processus cochleariformis and is inserted into the 

 handle of the malleus near its root. It pulls on the 

 malleus, thus drawing inward and making tense the 

 membrana tympani. Its nerve comes from the optic 

 ganglion. 



The stapedius muscle arises from the sides of its con- 

 taining cavity within the pyramid, and, emerging from 

 the apex, is inserted into the neck of the stapes. It 



