716 



HEARING. 



[CH. LIII. 



Fig. 540. Interior view of the tympanum, with 

 membrana tympani and bones in natural 

 position, i, Membrana tympani ; 2, Eusta- 

 chian tube ; 3, tensor tympani muscle ; 4, lig. 

 mallei exter. ; 5, lig. mallei super. ; 6, chorda- 

 tympani nerve; a, b, and c, sinuses about 

 ossicles. (Schwalbe.) 



processes : a short conical one, which receives the insertion of the 

 tensor tympani, and a slender one, processus gracilis, which extends 



forwards, and is attached 

 . 7 to the wall of the cavity 



at the Glaserian fissure. 



The incus, or anvil-bone, 



c shaped like a bicuspid 



ff molar tooth, is articulated 

 r 7 by its broader part, corre- 

 sponding with the surface 

 of the crown of the tooth, 



i- to the malleus. Of its two 



& fang-like processes, one, 



directed backwards, has a 

 free end lodged in a de- 

 pression in the mastoid 

 bone ; the other, curved 

 downwards, longer and 

 more pointed, articulates 

 by means of a roundish 



tubercle, formerly called os orbiculare, with the stapes, a little 

 bone shaped exactly like a stirrup, of which the base or bar fits 

 into the membrane of the fenestra ovalis. To the neck of the 

 stapes, a short process, corresponding with the loop of the stirrup, 

 is attached the stapedius muscle. 



The bones of the ear are covered with mucous membrane 

 reflected over them from the wall of the tympanum ; and are 

 movable both altogether and slightly one upon the other. The 

 malleus moves and vibrates with every movement and vibration of 

 the membrana tympani, and its movements are communicated 

 through the incus to the stapes, and through it to the membrane 

 closing the fenestra ovalis. 



The muscles of the tympanum are two in number. The tensor 

 tympani arises from the cartilaginous cud of the Kustachian 

 tube and the adjoining surface of the sphenoid and from the 

 sides of the canal in which the muscle lies ; the tendon of the 

 muscle bends at nearly a right angle over the end of the pro- 

 cessus cochleariformis and is inserted into the inner part of the 

 handle of the malleus. The Stapedius is concealed within a canal 

 in the bone in front of the aqueductus Fallopii. The tendon 

 issues from the aperture of this canal and is inserted into the 

 neck of the stapes posteriorly. 



The Internal Ear. The proper organ of hearing is formed 

 by the distribution of the auditory nerve within the internal 



