THE MIDDLE EAR. 



819 



ity looking outward, and oblique, inclining usually at an angle of forty-five degrees with 

 the perpendicular. This angle, however, varies considerably in different individuals. 

 The roof is formed by an exceedingly thin plate of bone. The floor is bony and is much 

 narrower than the roof. The inner wall, separating the tympanic cavity from the laby- 

 rinth, is irregular, presenting several small elevations and foramina. The fenestra ovalis, 

 an ovoid opening near its upper portion, leads to the cavity of the vestibule. This is 



FIG. 258. General view of the organ of hearing. (Sappey.) 



1, pinna ; 2, cavity of the concha, on the walls of which are seen the orifices of a great number of sebaceous glands ; 

 8, external auditory meatus ; 4, angular projection formed by the union of the anterior portion of the concha 

 with the posterior wall of the auditory canal ; 5, openings of the ceruminous glands, the most internal of which 

 form a curved line which corresponds with the beginning of the osseous portion of the external meatus ; 6, 

 membrana tympani and the elastic fibrous membrane which forms its border; 7, anterior portion of the incus; 

 8, malleus ; 9. handle of the malleus applied to the internal surface of the membrana tympani, which it draws 

 inward toward the projection of the promontory ; 10, tensor tympani muscle, the tendon of which is reflected at 

 a right angle to become attached to the superior portion of the handle of the malleus ; 11, tympanic cavity ; 12; 

 Eustachian tube, the internal, or pharyngeal extremity of which has been removed by a section perpendicular 

 to its curve; 13, superior semicircular canal; 14, posterior semicircular canal; 15, external semicircular canal, 

 16, cochlea; IT, internal auditory canal; 18, facial nerve; 19, large petrosal branch, given off from the ganglio- 

 form enlargement of the facial and passing below the cochlea to go to its distribution ; 20, vestibular branch of 

 the auditory nerve ; 21, cochlear branch of the auditory nerve. 



closed, in the natural state, by the base of the stapes and its annular ligament. Below, 

 is a smaller, ovoid opening, the fenestra rotunda, which leads to the cochlea. This is 

 closed, in the natural state, by a membrane, called the secondary membrana tympani. 

 In addition, the posterior wall presents several small foramina leading to the mastoid 

 cells, which are lined by a -continuation of the mucous membrane of the tympanic 

 cavity. The tympanic cavity also presents an opening leading to the Eustachian tube, 

 and a small foramen, which gives passage to the tendon of the stapedius muscle. The 

 Eustachian tube extends from the upper part of the pharynx to the tympanum. 



The small bones of the ear are three in number; the incus, the malleus, and the 

 stapes, forming a chain, connected together by ligaments (Fig. 259). These bones are 

 situated in the upper part of the tympanic cavity. The handle of the malleus (A, 2, 

 Fig. 259) is closely attached to the membrana tympani, and the long process (A, 3, Fig. 

 259) is attached to the Glasserian fissure of the temporal bone. The malleus is articu- 

 lated with the incus. The incus (B, Fig. 259) is connected with the posterior wall 

 of the tympanic cavity, near the openings of the mastoid cells. It is articulated with 

 the malleus, and, by the extremity of its long process (B, 2, Fig. 259), with the stapes. 

 The stapes (0, Fig. 259) is the most internal bone of the middle ear. It is articulated 



