SPECIAL SENSES 



the summit of the pyramid. In the canal in which this muscle is 

 lodged, its direction is vertical. At the summit of the pyramid it turns 

 at nearly a right angle, its tendon passing horizontally forward to be 

 attached to the head of the stapes. Like the other muscles of the ear, 

 this is enveloped in a fibrous sheath. Its action is to draw the head of 

 the stapes backward, relaxing the membrana tympani. This muscle 

 receives filaments from the facial nerve, by a distinct branch, the 

 tympanic. 



The posterior wall of the tympanic cavity presents foramina that 

 open directly into a number of irregularly-shaped cavities (mastoid 



cells) communicating freely 

 with each other in the mastoid 

 process of the temporal bone. 

 These are lined with a contin- 

 uation of the mucous mem- 

 brane of the tympanum. 

 There is, under certain con- 

 ditions, a free circulation of 

 air between the pharynx and 

 the cavity of the tympanum 

 through the Eustachian tube 

 and from the tympanum to 

 the mastoid cells. 



The Eustachian tube (12, 



Fig. 187. - The right temporal bone, the petrous par- Fig. I 8 5 ) is partly boiiy and 



tion removed, showing the ossicles seen from within re- 

 duced about one-half . From a photograph (Riidinger). 



^ 



party CartilagillOUS. Following 



. .. . r 



its direction from the tym- 



4, the incus, the short process of which is directed ., , r i 



nearly in a horizontal direction backward; 5, the long P aniC Cavlt Y> * P aSSCS forward, 



process of the incus, free in the tympanic cavity, articu- inward and slightly downward. 

 lated with the stapes ; 6, the malleus, articulated with the D . . . 



incus; 7, the long process of the malleus, in the Glasse- Its entire length IS about an 



rian fissure; 8, the stapes, articulated with the incus. 

 This is drawn somewhat outward ; otherwise the base of 



anc j a 



(38.! milli- 



the stapes alone would be visible. This figure shows the meters). Its Calibre gradually 

 handle of the malleus attached to the membrana tym- CQntracts from the tympanum 



to the spine of the sphenoid, 



and from this constricted portion it gradually dilates to its opening into 

 the pharynx, the entire canal presenting the appearance of two cones. 

 The osseous portion extends from the tympanum to the spine of the 

 sphenoid bone. The cartilaginous portion is an irregularly-triangular 

 cartilage, bent on itself above, forming a furrow, with its concavity pre- 

 senting downward and outward. The fibrous portion occupies about 

 one-half of the tube, beyond the osseous portion, and completes the 

 canal, forming its inferior and external portion. In its structure the 



