734: 



SPECIAL SENSES. 



tympani. The fibres of this, and of all the muscles of the middle ear, are of 

 the striated variety. The tensor tympani is supplied with motor filaments 



from the otic ganglion, which 

 are probably derived from the 

 facial nerve. 



The stapedius muscle is sit- 

 uated in the descending por- 

 tion of the acquseductus Fallo- 

 pii and in the cavity of the 

 pyramid on the posterior wall 

 of the tympanic cavity. Its 

 tendon emerges from a fora- 

 men at the summit of the pyra- 

 mid. In the canal in which 

 this muscle is lodged, its direc- 

 tion is vertical. At the sum- 

 mit 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 mus- 

 cles of the ear, this is enveloped 

 in a fibrous sheath. Its action 

 to draw the head of the 



is 



FIG. 265. The right temporal bone, the petrosal portion re- 

 moved, showing the ossicles seen from within. From a 



photograph (Riidinger). 

 4, the incus, the short process of which is directed nearly 



in a horizontal direction backward ; 5, the long process 



of the incus, free in the tympanic cavity, articulated 



with the stapes ; 6, the malleus, articulated with the 



incus; 7, the long process of the malleus, in the Glasse- 



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



This is drawn somewhat outward ; otherwise the base 



of the stapes alone would be visible. This figure shows 



the handle of the malleus, attached to the membrana 



tympani. 



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 several foramina, which 

 open directly into a number of irregularly shaped cavities communicating 

 freely with each other in the mastoid process of the temporal bone. These 

 are called the mastoid cells. They are lined by a continuation of the mucous 

 membrane of the tympanum. There is under certain conditions a free cir- 

 culation 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. 263) is partly bony and partly cartilagi- 

 nous. Following its direction from the tympanic cavity, it passes forward, 

 inward and slightly downward. Its entire length is about an inch and a 

 half (38-1 mm.). Its caliber gradually contracts 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 trian- 

 gular cartilage, bent upon itself above, forming a furrow with its concav- 

 ity presenting downward and outward. The fibrous portion occupies about 

 half of the tube beyond the osseous portion, and completes the canal, 

 forming its inferior and external portion.' In its structure the cartilage 



