TOPOGRAPHICAL ANATOMY OF THE EAE. 159 



closed, in its course, in a fibrous sheath. The laxator tym- 

 pani is generally believed to be muscular, though some au- 

 thorities deny that it is composed of true muscular fibres. Its 

 action would be to draw the malleus forward and outward, 

 producing relaxation of the membrana tympani. It is not 

 definitely known from what nerve this muscle derives its 

 motor filaments. 



The stapedius muscle is situated in the descending por- 

 tion of the aqueductus Fallopii and in the cavity of the pyra- 

 mid on the posterior wall of the tympanic cavity. Its ten- 

 don emerges from a foramen at the summit of the pyramid. 

 In the canal in which this muscle is lodged, its direction is 

 upward and vertical. At the summit of the pyramid, it turns 

 at nearly a right angle, its tendon passing horizontally for- 

 ward 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, relax- 

 ing the membrana tympani. This muscle receives filaments 

 from the facial nerve by a distinct branch, the tympanic. 1 , 



The posterior wall of the tympanic cavity presents sev- 

 eral foramina which open directly into numerous 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 mu- 

 cous membrane of the tympanum. There is, under certain 

 conditions, 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. 10) a is partly bony and 

 partly cartilaginous. Following its direction from the tym- 

 panic cavity, it passes forward, inward, and slightly down- 

 ward. Its entire length is about an inch and a half. Its 

 caliber gradually contracts from the tympanum to the spine 



1 See vol. iv., Nervous System, p. 149. 



a EUSTACHIUS, Epistola de Audituz Organis. Opuscula Anatomica, Venetiis, 

 1564, p. 148, et seq. 



141 



