THE MIDDLE EAR. 821 



Eustachian tube. It passes backward, through the Glasserian fissure, to be inserted into 

 the neck of the malleus, being enclosed, in its course, in a fibrous sheath. The laxator 

 tympani is generally believed to be muscular, though some authorities 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 portion of the aqueductus Fallopii 

 and in the cavity of the pyramid 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 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 several 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 mucous 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. 258) is partly bony and partly cartilaginous. Following 

 its direction from the tympanic cavity, it passes forward, inward, and slightly downward. 

 Its entire length is about an inch and a half. 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 triangular cartilage, bent upon itself above, 

 forming a furrow, with its concavity 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 of the 

 Eustachian tube is intermediate between the hyaline and the fibro-cartilage. 



The circumflexus, or tensor palati muscle, which has already been described in connec- 

 tion with deglutition, is attached to the anterior margin, or the hook of the cartilage. The 

 attachments of this muscle have lately been accurately described by Rudinger, who calls 

 it the dilator of the tube. The following excellent summary of the action of the muscles 

 upon the tube is taken from the report on otology, by Dr. J. Orne Green, contained in 

 the Transactions of the American Otological Society, 1870 : 



"The tensor palati muscle is a dilator of the tube; it is inserted along the whole 

 length of the hook of the cartilage, passing forward, inward, and slightly downward, and 

 its fibres spread out along the edge of the soft palate and on the side of the pharynx. In 

 contracting, it draws the hook of the cartilage forward and a little downward, thus en- 

 larging the caliber of the tube. The levator palati takes its origin from the temporal 

 bone just below the osseous tube, and passes along the floor of the tube, some of its fibres 

 arising from the lower end of the cartilage ; it is inserted in the uvula, and, in contracting 

 the belly of the muscle which lies along the floor of the tube, becomes thicker: the floor 

 of the tube is raised, and the fibres arising from the cartilage serve to draw the lower end 

 of this away from the opposite wall. 



" The palato-pharyngens rises from the posterior part of the lower end of the cartilage, 

 passes backward, and is inserted on the posterior wall of the pharynx. Its action would 

 be to draw the posterior wall of the tube backward; but. as it is often but slightly de- 

 veloped, it probably only serves to fix the cartilage, so that the other muscles can act 

 more effectively. 



