THE SENSE OF HEARING. 145 



pani. The incus, or anvil bone, articulates with the head of the malleus 

 by a capsular joint, and with the stapes by the end of its long process. 

 The stapes resembles a stirrup in shape ; it articulates externally with the 

 long process of the incus, and internally its oval base is applied to the 

 edges of the foramen ovale. 



The Fitnction of the chain of bones is to transmit the waves of sound 

 across the tympanum to the internal ear ; being surrounded by air, and 

 acting as a solid rod, they prevent the vibrations from losing but little in 

 intensity. 



The Tensor tympani muscle arises mainly from the cartilaginous part of 

 the Eustachian tube ; it then passes backward into the tympanic cavity, 

 where it bends at a right angle around a process of bone, and is inserted 

 into the root of the handle of the malleus. Its function is to draw the 

 handle of the malleus internally, and thus increase the tension of the mem- 

 brana tympani, so as to make it capable of vibrating with sounds of greater 

 or less intensity ; at the same time it tightens the joints of the chain of 

 bones, so that they may the better conduct waves of sound to the internal 

 ear, with but a slight loss of intensity. 



The Laxator tympani muscle, arising from the spinous process of the 

 sphenoid bone, passes backward through the Glasserian fissure, into the 

 tympanic cavity, and is inserted into the neck of the malleus, \\sftmction 

 is to draw the handle of the malleus outward, and so relax the membrana 

 tympani, and enable it to receive waves of sound of greater amplitude than 

 when it is tense. 



The Stapedius muscle, emerging from the cavity of the pyramid of bone 

 projecting from the posterior wall of the tympanum, is inserted into the 

 head of the stapes bone. Its function is to draw the stapes backward, 

 preventing too great movement of the bone, and at the same time relaxing 

 the membrana tympani. 



The Eustachian tube, by means of which the middle ear communicates 

 with the pharynx, is partly bony and partly cartilaginous in its structure. 

 It is about one and a half inches in length ; commencing at its opening in 

 the pharynx, it passes upward and outward to the spine of the sphenoid 

 bone, where it is slightly contracted ; it then gradually dilates as it passes 

 backward into the tympanic cavity. It is lined by mucous membrane, 

 which is continued into the middle ear and into the mastoid cells. 



The Eustachian tube permits the passage of air from the pharynx into 

 the middle ear; in this way the pressure of the air within and without the 

 membrana tympani is equalized, which is one of the essential conditions for 

 the reception of sonorous vibrations. 



