THE ORGANS OF SPECIAL SENSE. 63 



The Ear. The special organ of hearing is the ear, to 

 which there are three parts, the external, the middle, 

 and the internal ear. 



The external ear consists of the pinna or expanded car- 

 tilaginous portion, for the concentration and direction 

 of sound waves, and the external auditory canal, partly 

 cartilage, partly bone, which is directed forward, inward, 

 and downward and conveys sound to the middle ear. 



The middle ear or tympanum is an irregular cavity in 

 the petrous portion of the temporal bone. Its outer wall 

 is formed by the membrana tympani or drum, an oval 

 translucent membrane placed obliquely at the bottom of 



l-Os articular*, 

 CrLra., 



FIG. 21. The small bones of the ear; external view (enlarged). (After Gray.) 



the external auditory canal. The middle ear communi- 

 cates with the inner ear through the fenestra ovalis or 

 oval window and contains the ossicles, the malleus or 

 hammer, the incus or anvil, and the stapes or stirrup, 

 which are arranged in a movable chain from the drum to 

 the oval window. The malleus, which is connected with 

 the membrana tympani, articulates by its head with the 

 body of the incus, while the stapes articulates with the 

 incus by its head and is connected by its base with the 

 margin of the oval window. Connection is made be- 

 tween the middle ear and the pharynx and the pressure 

 of the air upon the drum made equal on either side by 

 means of the Eustachian tubes. These tubes are about 

 an inch and a half long, have cilia, and convey wax and 



