THE EAR 



365 



Malleus 



Stapes 



there it extends upward, where it is attached to the second 

 bone, the incus. This bone has something the shape of a 

 blacksmith's anvil (hence the name incus) and one projection 

 from it extends downward into the cavity again, connecting 

 with the third bone, the stapes (stirrup), which ends in a broad, 

 flat area, fitting into the 

 opening mentioned above 

 as the foramen ovale ; Fig. 

 182. 



The three bones are con- 

 nected by ligaments, and 

 there is practically no 

 hinge action except be- 

 tween the incus and 

 stapes. Two tiny muscles 

 are connected with these 

 bones; one, the tensor tym- 

 pani, leads from the wall of 

 the Eustachian tube to the 

 malleus bone. When it 

 shortens, it pulls on the 

 malleus and thus indirectly 

 tightens the drum. The other muscle, the stapedius, runs 

 from the wall of the cavity to the neck of the stapes, and 

 when it contracts pulls the stapes to one side, thus changing 

 the position of the membrane over the foramen ovale. The 

 value of these muscles will be discussed later. 



The Inner Ear. The foramen ovale is a short passage in 

 the bone and leads into a series of cavities which constitute 

 the inner ear. As shown in Figure 181, this cavity is com- 

 plicated and difficult to understand. Altogether it is no 

 larger than the end joint of the little finger, but it is the loca- 

 tion of the whole organ of hearing and of balancing. Inside 

 the foramen ovale membrane is a cavity called the vestibule, 

 filled with a thin watery fluid, the perilymph. The cavity ia 



Tympanic flembrane 

 FIG. 182. THE EAR BONES 



Stretching across the middle ear from the 

 tympanic membrane to the foramen orvale. 

 (Hensen) 



