A STUDY OF THE SENSES 



309 



The Middle Ear. Beyond the tympanum is a small cav- 

 ity, known as the middle ear. From this cavity a narrow 

 tube (the Eustaehian tube) about an inch and a half long 

 communicates with the upper part of the throat cavity. If 

 one were to go up on a high mountain, one would find that 

 the pressure of the air on the outside of the body, and there- 

 fore on the exterior of the eardrum, would become less, and 

 that if some of the air in the middle ear were not to escape, 

 the eardrums would be forced outward, and hence would be 

 ruptured. If, on the other hand, one should go into a deep 

 mine, the increased pressure on the outside of the drums 

 would force them inward. All these accidents are prevented 

 by the presence of the Eustaehian tubes, through which air 

 can pass into and out from the middle ear, and so the pres- 

 sure on both sides of the tympanum can be equalized. In 

 severe head colds, as we have already seen (p. 86), the open- 

 ing from the throat cavity into the Eustaehian tubes becomes 

 temporarily closed. We then are conscious of a ringing 

 sensation in the ears. 



The Bones of the Middle Ear. Within the cavity of the 

 middle ear are three tiny bones. 

 The first, from its fancied re- 

 semblance to a hammer, is 

 called the mal'le-us (Latin mal- 

 leus a hammer) ; the second 

 looks somewhat like an anvil, 

 and hence is known as the in'cus 

 (Latin incus = an anvil) ; the 

 third has the exact form of a 

 stirrup, and it has, therefore, 

 received the name sta'pes (Latin FIG. 143. Bones of Middle Ear. 

 stapes = a stirrup). These three 

 little bones are arranged in a 

 chain across the cavity of the 

 middle ear, for the handle of the malleus is connected with 

 the tympanic membrane, the flat part of the stirrup presses 



c = hammer (malleus) . 

 d anvil (incus). 

 /= stirrup (stapes). 



