310 THE SPECIAL SENSES 



number, and from their peculiar shapes bear the following 

 names : malleus, or the mallet ; incus, or the anvil ; and stapes, 

 or the stirrup. A fourth, the smallest bone in the body, in 

 early life intervenes between the incus and the stapes, but at 

 a later period it becomes a part of the incus. It is called the 

 orbicular bone. Small as are these ossicles and they, together, 

 weigh only a few grains they have their little muscles, carti- 

 lages, and blood-vessels, as perfectly arranged as the larger 

 bones of the body. One end of the chain of ossicles, the mal- 

 let, is attached to the membrane of the tympanum, or outer 

 drum-head, while the other end, the stirrup, is firmly joined 

 by its foot-piece to a membrane in the opposite side of the 

 cavity. The chain, accordingly, hangs suspended across the 

 drum between the two membranes; and when the outer one 

 vibrates under the influence of the sound-wave, the chain 

 swings inward and transmits the vibration to the entrance of 

 the inner ear. 



98. The musical instrument, the drum, is not complete if 

 the air within be perfectly confined; we therefore find in all 

 instruments of this kind a small opening in the side, through 

 which air may pass freely. By this means the pressure of the 

 air upon the vellum which forms the head of the duurn is made 

 equal upon all sides, and the resonance of the drum remains 

 unaffected by the varying density of the atmosphere. It will, 

 therefore, emit its proper sound, whether it be struck in the 

 rarefied air of the mountain-top, or in the condensed air of a 

 mine. The tympanum, or drum of the ear, in like manner has 

 an opening, by means of which it communicates freely with 

 the external air. This opening is a narrow canal, about an 

 inch and a half long, called the Eustachian tube, after the name 

 of its discoverer, Eustachius. 



99. The course of this passage is indicated in Fig. 71, 

 directed downward and inward: its other extremity opens 

 into the upper part of the throat. The passage itself is ordi- 

 narily closed, but whenever the act of swallowing or gaping 



98. The Eustachian tube ? Describe it, and state its use. 



99. What can you state of the action of the Eustachian tube ? 



