220 THE SPECIAL SENSES. 



its margin is fastened into a circular groove in* the adjacent 

 bone. Each wave of sound that impinges against this 

 delicate membrane causes it to vibrate, and it, in turn, 

 excites movements in the parts beyond. 



96. Within the tympanum is arranged a chain of re- 

 markable " little bones," or ossicles. They are chiefly three 

 in number, and from their peculiar shapes bear the follow- 

 ing 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 

 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, cartilages, and blood-vessels, as per- 

 fectly arranged as the larger bones of the body. One end 

 of the chain of ossicles, the mallet, is attached to the mem- 

 brane, 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. 



97. 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 drum 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 rarified air of the mountain- 

 top, or in the condensed air of a mine. The tympanum, 



96. What are the ossicles ? Their number and names ? Their arrangement ? 



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



