AUDITORY APPARATUS IN MAX. 189 



undulations of the air throw it into a similar state of 

 vibration. The structure of this part is muscular, being 

 thus designed to adapt itself to the force of the vibra- 

 tions'communicated to it from the external air. 



The cavity of the tympanum t, behind the Ear-drum 

 is always filled with air, but it would obviously defeat 

 the design of this organ, were the air confined to this 

 space, because unless it were in a state to expand and 

 contract, it could not remain in equilibrium with the 

 pressure of the atmosphere on the external surface of 

 the drum, which of course varies, according to the rise 

 and fall of the barometer. Hence, were this air con- 

 fined, an intense internal pressure on the drum would 

 be the consequence, whenever the external pressure 

 happened to be partially removed, as when one ascends 

 a mountain, or mounts up in a balloon. Against such 

 an evil, there is an effectual guard by the communication 

 between the internal Ear, and the atmosphere, by means 

 of the Eustachian tube, e. This tube, as the figure 

 shows, begins with a small orifice, and opens with a 

 wide mouth, back of the nostrils. It performs the same 

 office in the Ear, that the aperture in the side of a drum 

 does in that instrument ; that is, it forms a communica- 

 tion w r ith the external air, which appears to be as neces- 

 sary to the functions of the Ear, as it is for the sound of 

 the drum. When the air-hole of a drum is stopped, 

 the instrument not only does not sound as usual, but the 

 head is liable to be broken, by the re-action of the con- 

 fined air ; and when the Eustachean tube is obstructed, 

 as is often the .case during influenza, or colds, by which 

 this part is swollen, or its secretion is increased by inflam- 

 mation, then a partial deafness is the consequence. 

 This tube also appears to be the channel through which 

 sound may be admitted, or perhaps the hearing is more 

 perfect when there is an ample communication between 

 the external air and the tympanic cavity, for it is well 

 known that when one listens to a scarcely audible sound 



Where is the ear-drum situated 1 What effect do the undulations of 

 the air have upon the ear-drum ? What is the cavity within the tym- 

 panum called t What tube communicates with the cavity 1 What would 

 be the effect were the air of this cavity confined 1 Why does one open 

 his mouth in order to hear a distant sound 1 



