AUDITORY APPARATUS IN MAN. 185 



undulations of the air throw it into a similar state of vibra- 

 tion. The structure of this part is muscular, being thus 

 designed to adapt itself to the force of the vibrations com- 

 municated to it from the external air. 



575. The cavity of the tympanum t, behind the ear- 

 drum, is always filled with air, but it would obviously de- 

 feat 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 d^ens 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 communication 

 with the external air, which appears to be as necessary 

 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 sounds as usual, but the 

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

 fined air; and when the eustachian tube is obstructed, 

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

 this part is swollen or its secretion is increased by in- 

 flammation, 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 



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

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

 called? What tube communicates with the cavity? What would be the 

 effect were the air of this cavity confined ? Why does one open his mouth 

 in order to hear a distant sound ? 



16* 



