HEARING 171 



of air which reaches it through a small tube, the Eustachian 

 tube, connecting it to the back of the mouth. This tube 

 serves to keep the air pressure the same on each side of the 

 ear-drum ; a wide difference in pressure would damage the 

 drum, and even a small difference distorts it sufficiently to 

 be painful. The tube automatically opens every time we 

 swallow and that is why we equalize the pressures by swallow- 

 ing repeatedly when going up or down in a fast-travelling 

 lift, or rapidly climbing in a non-pressurized aeroplane. 

 Similarly, if you don't keep your mouth open when near 

 artillery in action the sudden change of pressure caused by 

 firing may actually burst the ear-drums. 



A chain of three very small bones stretches from the drum 

 across the middle ear. They are named from their shapes, 

 the hammer-, anvil- and stirrup-bones, and they form what 

 Professor Pumphrey has aptly called an impedance-matching 

 transformer incorporating automatic volume control. They 

 are joined together m such a way that movement of the 

 drum caused by the pressure of a sound wave is transmitted 

 by the hammer, which is attached to the drum by the end 

 of its handle, through the anvil to the stirrup. The leverage 

 is such that the stirrup moves through a distance decreased 

 by one-third, but produces a pressure increased by two-thirds 

 on the membrane to which it is attached on the inner side 

 of the middle ear cavity. Further, the joint between the 

 hammer and anvil is arranged so that it slips apart if there 

 is excessive movement of the drum, and the inner ear is 

 protected from damage. Finally two small muscles control 

 the movements of the bones to some extent, and take up any 

 backlash between them. 



The inner ear in mammals is embedded in bone, and con- 

 tains the nerve endings that are stimulated by the pressure 

 waves sent to them from the drum. The hearing part of the 

 inner ear — there is another part which we shall consider 

 later — consists of a tube filled with fluid ; in mammals it is 

 coiled into a spiral of two and a half turns known as the 

 "cochlea" from its likeness to a snail shell. A ridge of bone 

 runs along the inner side of the spiral, and two thin mem- 

 branes extend from it to the outer side so that the main tube 

 is subdivided into three parallel tubes. The upper and lower 



