SENSATIONS AND THE SENSE-ORGANS 147 



The part specialized for the translation of sound waves 

 into nerve-impulses is a spiral passage called the cochlea. 



Anatomists distinguish the external, the middle, and 

 the internal ear. The external ear includes the visible 

 part and the short passage that is terminated by the 

 tympanic membrane. This is what is commonly called 

 the eardrum, though it is strictly the drumhead. The 

 drum, regarded as a box, is represented by an irregular 

 cavity called the middle ear or tympanum in the temporal 

 bone. This contains air. Communication with the 

 exterior is by way of the Eustachian tube which, on 

 either side, leads from the tympanum to the upper part 

 of the throat or, as one may say with equal correctness, 

 to the back of the nose. 



The Eustachian tubes are very narrow; in fact they 

 are usually closed by the contact of their lining surfaces. 

 At certain moments, as when we swallow or blow the 

 nose, there is an effective connection. In order that 

 the tympanic membrane shall be free to vibrate nor- 

 mally the pressures on its two sides must be equal. The 

 pressure on the outside is that of the atmosphere and 

 subject to barometric changes. The tympanum holds 

 a small, isolated sample of the atmosphere and the 

 occasional opening of the Eustachian tube is neces- 

 sary to the standardizing of this sample. If the channel 

 of communication is blocked for any length of time an 

 inequality of pressure upon the two sides of the membrane 

 may be expected. It will be attended by the " stuffy" 

 sensation familiar in bad colds. When the tubes are 

 opened after long obstruction there is a noticeable snap 

 and a marked relief. 



Many persons whose Eustachian tubes do not open 

 readily may notice the resulting discomfort when rapidly 

 changing altitude, for example, in the ascent of a long 

 railroad grade. As the height is gained there is a lower 

 barometric pressure and the confined air in the tym- 

 panum is more dense than that outside. There must 

 be, under these circumstances, a slight outward bulging 



