600 MANUAL OF PHYSIOLOGY. 



rather wider as it approaches the tympanic cavity. It is usually 

 the seat of some short, stiff hairs, which help to prevent the 

 entrance of foreign matters. It is supplied with a peculiar 

 modification of sweat glands, which secrete a waxy material that 

 helps to keep the walls of the canal and the outside of the mem- 

 brane moist and soft. Upon the more ordinary sound vibrations, 

 however, the auditory canal has little or no effect. The elastic 

 column of air in any circumscribed space resounds more readily 

 to some one certain tone, which varies according to the capacity 

 of the space : thus are formed resonators of different pitch. Just 

 as different tubes or key barrels have different notes when blown 

 into, so the auditory canal has a note of its own, and if the canal 

 be short, the note is one of a very high pitch. When a tone of 

 the same pitch as that to which the canal is tuned strikes the ear, 

 it is unpleasantly magnified, and it is said that such sounds are 

 those which we commonly call shrill and disagreeable. 



The end of the auditory canal is closed by the membrana tym- 

 pani, which slopes obliquely from above downward and inward, 

 by which means its size is greater than if it were directly across 

 the canal. This membrane is not flat, but the central point is 

 drawn in by the handle of the malleus, which is firmly attached 

 to it along its entire length. The membrane is thus held in the 

 shape of a very blunt cone, somewhat like a Japanese umbrella, 

 the apex of which points inward toward .the cavity of the drum. 

 The peculiar form of the membrane of the drum is of great im- 

 portance for distinct hearing. 



As every confined volume of air has a certain tone of its own 

 to which it resonates more readily than to others, so a membrane 

 of a given size and tension has a certain self-tone, the vibration 

 periods of which it follows with great ease. This tone varies with 

 the tension, as may be seen in a common drum, the note of which 

 can be changed with the tension of its parchment the tenser the 

 membrane the higher the pitch. If the membrane of the drum 

 of our ears were thus set to one tone, our hearing would be most 

 imperfect and unpleasant, for we should be wearied by the reit- 

 eration and persistence of the note to which the tympanic mem- 

 brane was tuned. But this does not occur ; the tympanic mem- 



