External Ear

In man, the muscles are so poorly developed that he can hardly move the external ear or pinna perceptibly, and the part commonly called the ear is of little use. We know this, because the outer ear may be quite removed without materially affecting the power of hearing. The sound reflected from the pinna may be excluded, without reducing the intensity of that heard, by placing a little tube in the auditory canal. Birds hear well without any outer ear. But the movable ears of many animals are, no doubt, useful in helping them to ascertain the direction of a sound by catching more of the vibrations coming toward their pinna. That the external ear may be of some use, even to man, one is led to believe by the natural readiness with which a person with dull hearing supplements it by means of his hand. In this act the ear is pushed away from the head to an angle of about forty-five degrees, and its projection is considerably increased.

External Auditory Meatus

The auditory canal is a crooked and irregular passage, getting rather wider as it approaches the tympanic cavity. It is 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 membrane moist and soft.

The elastic column of air in any circumscribed space resounds more readily to some one tone, varying according to the capacity of the space; thus resonators of different pitch are formed. Different tubes 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 such sounds are called shrill and disagreeable. Upon the more ordinary sound vibrations, however, the auditory canal has little or no effect.