USE OF THE EXTERNAL MEATUS. 403 



the inequalities presented by the auricle are intended to receive at 

 right angles, and therefore favorably for complete conduction, sonor- 

 ous undulations from all directions. 



The external auditory meatus receives and conducts sonorous vibra- 

 tions to the membrana tympani. The impairment or loss of the power 

 of hearing sounds which come through the air, caused by stopping 

 the meatus with the finger, or by obstructing it with water, cotton-wool, 

 or other substances, indicates its function as a conductor of sound. 

 Owing to the curved direction of the canal, and to the partial conceal- 

 ment of its outer end, it is impossible for the atmospheric sound-waves 

 to pass straight down from the exterior to the membrana tympani; 

 they must undergo reflection many times, and at various angles, chiefly 

 from the internal surface of the concha and of the tragus, down into the 

 meatus, and from all sides of the latter, through the air within it, on to 

 the tympanic membrane. The walls of the meatus, like those of the 

 auricle, also conduct sounds; but their conducting power for atmos- 

 pheric sound-waves is but feeble, and, in the ordinary condition, is 

 probably specially provided against. If the meatus be closed exter- 

 nally with the tip of the finger, and the auricle or tragus be scratched, 

 or if a watch be held against these parts, the sounds produced are ex- 

 tremely loud. Through resonance, the sounds, in fact, appear to be 

 of increased intensity; for, the closed meatus constituting a resonant 

 chamber, the resounding vibrations, excited in the air contained within 

 it, act on the membrane of the tympanum, and, in this way, intensify 

 the original sound. It is in a similar manner, that those sounds which 

 pass through the teeth, or cranial bones, and which are known as head- 

 sounds, are also rendered very much louder. The resonance of the 

 mass of air contained within the meatus, also gives increased strength 

 to the intensity of atmospheric sounds. If, indeed, a tube be added to 

 the meatus, so as to lengthen the auditory passage, the intensity of all 

 sounds becomes much greater. 



The membrana tympani is admirably adapted for the reception of 

 atmospheric sound-waves, and, although it may, to a certain extent, 

 be thrown into vibrations through the osseous ring in which it is set, 

 it is principally intended to be acted upon by the atmospheric undula- 

 tions received through the auditory meatus. Its area is equal to about 

 i^th of a square inch. The slanting position of the membrane at the 

 bottom of this canal, not only serves to increase its area, but is prob- 

 ably intended to adapt it for the reception of more numerous vibra- 

 tions from the walls of the meatus, it may be, at some given angle, or 

 in a perpendicular direction. In its usual condition, the membrane is 

 in a state of moderate tension, due partly to its own structure, and 

 partly to the support afforded it by the long process of the malleus. 

 For a wide range of notes, its state of tension must be constantly 

 undergoing variations; thus, for low sounds, it must be relaxed; for 

 high sounds it must be rendered tense. If, e. g., we close the mouth 

 and nostrils, and force air into the tympanum, through the Eustachian 

 tube, by means of an expiratory effort, or if we exhaust the air in the 

 tympanic cavity, by an inspiratory effort, we increase the tension of 

 the tympanic membrane; in this state, grave sounds are rendered less 



