AURICLE. EXTERNAL AUDITORY CANAL. 887 



If / is the wave-length of a tone, t the time in seconds of an oscillation 

 of the body producing the wave, then / = nt, in which n = 340.88 meters 

 the velocity in each second of sound- transmission through the air. The ve- 

 locity of sound-transmission in water has been found to be 1435 meters 

 in each second or about four times as great as in air, In solid sonorous bodies, 

 it is from 7 to 18 times greater than in air. Sound is conducted best when it 

 remains in one medium; if it passes through different media, it is always weakened. 



Reflection of sound-waves occurs when they strike a solid obstacle, in which 

 case the angle of reflection is always equal to the angle of incidence. 



At this place some additional facts relating to wave-movements may be stated. 

 Two varieties of wave-movements are distinguished: 



I. Progressive Wave-movements. These can appear in two different forms: 

 As longitudinal waves, in which the individual particles of the oscillating 

 body vibrate about their center of gravity in the direction of the propagation 

 of the wave. To these belong the waves in water and in air. In this form of 

 motion, the particles are, of necessity, heaped up in certain places, for example, 

 on the crests of water-waves, while in other places they are diminished in number. 

 This form of wave is, therefore, called a wave of condensation and rarefaction. 

 If, however, each particle in the advancing wave moves only up and down ver- 

 tically, that is transversely to the direction of propagation of the wave, then 

 there result simple transverse waves or progressive flexion-waves, in which there 

 is no condensation or rarefaction in the direction of propagation, as the particles 

 are merely displaced laterally. An example of this wave-motion is afforded by 

 the progressive waves in a rope. 



II. Stationary Flexion-waves. If all the particles of an elastic vibrating 

 body oscillate in such a manner that they are always in the same phase of move- 

 ment, like the two prongs of a sounding tuning-fork, or a twanged cord, the resulting 

 movements are designated stationary flexion-waves. As bodies of little extent in 

 the direction of oscillation vibrate to and fro in stationary flexion -waves, it is 

 evident that the small parts of the auditory apparatus also (tympanic membrane, 

 auditory ossicles, endolymph) oscillate in stationary flexion -waves. Stretched 

 strings, interrupted by nodal points, can also execute stationary flexion-waves in 

 individual segments. 



AURICLE. EXTERNAL AUDITORY CANAL. 



When the cartilaginous (elastic) auricle is absent, the acuteness of hearing 

 is but little altered. Consequently the auricle is physiologically of minor im- 

 portance. It has been supposed that the elevations and depressions of the auricle 

 have a favorable action in reflecting the sound-waves. Many of the latter are 

 manifestly reflected outward again, and those that reach the deep part of the 

 concha are supposed to be thrown against the tragus, to be reflected from this into 

 the external auditory canal. It has also been suggested that the auricle intensi- 

 fies the sound by oscillating in unison with it. By filling the depressions of the 

 auricle with wax, up to the meatus, Schneider claims to have reduced the acute- 

 ness of hearing, but Harless and Esser found it unchanged. Against the assump- 

 tion that there is an effective reflection of the sound-waves both from the parts 

 of the auricle and from the walls of the canal, Mach with justice raises the objection 

 that the dimensions of these parts are too small in comparison to the wave-lengths 

 of sounds. Finally, it has been assumed that the auricle, as an independent, 

 elastic plate, takes up the sound-waves, and conducts them to the cranial bones; 

 so that, in this way, the stimulation of the auditory nerves is strengthened. As, 

 however, the conduction of sound through the bones of the skull, from the air, is 

 exceedingly slight, no serious consideration can be given to such a theory. 



According to Kessel there are in the auricle five situations from which the 

 sound is conducted to the ear, in varying degree, when the head is held still; 

 or if the head is moved, variations in intensity will occur. If the posterior surface 

 of the auricle is covered with rubber, the acuity of hearing and the ability to local- 

 ize sound-impressions coming from behind are decreased. 



Muscles of the External I'.ar. (i) The entire auricle is moved by the retrahens, 

 attrahens, and attolens. (2) The form of the auricle may be altered by the 

 tragicus, an titragicus, helicis major and minor internally; and by the transversus 

 and obliquus auriculae externally. Individuals who can move their ears observe 

 no alteration in hearing during the movement. The helicis major and minor 



