THE EAR MUSCLES AND EXTERNAL AUDITORY MEATUS. 



799 



to the direction of the propagation of the wave, then we have the simple transverse waves, or 

 progressive waves, in which there is no condensation or rarefaction in the direction of propaga- 

 tion, as each particle is merely displaced laterally. An example of this is the progressive waves 

 in a rope. 



II. Stationary Flexion Waves. When all the particles of an elastic vibrating body so 

 oscillate that all of them are always in the same phase of movement as the limbs of a vibrating 

 tuning-fork or a plucked string, then this kind of movement is described as stationary flexion 

 waves. As bodies, whose expansion in the direction of oscillation is very slight, vibrate to and 

 fro in the stationary flexion wave, so we see that the small parts of the auditory apparatus 

 (tympanic membrane, ossicles, lymph of the labyrinth) oscillate in stationary flexion waves. 



408. EAR MUSCLES EXTERNAL AUDITORY MEATUS. When the external ear is 



absent, little or no impairment of the hearing is observed ; hence, the physiological functions 

 of these organs are but slight. Boerhaave thought that the elevations and depressions of the 

 outer ear might be connected with the reflection of the sound-waves. Numerous sound-waves, 

 however, must be again reflected outwards ; and those waves which reach the deep part of the 

 concha are said to be reflected towards the tragus, to be reflected by it into the external 

 auditory meatus. According to Schneider, when the depressions in the ear are filled up with 

 wax, hearing is impaired ; other observers, however, have found the hearing to be unaffected. 

 Mach points out that the dimensions of the external ear are proportionally too small to act as 

 reflecting organs for the wave-lengths of noises. 



Muscles of the External Ear. (1) The whole ear is moved by the retrahentes, attrahens, and 

 attollens. (2) The form of the ear may be altered by the tragicus, antitragicus, helicis major 

 and minor internally ; and by the transversus and obliquus auriculae externally. Persons who 

 can move their ears do not find that the hearing is influenced during the movement. The Mm. 

 helicis major and minor are regarded as elevators of the helix, the transversus and obliquus 

 auricula? as dilators of the concha ; the tragicus and antitragicus as constrictors of the meatus. 

 In animals, the external ear and the action of its 

 muscles have a marked effect upon hearing. The 

 muscles point the ear in the- direction of the sound, 

 while other muscles contract or dilate the space within 

 the external ear. In many diving animals, the meatus 

 can be closed by a kind of valve. 



The external meatus is 3 to 3*25 cm. long 

 \\.\ to 1J inch], 8 to 9 mm. high, and 6 to 8 

 mm. broad at its outer opening (fig. 578). It 

 is the conductor of the sound-waves to the 

 tympanic membrane, so that almost all the 

 sound-waves first impinge upon its wall, and 

 are then reflected towards the tympanic mem- 

 brane. To see well down into the meatus, we 

 must pull the auricle upwards and backwards. 

 Occlusion of the meatus, especially by a plug 

 of inspissated wax ( 287), of course interferes 

 with the hearing, [and when it presses on the Fig. 578. 



membrana tympani may give rise to severe The external auditory meatus and the 

 vertigol. tympanic cavity. M, osseous spaces in 



the temporal bone ; Pc, cartilaginous 



409. TYMPANIC MEMBRANE. The part of the meatus ; L, membranous 



tympanic membrane, which is tolerably laxly union between both ; F, auricular sur- 

 fixed in a special osseous cleft, with a thickened face for the cond y le of the lower J aw - 

 margin, is an elastic, unyielding, and almost non-extensible membrane of about 0*1 

 mm. in thickness, and with a superficial area of 50 square millimetres (fig. 581). 

 It is elliptical in form, its greater diameter being 9*5 to 10 mm., and its lesser 8 

 mm., and it is fixed in the floor of the external meatus obliquely, at an angle of 

 being directed from above and outwards, downwards and inwards. 



40 



Both 



tympanic membranes converge anteriorly, so that if both were prolonged they 

 would meet to form an angle of 130 to 135. The oblique position enables a larger 

 surface to be presented than would be obtained if it were stretched vertically, so 

 that more sound-waves can fall vertically upon it. The membrane is not stretched 

 flat, but a little under its centre (umbilicus) it is drawn slightly inwards by the 



