79 g CONDUCTION OF SOUND TO THE LABYRINTH. 



The sonorous vibrations may set the perilymph in vibration in three different 



"Y* Conduction through the Bones of the Head. This occurs especially when 

 the 'vibrating solid body is applied directly to some part of the head, e.g., a tuning- 

 fork placed on the head, the sound being propagated most intensely m the 

 direction of the prolongation of the handle of the instrument also when the sound 

 is conducted to the head by means of fluid, as when the head is ducked under 

 water Vibrations of the air, however, are practically not transferred directly to 

 the bones of the head, as is shown by the fact that we are deaf when the ears are 

 stopped. 



The <oft parts of the head, which lie immediately upon bone, conduct sound best, and of the 

 projecting part, the best conductor is the cartilaginous portion of the external ear But even 

 under the most favourable circumstance, conduction through the bones of the head is far less 

 effective than the conduction of the sound-waves through the external auditory meatus. If a 

 tuning-fork be made to vibrate between the teeth until we no longer hear it, its tone may still 

 be heard on bringing it near the ear (Mnne). The conduction through the bones is favoured 

 when the oscillations are not transferred from the bones to the tympanic membrane, and are 

 thus transferred to the air, in the outer ear. Hence, we hear the sound of a tuning-fork applied 

 to the head better when the ears are stopped, as this prevents the propagation of the sound- 

 waves through the air in the outer ear. If, in a deaf person, the conduction is still normal 

 through the cranial bones, then the cause of the deafness is not in the nervous part of the ear, 

 but in the external sound-conducting part of the apparatus. 



2. Normal hearing takes place through the external auditory meatus. The 

 enormous vibrations of the air first set the tympanic membrane in vibration (fig. 

 577, T); this moves the malleus (h), whose long process is inserted into it; the 

 malleus moves the incus (a), and this the stapes (*), which transfers the movements 

 of its plate to the perilymph of the labyrinth. 



3. Direct Conduction to the Fenestra. In man, in consequence of occasional disease of the 

 middle ear, whereby the tympanic membrane and auditory ossicles may be destroyed, the 

 auditory apparatus may be excited, although only in a very feeble manner, by the vibrations of 

 the air being directly transferred to the membrane of the fenestra rotunda (r), and the parts 

 closing the Fenestra ovalis (o). The membrane of the fenestra rotunda may vibrate alone, even 

 when the oval window is closed with a rigid body ( Wcber-Liel). 



407- PHYSICAL INTRODUCTION. Sound is produced by the vibration of elastic bodies 

 capable of vibration. Alternate.condensation and rarefaction of the surrounding air are thus 

 produced ; or, in other words, sound-waves in which the particles vibrate longitudinally or in 

 the direction of the propagation of the sound are excited. Around the point of origin of the 

 sound, these condensations and rarefactions occur in equal concentric circles, which conduct 

 the sound vibrations to our outer ear. The vibrations of the sounding body are so called 

 "stationary vibrations," i.e., all the particles of the vibrating body are always in the same 

 phase of movement, in that they pass into movement simultaneously, they reach the maximum 

 of movement simultaneously, e.g., in the particles of a sounding vibrating metal rod. Sound 

 is produced by the stationary vibrations of elastic bodies ; it is propagated by progressive 

 wave-motion of elastic media, generally the air. The wave-length of a tone, i. e. , the distance 

 of one maximum of condensation to the next one in the air, is proportional to the duration of 

 the vibration of the body, whose vibrations produce the sound-waves. 



If X is the wave-length of a tone, t in second the durations of a vibration of the body 

 producing the wave, then \ n t, where 71 = 340*88 metres, which is the rate per second of 

 propagation of sound-waves in the air. The rapidity of the transmission of sound-waves in 

 water- 1435 metres per second, i.e., nearly four times as rapid as in air; while, in solids 

 capable of vibration, it is propagated from seven to eighteen times faster than in the air. 

 Sound- waves are conducted Dest through the same medium; when they have to pass through 

 several media they are always weakened. 



Reflection of the sound-waves occurs when they impinge upon a solid obstacle, in which case 

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



Wave Movements. We distinguish I. Progressive wave movements which occur in two 

 forms (1) As longitudinal reaves, in which the individual particles of the vibrating body 

 vibrate around their centre of gravity in the direction of the propagation of the wave ; examples 

 are the waves in water and air. This movement causes an accumulation of the particles at 

 certain places, e.g., on the crests of the waves in water-waves, while at other places they are 

 diminished. This kind of wave is called a wave of condensation and rarefaction. (2) If, 

 however, each particle in the progressive wave moves vertically up and down, i.e., transversely 



