886 



PRELIMINARY PHYSICAL CONSIDERATIONS. 



as the transmission is thus restricted. If in persons hard of hearing conduction 

 and hearing through the bones of the skull are still normal, the cause of the deaf- 

 ness is not in the nervous parts of the ear, but in the external sound-conducting 

 part of the apparatus. 



2. Normal conduction, in ordinary hearing through the external 

 auditory canal, takes place as follows: the vibrations of the air first set 



the tympanic mem- 

 brane (Fig. 314, T) 

 into vibration ; this 

 in turn moves the 

 malleus (h), the 

 incus (a), and the 

 stapes (s), the last 



of which transmits 

 the vibrations of 

 its base to the fluid 

 of the labyrinth. 



3. In individ- 

 uals, in whom as 

 a result of destruc- 

 tive disease of the 

 middle ear, the 

 tympanic mem- 

 brane and the 

 ossicles are de- 

 stroyed, stimula- 

 tion of the auditory 

 apparatus can take 

 place (to be sure, 

 only in an impaired 

 degree) also by a 

 transmission of the 



FIG. 314. Diagrammatic Representation of the Auditory Apparatus: AG, 

 external auditory canal', T, tympanic membrane, K, malleus, with its 

 head (h), short process (kf) and manubrium (m); a, incus, with its 

 short process (x) and long process, which is united with the stapes (s) 

 by the os orbiculare (ossicle of Sylvius); P, tympanic cavity, o, oval 

 window; r, round window, X, beginning of the lamina spiralis of the 

 cochlea; pt, the scala tympani, and vt, the scala vestibuli; V, vesti- 

 bule; S, saccule; U, utricle; H, semicircular canals; TE, Eustachian 

 tube. The long arrow indicates the direction of action of the tensor 

 tympani muscle, the short curved arrow that of the stapedius muscle. 



atmospheric vibra- 



tions directly to the membrane covering the fenestra rotunda (r) and the 

 structure closing the fenestra ovalis (o). The membrane of the fenes- 

 tra rotunda can, in fact, be set in vibration alone, even if the parts clos- 

 ing the fenestra ovalis have become unyielding. 



PRELIMINARY PHYSICAL CONSIDERATIONS. 



Sound is produced by the oscillations of elastic bodies capable of vibration. 

 These oscillations cause alternate condensations and rarefactions of the surround- 

 ing air; or in other words waves, in which the particles vibrate longitudinally, 

 that is in the direction of transmission of the sound. These condensations and 

 rarefactions form concentric hollow spheres around the point of origin of the 

 sound, which propagate the sound-vibrations to the ear. The vibrations of 

 sonorous bodies are called stationary vibrations, that is all of their particles are 

 always in the same phase of movement, as they begin to move simultaneously, 

 reach the maximum of vibration at the same time, and begin the return motion 

 at the same time as, for example, the particles of a sounding, vibrating metallic 

 rod. Sound is produced, therefore, by the stationary vibrations of elastic bodies, 

 and it is propagated by advancing wave-motions of elastic media (ordinarily of 

 the air). The wave-length of a tone, that is the distance between one maximum 

 of condensation and the succeeding one in the air (or between two condensation- 

 spheres of the air) is proportional to the time of oscillation of the body whose 

 vibrations produce the sound-waves. 



