596 MANUAL OF PHYSIOLOGY. 



a nerve impulse. The impulse is transmitted along the nerve to 

 the brain, and there gives rise to the sensation with which we 

 are familiar as sound. 



The vibrations of the air are wave-like movements depending 

 upon a series of changes of density in the gases, the particles of 

 which move toward or from one another, and transmit the mo- 

 tion to their neighbors, so as to propagate the sound wave. To 

 demonstrate these vibrations, a special apparatus must be used. 



When a tuning-fork is struck it is thrown into vibration, and 

 a sound is given forth. But the vibrations are often so rapid and 

 so small that the motion of the tuning-fork cannot be appreciated 

 by the eye. But if a fine point be attached to one prong of the 

 tuning-fork or, indeed, any elastic body, such as a bar of metal 

 and this point be brought into contact with a moving smoked 

 surface, such as has been already described for similar records, a 

 little wavy line is drawn, showing that the vibrating fork moves 

 up and down at an even and regular rate. Each up and down 

 stroke indicates a vibration. The length of the wave, as drawn 

 on the evenly-moving surface of the recorder, shows the amount 

 of time occupied by each vibration. This is always found to be 

 the same, for a tuning-fork of a given pitch, and thus the record- 

 ing fork is in constant use by the physiologist as an exact measure 

 of small intervals of time. The pitch of the note, then, depends 

 upon the rate or period of vibration, a note or tone of a certain 

 pitch being simply a sound caused by so many vibrations per 

 second. The quicker the vibration the higher the note, and the 

 slower the deeper, until, at the rate of about thirty per second, 

 no sound is any longer audible. Whether a note be produced 

 by a metal fork, a tense string, or any other vibrating body, if 

 the number of vibrations per second be the same, the note must 

 have the same pitch. 



The elevation of each vibration as seen in the tracing made by 

 a recording fork is different at different times. When the fork 

 is first struck the waves are high and well marked, and the ex- 

 cursions of the recording prong can be seen to become less and 

 less extensive as the fork gradually ceases to vibrate, and the 

 sound becomes faint ; or, in other words, as the sound produced 



