824 SPECIAL SENSES. 



ossicles and of their ligaments and muscles is essential to the proper conduction of sound 

 to the labyrinth ; the presence of liquid in the labyrinth is a condition essential to the 

 conduction of the waves to the filaments of distribution of the auditory nerves ; and, 

 finally, from the labyrinth, the nerves pass through the internal auditory meatus to the 

 brain, where the auditory impressions are appreciated. 



Most of the points in acoustics which are essential to the comprehension of the physi- 

 ology of audition are definitely settled. The theories of the propagation of sound involve 

 wave-action, concerning which there is no dispute among physicists. For the conduc- 

 tion of sound, a ponderable medium is essential ; and it is not necessary, as in the case 

 of the undulatory theory of light, to assume the existence of an imponderable ether. 

 The human ear, although perhaps not so acute as the auditory apparatus of some of the 

 inferior animals, not only appreciates irregular waves, such as produce noise as distin- 

 guished from sounds called musical, but is capable of distinguishing regular waves, as in 

 simple musical sounds, and harmonious combinations. 



In music, certain successions of regular sounds are agreeable to the ear and constitute 

 what we call melody. Again, we are able to appreciate, not only the intensity of sounds, 

 both noisy and musical, but we recognize pitch and different qualities, particularly in 

 music. Still farther, we find that musical notes may be resolved into certain invariable 

 component parts, such as the octave, the third, fifth, etc. These components of what 

 are usually supposed to be simple sounds which may be isolated by artificial means, to 

 be described farther on are called tones ; while the sounds themselves, produced by the 

 union of the different tones, are called notes, which may themselves be combined to form 

 chords. 



The quality of musical sounds may be modified by the simultaneous production of 

 others which correspond to certain of the components of the predominating note. For 

 example, if we add to a single note, the third, fifth, and octave, we produce a major 

 chord, the sound of which is very different from that of a single note or of a note with 

 its octave. If we diminish the third by a semitone, we have a different quality, w^ich 

 is peculiar to minor chords. In this way, we can form an immense variety of musical 

 sounds upon a single instrument, as the piano. And still farther, by the harmonious 

 combinations of the notes of different instruments and of different registers of the 

 human voice, as in grand choral and orchestral compositions, shades of effect, almost 

 innumerable, may be produced. The modification of tones in this way constitutes har- 

 mony ; and an educated ear, not only experiences pleasure from these musical combina- 

 tions, but can distinguish their different component parts. 



A chord may convey to the ear the sensation of completeness in itself or it may lead 

 to a succession of notes before this sense of completeness is attained. Different chords 

 of the same key may be made to follow each other, or we may, by transition-notes, pass 

 to the chords of other keys. Each key has its fundamental note, and the transition from 

 one key to another, in order to be agreeable to the ear, must be made in certain well- 

 defined and invariable ways. These regular transitions constitute modulation. The ear 

 becomes fatigued by long successions of notes always in one key, and modulation is essen- 

 tial to the enjoyment of elaborate musical compositions ; otherwise, the notes would not 

 only become monotonous, but their correct appreciation would be impaired, as the ap- 

 preciation of colors becomes less distinct after looking for a long time at an object pre- 

 senting a single vivid tint. 



Laws of Sonorous Vibrations. 



As we have already remarked, sound is produced by vibrations in a ponderable me- 

 dium. The sounds ordinarily heard are transmitted to the ear by means of vibrations 

 of the atmosphere. A simple and very common illustration of this fact is afforded by 

 the experiment of striking a bell carefully arranged in vacua. Although the stroke and 

 the vibration can readily be seen, there is no sound ; and, if air be gradually introduced, 



