656 THE SENSES 



the solid structures of the body as a whole for the propagation of sound to 

 the internal ear to be adequately effected by that means alone. In passing 

 from air directly into water, sonorous vibrations suffer also a considerable 

 diminution of their strength; but if a tense membrane exists between the 

 air and water, the sonorous vibrations are communicated from the former 

 to the latter medium with very great intensity. This fact, of which Miiller 

 gives experimental proof, furnished at once an explanation of the use of the 

 fenestra ovalis and of the membrane closing it. It is the means of com- 

 municating, in full intensity, the vibrations of the ear bones, or, in their 

 absence, of the air in the tympanum, to the fluid of the labyrinth. The 

 vibration of the fluid, the perilymph and endolymph, of the internal ear, sets 

 the basilar membrane in vibration and in consequence stimulates the sensory 

 apparatus resting upon it. This last is the essential stimulating act, while 

 all that precedes is more or less accessory or contributory to this act. Just 

 what the accessory apparatus contributes can be best understood by an ex- 

 amination of the stimulus and the sensation which results from its action. 



Sound. Any elastic body, e.g., air, a membrane, or a string, performing 

 a certain number of regular vibrations per second, gives rise to what is termed 

 a musical sound or tone. We must, however, distinguish between a musical 

 sound and a mere noise; the latter being due to irregular vibrations. 



Musical sounds are distinguished from each other by three qualities: 

 i. Strength or intensity, which is due to the amplitude or length of the wave 

 of vibrations. 2. Rate, the number of vibrations in a second. 3. Quality, 

 or timbre, the peculiar property by which we distinguish the same note 

 sounded on two instruments, e.g., a piano and a flute. It has been proved 

 by Helmholtz to depend on the number of secondary tones, termed harmonics, 

 which are present with the predominating or fundamental tone; that is, 

 rhythmic vibrations are either simple in form, like the vibrations of a reed 

 or tuning-fork, or compound, like the vibrations of a violin or piano string. 

 If the string of a violin is plucked it not only vibrates as a whole, but in seg- 

 ments in the ratio of one, two, three, etc. The form of air wave that is pro- 

 duced by several such vibrating bodies is very complex indeed, as, for exam- 

 ple, when an orchestra is playing. 



The compound wave can be analyzed into its constituent elements by a 

 system of resonators, on the principle of sympathetic vibration. If one 

 sounds a series of musical notes before such a system of resonators it will be 

 found that the tones and overtones are selected by the resonators and made 

 more prominent so that they can be identified. 



The sensation of sound has in it certain elements that correspond closely 

 with the physical properties of sound, i.e., loudness, pitch, and quality. 

 Loudness is dependent merely on the intensity of the stimulation. A sound 

 wave of great energy, for example, produces a larger movement of the tym- 

 panic membrane, and it, through the chain of bones and the fluid of the 



