HEARING. 401 



or excite in, the opposing body, according to its elasticity or suscepti- 

 bility, vibrations similar to their own. 



The communication of sounds merely consists in the conveyance of 

 sonorous vibVations from one body to another; such communication is 

 common to both noises and definite tones. The excitation of sound by 

 one body in another, is a different phenomenon, and occurs, in its 

 purest forms, only with regular or definite tones. It is essential for 

 this, that the natural note, emitted by the exciting and excited body, 

 when struck or sounded, be identical. If two strings, e. g., tuned to 

 the same note, be placed side by side, and one be made to vibrate, the 

 other is at once thrown into corresponding undulations, and gives forth 

 the same note; this is called the reciprocation of sounds; the bodies are 

 called reciprocating, and the sounds reciprocal. In the same manner, 

 dry stretched membranes reciprocate their corresponding or natural 

 notes. When a sounding 'body, instead of exciting its own fundamen- 

 tal note, in another body or in parts of it, excites other notes bearing 

 certain harmonious relations to it, the latter body is said to resound, 

 and is called a resonant body. This form of excitation of sound is not 

 so pure as the one previously mentioned. The air itself is, in this 

 sense, both a reciprocating and resonant body, more particularly when 

 it is isolated in tubes, or is confined in closed chambers. 



It has been stated by Helmholz that although certain sounds con- 

 sist merely of a fundamental note produced by a single set of uniform 

 vibrations, yet that most sounds are caused by combinations of the 

 fundamental note with certain secondary or harmonic notes; and that 

 the timbre or quality of sounds is dependent on the manner in which 

 these secondary sounds are associated together in groups, named by 

 Helmholz, sound colors. 



Those sound-waves, the number of undulations of which corre- 

 sponds or bears a certain definite numerical proportion to each other, 

 are more or less agreeable to the ear, and are named concords; those 

 which do not, are disagreeable when heard together or in succession, 

 and are called discords. 



Searing. 



In Man and air-breathing animals, sounds excited in the atmos- 

 phere reach the fluid of the labyrinth by two paths. First, through 

 the mixed membranous and osseous tympanic apparatus ; and sec- 

 ondly, through the cranial bones. The passage of such sounds 

 through the tympanum is effected readily, and with great range and 

 delicacy of appreciation. Through the solid bones of the head, 

 however, the transmission of sounds excited in the atmosphere is ac- 

 complished with difficulty ; were it not so, the noises which would thus 

 be produced would be unbearable; and they would, moreover, confuse 

 the sounds received through the tympanum. It is through the bones 

 of the head that sounds are transmitted to the internal ear, when, 

 from any cause, the tympanum ceases to conduct sound. In speaking 

 and singing, the hand placed on the head distinctly feels the vibra- 

 tions of the cranial bones, and the auditory nerve is excited by them 

 when the ears are closed. 



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