SENSE OF HEARING. 649 



tions. The impressions received in this way depend upon the vibrations 

 excited in the atmosphere by sonorous bodies, which are themselves 

 thrown into vibration by various causes, and which then communi- 

 cate similar undulations to the surrounding air. These undulations 

 are of such a kind that they cannot be directly appreciated by the 

 organs of general sensibility ; but when communicated to the auditory 

 apparatus they produce, through it, the sensation of sound. 



Organ of Hearing. The organ of hearing consist of, first, the ex- 

 ternal ear, a conch or trumpet-shaped expansion, destined to collect the 

 sonorous impulses coming from various quarters, and to conduct them 

 into its tubular continuation, the external auditory meatus ; secondly, 

 a membranous sheet or drum-head, the membrana tympani, stretched 

 across the bottom of the external auditory meatus, by which the sono- 

 rous vibrations are received and transmitted, through the chain of bones 

 or auditory ossicles, across the cavity of the tympanum or middle ear, 

 to the third portion of the auditory apparatus, namely, the labyrinth, 

 or internal ear ; a cavity excavated in the petrous portion of the tem- 

 poral bone, filled with fluid, and containing various membranous sacs 

 and canals, upon which are distributed the filaments of the auditory 

 nerve. 



Thus the delicate terminal expansions of the auditory nerve, deeply 

 concealed in their bony cavities, and sustained by the surrounding fluid, 

 are protected from all other mechanical impressions, but are so placed 

 as to receive the impulse of sonorous vibrations. 



External Ear. The external ear consists of a cartilaginous frame- 

 work, covered with integument, loosely attached to the bones of the 

 head, and more or less movable by means of various muscles, which, 

 by their contractions, tend to turn its concavity in various directions. 

 In man, notwithstanding the existence of these muscles, their functional 

 activity is nearly imperceptible ; and it is only in exceptional cases that 

 thej 7 are capable of producing a partial sliding or rotatory movement of 

 the external ear. In most of the quadrupeds, on the other hand, these 

 movements are vigorous and extensive, and play an important part, not 

 only in the changes of expression by varying the attitude of the ex- 

 ternal ear, but also in aid of the sense of hearing, by enabling the 

 animal to catch distinctly the sonorous vibrations, from whatever quarter 

 they may come. By their assistance the direction of a sound is also 

 appreciated, since the animal ascertains, in placing the ear in different 

 positions, the region from which it is received with the greatest distinct- 

 ness. 



Membrana Tympani and the Chain of Bones The membrana tym- 

 pani is a fibrous sheet of circular form, composed of a principal la}^er 

 not more than 0.05 millimetre in thickness, but quite strong, and con- 

 sisting of circular and radiating tendinous fibres with a trace of inter- 

 mingled elastic fibres. Its external and internal surfaces respectively 

 are covered by thin continuations of the integument of the external 

 auditory meatus on the one hand, and of the mucous membrane of the 

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