554 THE NERVOUS SYSTEM. 



star, in order to get a distinct view, its light disappears. It can only 

 be seen as an evanescent object by indirect vision. 



Sense of Hearing. 



By the sense of hearing we receive the impressions of sound, and 

 appreciate their intensity, their higher or lower notes, and their quality, 

 that is, the different character of sounds of similar pitch and intensity, 

 produced by different means, as by reeds, strings, or wind instruments, 

 or the concussion of solid or liquid bodies. Our idea of time, or the 

 succession of events, seems also especially connected with auditory 

 sensations. Impressions received in this way depend on the vibrations 

 excited in the atmosphere by sonorous bodies, which are themselves 

 already in vibration. These undulations, when communicated to the 

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



Organ of Hearing. The organ of hearing consists of, first, the exter- 

 nal ear, a trumpet-shaped expansion, which collects the sonorous im- 

 pulses coming from various quarters, and conducts them into its tubu- 

 lar continuation, the external auditory meatus ; secondly, a membranous 

 sheet or drum-head, the membrana tympani, stretched across the audi- 

 tory meatus, by which the vibrations are received and transmitted, 

 through the chain of bones in the tympanum, to the labyrinth, or 

 internal ear ; a cavity in the petrous portion of the temporal bone, con- 

 taining various membranous sacs and canals, upon which are distributed 

 the filaments of the auditory nerve. 



Thus the terminal expansions of the auditory nerve, deeply concealed 

 in their bony cavities, and sustained by the surrounding fluids, while 

 protected from all other mechanical impressions, are so placed as to 

 receive the impulse of sound. 



External Ear. The external ear is a cartilaginous framework, 

 covered with integument, and more or less movable by various muscles, 

 which turn it in various directions. In man, these muscles are nearly 

 inactive ; though in exceptional cases they can produce a partial sliding 

 or rotatory movement of the ear. In most quadrupeds, on the other 

 hand, the movements are vigorous and extensive, and greatly aid in 

 the sense of hearing, by enabling the organ to catch distinctly the son- 

 orous vibrations, from whatever quarter they come. They also serve 

 to indicate the direction of a sound, since the animal ascertains, by 

 placing the ear in different positions, the region from which it is received 

 with greatest distinctness. 



Membrana Tympani and Chain of Bones. The membrana tympani 

 is a circular fibrous sheet not more than 0.05 millimetre in thickness, 

 but quite strong, consisting of circular and radiating tendinous fibres, 

 with a trace of intermingled elastic tissue. Its outer and inner surfaces 

 respectively are covered by thin continuations of the integument of the 

 external auditory meatus, and of the lining membrane of the tympanic 

 cavity; and the three layers combined form a membrane about 0.10 

 millimetre in thickness. 



