TRANSMISSION OF SONOROUS VIBRATIONS. 403 
munication is rendered much more easy by the intervention 
of a membrane extended between them. 
512 . The Auditory nerve, or nerve of Hearing, is adapted 
to receive and transmit to the brain the sonorous undulations 
produced in the surrounding medium by vibrating bodies. 
How, it is obvious that it may be affected by these in various 
ways, especially in animals that inhabit the water. The 
vibrations excited in the liquid will be transmitted to the 
solid parts of the head, and thence to the nerve contained in 
it, without much interruption; and this independently of any 
special apparatus of hearing. Indeed, the simplest form of 
this apparatus is only designed to give increased effect to the 
vibrations thus excited in the solid parts of the head; for it 
consists merely of a cavity excavated in their thickness, which 
cavity is filled with fluid, and is lined by a membrane whereon 
the auditory nerve is minutely distributed. This is the con¬ 
dition of the organ of hearing in the Mollusca, where any such 
exists , and also in many of the Crustacea. In those of the 
latter class which chiefly inhabit air, however, this cavity is 
excavated in the surface of the shell covering the head, and is 
shut-in by a membrane which is exposed to the surrounding 
medium. According to the principle (iv.) mentioned in the 
last paragraph, the liquid contained in the chamber will be 
thrown into undulation by vibrations in air, as well as by 
those of water; so that those animals which possess this kind 
of apparatus are able to hear much better in air, than are those 
in which the cavity is completely shut-in by stony walls. Of 
the degree in which sonorous vibrations may be communicated 
to our own auditory nerves through the solid parts of the 
skull, we may easily satisfy ourselves by closing the ears 
carefully, and placing any part of the head against a solid 
body which communicates with the one in vibration. In this 
manner we may hear the sounds produced by the latter with 
considerable distinctness, though accompanied by an unpleasant 
jarring . A deaf gentleman was once agreeably surprised 
to find that, when smoking his pipe, with the bowl resting on 
his daughter’s pianoforte, he could distinctly hear the music 
she was producing from it; and many deaf persons may be 
made to hear conversation, by holding a piece of stick 
between their own teeth, and placing it against the teeth of 
the person speaking. 
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