TRANSMISSION OP SONOROUS VIBRATIONS. 403 



munication is rendered muck 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. 

 Now, 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 j 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. 



DD 2 



