SENSE OF HEARING. 541 



heard in the neighbourhood of these bodies, than it would otherwise 

 have been. 



III. The sonorous vibrations are much more weakened in the trans- 

 mission of solids to air ; and those of air make but little impression on 

 solids. 



IV, Sonorous vibrations in water are transmitted but feebly to air ; 

 and those which are taking place in air are with difficulty communicated 

 to water ; but the communication is rendered more easy by the inter- 

 vention of a membrane extended between them. 



.The application of these conclusions, in the Physiology of Hearing, 

 will be presently apparent. 



950. It is on the Auditory nerve (commonly termed the Portio 

 Mollis of the 7th pair), that the -sonorous undulations make their im- 

 pression ; but we invariably find, that this impression is made through 

 the medium of a liquid, contained in a cavity, on the walls of which 

 the ultimate branches of this nerve are distributed. The simplest form 

 of the organ of Hearing, such as we find in Cephalopoda and in certain 

 Fishes, consists merely of a cavity excavated in the solid framework 

 of the head ; which cavity is filled with liquid, and lined by a membrane 

 on which the auditory nerve is distributed. These animals are inhabi- 

 tants of the water ; and the sonorous vibrations excited in this medium, 

 being communicated to the solid parts of the head, will be by them 

 again transmitted to the contained fluid, without much diminution of 

 their intensity ; according to principles I. and II. In certain Crustacea, 

 however, whose organ of hearing is contained in the base of the anten- 

 nae, as well as in most Fishes, we find the auditory cavity or vestibule 

 no longer entirely closed ; but having an aperture on its external ' side, 

 which is covered in by a membrane. Here the vibrations of the liquid 

 within the cavity will be more directly excited by those of the sur- 

 rounding medium, for if this be water, it will propagate its undulations 

 into the cavity, with little interruption from the membrane stretched 

 across its mouth ; whilst, if it be air, the interposition of this very mem- 

 brane will greatly assist in the transmission of the vibrations to the 

 water of the auditory cavity, according to principle IV. In most of the 

 animals which have the- organ of hearing constructed upon this simple 

 plan, the force of the vibrations of the fluid within the cavity is increased 

 by several minute stony concretions (termed otolithes\ which are sus- 

 pended in it. These act according to principle II. Some traces of 

 them are found in the higher animals ; in which they are for the most 

 part superseded, however, by an apparatus better adapted to augment 

 the intensity of the sonorous vibrations. 



951. This apparatus consists, in all Vertebrated animals above the 

 inferior Reptiles, of the tympanum or drum, with its membrane' and 

 chain of bones ; together with, in the Mammalia, the external ear ; 

 which is adapted to direct itself, more or less completely, towards the 

 point from which the sonorous vibrations proceed, and to give them a 

 degree of preliminary concentration. The tympanic apparatus is 

 interposed between the external ear and the membrane covering the 

 foramen ovale, which is the entrance to the real auditory cavity ; and 

 its purpose is evidently, to receive the sonorous vibrations from the air, 



