250 



so as to admit a greater or smaller number of its rays, according to the 

 impression which they produce, 59 by the relaxation or tension of the 

 membrane of the tympanum, or of tlje fenestra ovalis, the ear reduces or 

 increases the strength of sounds whose violence would affect its sensibili- 

 ty, in a painful manner, or whose impression would be insufficient. The 

 iris and the muscles of the stapes and of the malleus are, therefore, 

 the regulators of the auditory and visual impression ; there is as close a 

 sympathetic connexion between these muscles and the auditory nerve, 

 as between the iris and the retina*. The air which fills the tympanum 

 is the true vehicle of sound ; this air diffuses itself over themastoid cells, 

 the use of which is to augment the dimensions of the tympanum, and the 

 force and extent of the vibrations which the air experiences within it. 



These vibrations transmitted by the membrana tympani, are commu- 

 nicated to those membranes which cover the fenestra ovalis, and the fe- 

 nestra rotunda, then, by means of these, to the fluid which fills the dif- 

 ferent cavities of the internal ear, and in which lie the soft and delicate 

 filaments of the auditory nerve or of the portio mollis of the seventh 

 pair. 



The agitation of the fluid affects these nerves and determines the sen- 

 sation of grave or acute sounds, according as they are slower or more 

 rapid. It appears that the diversity of sounds should rather be attribu- 

 ted to the more or less rapid oscillations, and to the undulations of the 

 lymph of Cotunni, than to the impressions on filaments of different 

 lengths, of the auditory nerve. These nervous filaments are too soft and 

 too slender to be traced to their extreme terminations. It is, however, 

 probable, that the various forms of the internal e ar (the semi-circular canals, 

 the "vestibule^ and the cochlea^) have something to do with the diversity of 

 sounds. It must also be observed, th&t the cavities of the ear are con- 

 tained in a bony part, harder than any other substance of the same kind, 

 and well fitted to maintain, or even to augment by the reaction of which 

 it is capable, the force of the sonorous rays. 



The essential part of the organ of hearing, that which appears exclu- 

 sively employed in receiving the sensations of sound is, doubtless, that 

 which exists in all animals endowed with the faculty of hearing. This 

 part is the soft pulp of the auditory nerve, floating in the midst of a ge- 

 latinous fluid, contained in a thin and elastic membranous cavity. It is 

 found in all animals, from man to the sepice. In no animal lower in the 

 scale of animation, has an organ of hearing been met with, although some 

 of these inferior animals do not seem to be absolutely destitute of that 

 organ. This gelatinous pulp is, in the lobster, contained in a hard and horny 

 covering. In animals of a higher order, its internal part is divided into 

 various bony cavities, in birds there is interposed a cavity between that 

 which contains -the nerve of hearing and the outer part of the headf, in 

 man and in quadrupeds, the organ of hearing is very complicated ; it is 

 enclosed in an osseous cavity, extremely hard, situated at a considerable 



* It may be added, that this connexion is formed very nearly in the same way by 

 means of the retrograde branch of the fi i'th pair passing through the internal ear, as the 

 supra orbitar ID ranch through the orbit, to assist in supplying the iris. Guelman. 



\ In some birds, especially the crow, magpie, and raven, the semicircular canals are 

 remarkably large, and cross each other in a very singular manner, as I have frequently 

 observed. In birds of the goose or duck tribe they are small and less peculiar, whence 

 it may be inferred that in birds the acuteness of hearing is very much dependent on the 

 size of these canals. Gasman. 



