moveable bridge of the monochord, adjusting it to the vibrations required to be produced. Thus the combined effects of the action of these muscles give the perceptions of grave and acute tones; and in proportion as their original conformation is more or less perfect, so will their action be, and consequently the perceptions of sound which they communicate.
This mode by which the membrana tympani is capable of being adjusted to certain tones, or rather musical keys, will it is thought fully account for the difference between a musical ear, and one which is too imperfect to discriminate different notes with any degree of nicety. This delicacy of the ear, as it is found to depend on muscular action, may therefore be in some measure acquired, and is likewise liable to be impaired by illness or other accidental causes, of which some striking instances are here related.
In endeavouring to explain the uses of the more internal parts of the ear, considerable advantage, it is thought, may be derived from classing them in two divisions, namely, those which are formed for the purpose of receiving impressions conveyed through the medium of liquids or of solid substances; and those adapted to receive impressions made by the impulses of an elastic fluid such as common air. The former are the ears of fish, which are found to have fewer parts than those of birds, quadrupeds and man; but in the latter we find that the organ is susceptible of impressions by both vehicles. Thus men can hear the ticking of a watch by applying it to the forehead, and shutting the ears: the sound in this instance being evidently conducted through the bones of the skull, it appears manifest that only the interior parts of the ear, namely, the vestibulum and semi-circular canals, co-operate to produce this sensation; and these in fact are the principal parts of that organ in fish.
In birds the membrana tympani has no tensor muscle to vary its adjustment, and hence their scale of sounds cannot descend so low as in the human ear. The cochlea, which has hitherto been considered as the part of the organ by which sounds are modulated, is also wanting in birds, which, however, are known to have a singular nicety in discriminating inarticulate sounds; and hence, as well as on account of its being filled with water instead of air, which renders it less capable of modifying sounds, it is manifest that this is not the real use for which the cochlea is destined. What is its precise use, as well as of the semicircular canals, remains yet to be investigated.
Lastly, it is observed that in the elephant there is no bony septum separating the cells of the skull belonging to one ear from those which open into the other, but a free communication exists between them: from this, the enlarged proportions of the organ and some other circumstances here mentioned, it is inferred that the sense of hearing must be quicker in this than in any other animal. And in fact some curious instances are mentioned Which seem fully to confirm this assertion.
