moveable bridge of the monochord, adjusting it to the vibrations re- 

 quired 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 mus- 

 cular 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 im- 

 pressions 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 ca- 

 pable 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 as- 

 sertion. 



B -2 



