418 ORGANS OF THE SENSES. 



of the membrane serves to diminish the effect of the sound 

 upon it (ns the more a membrane is stretched, the less full 

 will be the vibrations), and to soften certain disagreeable 

 sounds. On the other hand, this tension makes the mem- 

 brane vibrate more readily to acute sounds; and to hear 

 these demands very close attention (the greater the tension 

 of a membrane, the more numerous its vibrations). 



Next to the membrane of the tympanum comes the chain 

 of small bones, connecting it with the membrane of the oval 

 fenestra (base of the stapes). In the inferior animals, this 

 chain simply consists of a straight and rigid stalk (as found 

 in some anourous amphibians, ihepipa, for instance) ; in frogs 

 it takes the form of a broken line, a small bone, long and 

 curved, and which is called the columella ; in man, finally, it 

 is formed by the junction of three small bones or ossicles (the 

 malleus or hammer, the incus or anvil, and the stapes) : these 

 bones are articulated, but, in regard to the transmission of 

 sound, they may be considered as anchylosed, it having been 

 proved that these articulations do not directly operate lor the 

 transmission of sounds. 



The chain of ossicles through which the sound waves 

 chiefly pass, crosses a drum filled with air, called the tym- 

 panic cavity; it is flattened from without inwards, and, like 

 the membrana tympani, exhibits a plane oblique to the exter- 

 nal auditory canal. It is supposed that the sound waves are 

 not only transmitted by this jointed connecting-rod of bones, 

 but that the air in the drum also serves to convey them to the 

 fenestra rotunda ; this is possible, but not very probable, and 

 at all events, this mode of progression must be only secondary, 

 for the round fenestra avoids the sound waves, as it were, 

 being hidden beneath the promontory (tuber cochlea3, or 

 projection outwards of the first turn of the cochlea directly 

 opposite the protuberance of the membrana tympani) ; the 

 correspondence of this round fenestra with one of the open- 

 ings of the cochlea, which communicates on the other side 

 with the vestibule, appears to be intended for the purpose of 

 giving free play to the fluid waves which pass through this 

 complicated organ. Finally, since sound is more readily 

 transmitted through solids than through fluids, the chain of 

 small bones must have a much more important office to fulfil 

 than the air, which latter,, no doubt, serves only as an insu- 

 lating apparatus. The membrana tympani, and these ossi- 

 cles, with the exception of the stapes, may be destroyed with- 

 out the complete abolition of hearing; it will only be more 



