EXTERNAL STRUCTURE. 83 



tion reaches the oval window, trebled in intensity. The same object is 

 pursued within the labyrinth. A liquid is placed there, because sound is 

 propag-ated throug-h it, with greater rapidity. While sound travels through 

 air at the rate of 1132 feet in a second, it passes through water at the rate 

 of more than 4000 feet in the same time. The impulse communicated to the 

 water by the membrane, is thus more suddenly spread over the whole of the 

 labyrinth. There is, besides, a law regulating the pressure of fluids, by which 

 this impulse must be spread over the whole of the labyrinth, and every por- 

 tion of the expansion of the nerve will be affected by it, which would not 

 be the case in a fluid so rare and so expansible as air. 



The strongest reason, however, remains to be stated, — the impression or 

 vibration is rendered more intense, by travelling through water. That sound 

 which would scarcely be heard in the air, is almost deafening under water. 

 It is a common practice for boys when they bathe, to dive with a stone in 

 each hand, and the rubbing of them together under water produces a rum- 

 bhng sound of extraordinary loudness. This is contrary to the old opinion ; 

 and even philosophers, of no mean repute, have denied that fishes had ears, 

 because they were placed in a medium, through which sound could scarcely 

 be conveyed, and where their ears would be of little use to them. Later 

 and better observers have proved that sound is propagated far more in- 

 tensely through water, than through air ; and therefore, an aqueous fluid 

 occupies those chambers of the ear, on the walls of which the auditory nerve 

 is expanded. 



The oval window opens into the labyrinth, which is divided into three 

 compartments. First is the vestibule, h, the hall of entrance, not more 

 than a quarter of an inch wide in the actual subject, but magnified in our 

 cut, for the purpose of illustration. Over the whole of the membrane by 

 which it is lined, there are spread expansions of the soft portion of the 

 seventh pair of nerves. 



On the upper side are several foramina or holes, k, which conduct to 

 the semi-circular canals, i, containing also water, lined by the same mem- 

 brane, and that membrane likewise covered, although not so thickly, with 

 nervous pulp. The posterior one is a perfectly semi-circular canal, with two 

 openings into the vestibule. The other two run into each other in a part 

 of their course, and ha^e one ct iimon opening, and one peculiar to each ; 

 so that these canals open into the vestibule by five apertures. 



These canals contain a singular mechanism. In the part of the vesti- 

 bule at the opening of the canals, k, is suspended a little bag filled with a 

 very clear fluid, and from which branches go into, and occupy the canals, 

 not filling them, but floating in the fluid which they contain ; and on these 

 bags the portion of the nerve belonging to the canal is principally distri- 

 buted. The membrane composing these bags is exceedingly thin. Thus 

 floating in the fluid of the canals, and richly supplied with nervous matter, 

 the slightest vibration or motion communicated to the fluid, by the stirrup 

 on the oval window, will be immediately and powerfully felt. 



On the other side is, if possible, a more complex mechanism. At m is 

 the cochlea, so termed from its likeness to the convolutions of a shell. It, 

 however, more resembles a spiral lamina, or narrow and thin plate, partly 

 bony, and partly membranous, running round a column in the centre. It 

 is a spiral staircase in a round tower. The base of it rests on the internal 

 passage, n, through which both portions of the seventh pair of nerves pass 

 into the ear. Its apex, or top, approaches the Eustachian tube, o. The 

 soft, or auditory portion of the nerve, penetrates through the cribriform or 

 sieve-like termination of the passage, and a part of it runs up the central 



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