EXTERNAL STRUCTURE. ^ 



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

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

 propagated through 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 portion of the expansion of the nerve will be affected by it, which 

 would not be the case in a fluid so rare and 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 rumbling 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 intensely through water than through air; and therefore an aque- 

 ous .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 membrane, 

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

 pulp. The posterior one is a perfeqt semi-circular canal, with two openings 

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

 course, and have one common opening, and one peculiar to each ; so thai 

 these canals open into the vestibule by five apertures. 



These canals contain a singular mechanism. In the part of the vestibule 

 at the openings 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 pTincipally distributed. 

 The membrane composing these bags is exceedingly thin. Thus floating in 

 the fluid of the canals, and richly supplied with nervous matter, the slight- 

 est vibration or motion communicated to ihe 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 stair-case 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 



