[Sidenote: 99. What other purpose does the Eustachian tube serve? How is this shown? "Throat-deafness?" Primary use of the Eustachian tube?] 99. The Eustachian tube serves, also, as an escape-pipe for the fluids which form within the middle ear; and hence, when its lining membrane becomes thickened, in consequence of a cold, or sore throat, and the passage is thus more or less choked up, the fluids are unable to escape as usual, and therefore accumulate within the ear. When this takes place, the vibrations of the membrane are interfered with; the sounds heard appear muffled and indistinct; and a temporary difficulty of hearing, which is known as "throat-deafness," is the result. This result resembles the effect produced by interrupting the vibrations of a sonorous body, such as all are familiar with; if the finger be placed upon a piano-string or bell when it is struck, the proper sound is no longer fully and clearly emitted. But the primary use of this tube is to afford a free communication between the middle ear and the external atmosphere, and thus secure an equal pressure upon both sides of the membrane of the drum of the ear, however the density of the atmosphere may vary. If, from undue tension of the membranes, pain is experienced in the ears, when ascending into a rare atmosphere, as in a balloon, or descending into a dense one, as in a diving-bell, it may be relieved by repeating the act of swallowing, from time to time, in order that the inner and outer pressure may thus be promptly equalized. [Sidenote: 100. The essential part of the organ of hearing? Its location? Formation?] 100. THE INTERNAL EAR, OR LABYRINTH.--The most essential part of the organ of hearing is the distribution of the _auditory nerve_. This is found within the cavity of the internal ear, which, from its exceedingly tortuous shape, {223} has been termed the _labyrinth_ (see Fig. 53, C). This cavity is hollowed out in dense bone, and consists of three parts; the _vestibule_ (_a_), or ante-chamber, which is connected with the other two; the _cochlea_ (_b_), or snail's shell; and the three _semicircular canals_ (_c_). The manner in which the nerve of hearing is distributed is remarkable, and is peculiar to this nerve. In the vestibule and the canals its fibres are spread out over the inner surface, not of the bony cavity but of a membranous bag, which conforms to and partially fills that cavity; and which floats in it, being both filled and surrounded with a clear, limpid fluid. [Sidenote: 101. Where is the "ear-sand" found? Give the theory as to its use.]