THE SPECIAL SENSES 299 



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 communi- 

 cation between the middle ear and the external atmosphere, 

 and thus secure an equal pressure upon both sides of the mem- 

 brane of the drum of the ear, however the density of the atmos- 

 phere may vary. If, from undue tension of the membrane, 

 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. 



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 winding shape, has been termed 

 the labyrinth (see Fig. 70, 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 coch- 

 lea (6), 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 con- 

 forms to and partially fills that cavity, and which floats in it, 

 being both filled and surrounded with a clear, limpid fluid. 



101. A singular addition to the mechanism of hearing is 

 observed within this membranous bag of the labyrinth. This 

 consists of two small oval ear-stones, and a quantity of fine 

 powder of a calcareous nature, which is called "ear-sand." 

 When examined under the microscope, these sandy particles 



100. The essential part of the organ of hearing ? Its location ? Formation ? 



101. Where is the " ear-sand " found ? Give the theory as to its use. ' 



