THE SPECIAL SENSES 



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of hearing are located in a bony chamber, half the size of the 

 finger tip, in each side of the head. This chamber is filled 

 with a fluid into which certain delicate filaments from the 

 nerves project, and, when the fluid is caused to vibrate, the 

 filaments transmit sensations of sound to the brain. A narrow 

 passage leads from the outside to this chamber and at its inner 

 end is closed by a membrane commonly known as the ear 

 drum. When vibrations fall on the ear drum, three tiny bones 

 carry them across the short 

 space between the ear drum 

 and the bony chamber and 

 cause vibrations in its con- 

 tents. A tiny tube, the 

 eustachian tube, leads from 

 the throat to the inner side 

 of each ear drum and serves 

 to equalize the pressure of 

 the air on the two sides of 

 the membrane. The eyes, 

 the only two parts of the 

 body that are sensitive to 

 light rays, function like a set 

 of lenses to focus the light on 

 an inner sensitive part, the 

 retina, from which sensations of sight go to the brain. The 

 interior of the eye is lined with a tissue containing pigment 

 which absorbs such light rays as do not fall upon the retina. 

 In the front of the eye, this pigment is visible and forms the 

 iris. The dark spot or pupil of the eye is really an opening 

 through the iris, and the iris itself is able to contract or expand 

 and thus modify the amount of light admitted. Behind the 

 iris, an organ called the crystalline lens aids in focussing the 

 light rays. It is unable to move forward and back as camera 

 lenses do, but accomplishes the same end by becoming thicker 



FIG. 95. The ear. 



