CHAPTER XXIX. 

 THE SPECIAL SENSES. 



The sensory nerve terminations, or afferent receptors, that 

 are scattered over the skin are affected by stimuli which come in 

 actual contact with the surface of the body. In order that the 

 stimuli transmitted from a distance, such as those of light, sound 

 and smell, or the projicient sensations as they are called, may 

 be appreciated by the nervous system, specifically designed or- 

 gans, called the organs of special sense, are required. These 

 organs collect the stimuli in such a way as to cause them to act 

 effectively on receptors which have been especially adapted to 

 react to them. 



Although not really a projicient sensation, taste is conven- 

 iently considered along with the above. 



Vision. 



Light is due to vibration of the ethereal particles that oc- 

 cupy space. The vibrations occur at right angles to the rays 

 of light, and these travel at high velocity in straight lines from 

 the source of the light. The rate of vibration of the rays is not 

 always the same, and on this difference depends the color of the 

 light, red light vibrating much slower, and its waves being 

 accordingly much longer, than those of violet light. The termi- 

 nations of the optic nerve have been specially developed as the 

 retina, so as to receive the light waves. But in order that a 

 comprehensive picture of everything that is to be seen may be 

 projected on the retina, an optical apparatus, consisting of the 

 cornea and lens, is situated in front of it. The retina and the 

 optical apparatus are built into a globe the eyeball which, 

 pivoting on the attachment of the optic nerve, can be so moved 

 that images from different parts of the field of vision may be 



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