CHAPTER XXX. 



SPECIAL SENSES. 



It has been pointed out that the sensory nerves receive im- 

 pressions from without and carry the impulse thus excited more 

 or less directly to certain nerve cells in the brain, where it becomes 

 a sensation. The afferent nerves are, then, the means by which 

 the mind becomes acquainted with occurrences in the outer world, 

 as well as the channels along which a variety of stimuli pass to 

 nerve centres, whence they are reflected to different organs and 

 parts, without causing any definite sensation in the nerve cells of 

 the sensorium. 



The ordinary sensory nerves are brought into such relationship 

 to the surface that they are affected by slight mechanical stimuli, 

 which throw the nerve fibres into activity, and send impulses to 

 the brain. But we are capable of appreciating many other im- 

 pressions besides mechanical stimulation. We can distinguish 

 between degrees of heat and cold, when the difference is far too 

 slight to act as a direct nerve stimulus. We can appreciate light, 

 of which no degree of intensity is capable of exciting a nerve fibre 

 to its active state, or of stimulating an ordinary nerve cell in the 

 least degree. We recognize the delicate air vibrations called 

 sound, which would have no effect on an ordinary nerve ending. 

 We can also distinguish several tastes ; and, finally, we are con- 

 scious of the presence of incomprehensibly small quantities of 

 subtle odors floating in the air. When the amount of the sub- 

 stance is too small to be recognized even by spectrum analysis, 

 which detects extraordinarily minute quantities, we can perceive 

 an odor by our olfactory organs. 



There must then be a special apparatus for the reception of each 

 of these special impressions in order that the nervous system be 

 accessible to such slender influences. In fact, special mechanisms 

 must exist by means of which heat, light, sound, tastes, and odor 

 are enabled to act as nerve stimuli. These peculiar nerve termi- 



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