CHAPTER XVII. 



GENERAL CONSIDERATIONS CONCERNING THE NERVOUS 



SYSTEM. 



Nerve Stimuli. Natural nerve impulses that run out- 

 ward are ordinarily started by the action of some nerve 

 cell or cells, as from the gray matter of the brain or of 

 the spinal cord. 



Nerve impulses coming inward may be started in sev- 

 eral ways. Ordinarily by some one of a few forces that 

 are capable of affecting the nerve endings. Mechanical 

 force, as pressure, acts on the nerve endings of the skin, 

 and starts nerve impulses which are carried to the brain 

 and rouse certain cells to activity, and give us the sensa- 

 tion of touch. The vibrations known as light excite the 

 special nerve endings in the retina, but affect no other 

 nerve endings. Sound is appreciated only by the endings 

 of the auditory nerve. Certain gases or fine particles 

 affect the olfactory nerve endings, and certain substances 

 may give the sense of taste by acting on the ends of nerves 

 in the mouth. Different nerves, then, are adapted to re- 

 ceiving impressions from the action of different forces. 



Kinds of Nerve Stimuli. There are four kinds of 

 nerve stimuli, electrical, mechanical, thermal, and chemi- 

 cal. In experiment, electricity is usually the best stimulus ; 

 mechanical stimuli, as used in the experiments with the 

 muscle-nerve preparation from the frog, by cutting or 

 pinching the nerve, may be employed ; heat, as in touch- 



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