CHAPTER III 

 THE ELEMENTS OF NERVE PHYSIOLOGY 



IT was impossible to discuss the structure of the nervous 

 tissues without anticipating to some extent an exposition 

 of their working. We can now proceed further with this 

 subject. We have chosen the word "coordination" to 

 express the broad function of the system, and we have 

 siid that the chief property of its elements is conductivity. 

 The simplest statement of the duty discharged has been 

 said to be something like this: The receptors lie exposed 

 to external stimulation, thence the effects of this stimula- 

 tion are conveyed to the adjustors in the central axi-. 

 the adjusters distribute to efferent neurons, and by these 

 the responses of muscles and glands are secured. It is imw 

 time to inquire into the nature of the external stimuli and 

 the transmitted energy. 



Stimuli. A stimulus is a change, physical or chemical, 

 which is capable of producing physiologic reactions. Rais- 

 ing the temperature of a selected area of the skin is a 

 means of stimulation. Lowering the temperature may 

 have equally positive re.-ults. The application of press- 

 ure at any spot whence it may be conducted to underly- 

 ing receptor- i- an instance of mechanical stimulation. 

 There are in the surface of the tongue receptors which 

 can be excited by various dissolved substances. Here 

 \ve have an example of chemical stimulation, \\hen the 

 retina i- aeted on by light and impulses (low along the 

 optic nerve to the brain there are probably two steps in 

 the process: first, the light causes a chemical change, as 

 it does in the photographic plate; >econd. some product 

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