THE SPECIAL NERVOUS MECHANISMS 107 



exposed to stimulation by chemical substances floating in the 

 inspired air or dissolved in the liquids taken into the mouth, 

 and they are highly sensitive to such chemical stimuli. On the 

 other hand, they are not at all affected by light or sound vibra- 

 tions, and mere physical contact with them of some solid in- 

 soluble substance only evokes a sensation of touch. All sensory 

 surfaces are stimulated by electric discharges, as one may find 

 by placing the electrodes from a battery cell on the tongue, but 

 in such cases the sensation has not the quality of that which is 

 evoked by the special agency appropriate to the sense organ. 



A special sense organ consists, therefore, of the terminations 

 (dendrites) of a nerve cell. The axon of this nerve cell forms 

 one of the fibres in the sensory or afferent nerve which conveys 

 into the central nervous system the impulses generated by the 

 stimulation of the dendrites. As a rule the essential or special 

 part of the sense organ is not so simple as we have indicated. 

 Thus we have in the retina, or essential part of the organ of 

 vision, the following structures at least : 



To Obfic f 



\ ^ ~LT; ~\JntGr/or of 



Thickness cf the retina ^ 



Fig. 30. — A Diagram of the Nervous Elements of the Retina. 



The latter is supposed to be seen in section, the concave surface being 

 that receiving the light from the pupil and the convex surface being 

 that turned to the back of the eye. 



The actual nerve terminations that receive the rays of light 

 are the nerve cells, called the rods and cones (1). From these 

 elements axons go off and make synapses with the dendrites of 

 another nerve cell (2), which gives off an axon that enters into 

 a synapse with a third nerve cell (3). The axon of this is pro- 

 longed out into the optic nerve, and so passes up into the brain. 

 None of these nervous elements, or neurones, is in actual physical 

 contact with another. 



