100 PHYSICAL EXPKESSIOX. 



A further example of " retentiveness " to light 

 impressions is seen in similar repetitions of facial 

 expression, following the sight of the same object. 



One effect of light, then, is to excite, stimulate, or 

 stir up certain reflex actions. 



A second effect of light is to produce new, not 

 inborn, reflex actions, as indicated by the produc- 

 tion of retentiveness in the nerve-mechanism to the 

 effects of certain light impressions. 



A third effect of light is to control or modify 

 previously existing movements. Examples are 

 seen in the effect of the sight of a bright light, 

 or the sight of certain objects, in co-ordinating or 

 arresting spontaneous movements. 



A light, or the sight of an object, may inhibit 

 movements instead of increasing their amount and 

 frequency (see tracing, Fig. 9). 



The following phenomena may then be observed 

 as produced by the action of light on man : 



(1) Keflex action. 



(2) Transmitted effects. 



( (a) Excitant, 



(3) Action on spontaneous movements < (&) Inhibitory, 



( (c) Co-ordinating. 



(4) Retentiveness of the effects of light. 



(5) The evolution or building up of mentation. 



The action of light may be spoken of as extrinsic, 

 inasmuch as it is a stimulus originating out- 

 side the organism, and producing changes in its 

 structures. In contrast we may speak of the 

 " passions " and " feelings " as intrinsic stimuli, as 

 it may be supposed that they originate in some 

 part of the organism. 



