ACTION OF LIGHT ON BRAIN. 91) 



of movements; this is the effect of the visual 

 stimulus. 



The sight of an object not only produces move- 

 ments of the eyes and head, but also causes 

 such movements in the upper extremity, as result 

 in the hand approaching and grasping the object 

 which reflects the beam of light and excites the 

 movements ; the movements spread to a wide area. 



The sight of an object that has many times been 

 seen, especially if some pleasure has accompanied 

 the sight, produces a repetition of movements ; the 

 reflex action recurs on the recurrence of the same 

 light stimulus. This indicates " retentiveness ; " it is 

 evidence of the fact that nerve-currents proceed 

 along certain paths when they are produced by a 

 similar stimulus. 



In all these phenomena a moving beam of light 

 produces more marked effect than a stationary 

 beam ; that is to say, a moving light, a moving 

 object, attracts the most attention. 



The brain of an imperfect subject may be so 

 constructed as not to give the phenomena described 

 above. The child may not be attracted by the 

 light ; it may, when six months old, not notice or 

 attempt to grasp an object presented. We see that 

 here the central nerve-mechanism is not duly sus- 

 ceptible to the reflex actions of light, and this 

 indicates that it is not normal in its construction. 

 These are some of the methods by which we judge 

 whether a child has such a nerve-mechanism as will 

 probably develop the clue or normal functions of 

 mentation in its after growth. 



