96 PHYSICAL EXPRESSION. 



p. 101). The action of light on vegetable organisms, 

 and its effects in producing movements of various 

 kinds, have also been carefully studied. I have, 

 therefore, put together the following notes on the 

 effects of light. 



In such a subject as the present, as in so many 

 other questions concerning the action of physical 

 forces upon living organisms, it seems to me use- 

 ful to consider the effects of light upon the simple 

 cellular vegetable organisms, where these changes 

 can be studied with much exactness, as well as in 

 animals. 



The action of light may be roughly described as 

 trophic or kinetic; that is, mainly producing growth, 

 or mainly causing movements and not growth. 



Take a sample of the human species at the 

 earliest period of possible observation, directly after 

 birth. If we examine a new-born infant, we find 

 that when a light shines upon its face it screws up 

 its eyes (i.e. the orbiculares oculi muscles contract 

 strongly), and it corrugates the forehead (i.e. corru- 

 gators contract). If the eyelids be opened, the iris 

 contracts to light ; in some cases, when the infant 

 is a few days old, there seems to be a tendency to 

 turn the head from the light, i.e. the skull rotates 

 from a lateral light. 



The action of light upon the iris needs only to be 

 mentioned. It is commonly admitted that this is a 

 purely reflex act in which the optic is the afferent 

 nerve ; the third, or motor-oculi, the efferent nerve ; 

 and the centre is some portion of the corpora 

 quadrigemina. 



