INTEGRATTVTE ACTION OP THE CEREBRUM 



953 



brum are differentiated from those of the loAver levels of the nervous 

 system. 



The relation which the distance receptors bears to the cerebral proc- 

 esses is well illustrated by a comparison of the responses of animals of 

 different kinds toward light. Many of the lower organisms possess eyes 

 incapable of forming an image such as is produced in the eye of the 

 vertebrate. Their eyes are so arranged as to be stimulated by light 

 coming only from a certain direction. Consequently the eye may form 

 a guide which determines the direction of progression, which will be 

 either toward or away from the light according to the kind of animal 

 which is studied. Such an animal obviously cannot act with much 

 discretion in regard to light from different sources, since the shape of the 

 source of light and perhaps the color of the light make no appeal to 

 it. The insects and Crustacea possess eyes which are capable of forming 

 an image and consequently might be capable of distinguishing between 

 the light of a candle and that from an open window. The development 

 of the nervous system of these animals has not kept pace, however, with 



Fig. 243. Postures assumed by the robber fly when the eyes are unequally illuminated: illustrat- 

 ing the influence of light on the tone of the muscles. A, the lower half of each eye is blackened; 

 B, the upper half of each eye is blackened; C, the right eye is blackened. (After Garry.) 



the optical perfection of the ocular mechanism. Each part of the retina 

 is connected in a rather inflexible way with certain parts of the motor 

 mechanism. Unequal stimulation of the different parts of the retina 

 causes certain parts of the musculature to become more active than other 

 parts in other words sets up a characteristic reflex figure (Fig. 243). 

 In the robber fly, for example, light falling on the upper half of the 

 retina, when the lower half is covered by opaque cement, causes the trunk 

 to be curved upward, the forelegs extended and the hind legs flexed. 

 On taking flight the insect tends to swerve upward and backward 

 and consequently loops the loop. Blinding the upper half of the retina 

 has the reverse effect. If one eye only is covered, the legs of that side 

 are extended, those of the opposite side flexed, and in walking the fly 

 tends to circle toward the side on which the eye is exposed to light. By 

 virtue of the physiological influence of the eyes on the muscles the 

 direction of locomotion of these insects is guided with mechanical pre- 



