308 THE NERVOUS SYSTEM 



first portions of the brain to receive visual impressions 

 through the optic tracts; but just what share they have in 

 vision is still not fully determined. We know that they are 

 concerned in the movements of the iris and of the ciliary 

 muscle, and there are indications that they have much 

 to do with the consciousness of light and color. It may 

 be that these and other neighboring ganglia, having re- 

 ceived direct impressions from the retina, originate reflex 

 movements without waiting for voluntary, conscious action 

 of the higher centers. This would explain the uncon- 

 scious sight and movements of the somnambulist, who 

 sees, without knowing it, how to direct his steps along 

 dangerous and intricate paths. . Destruction of the cor- 

 pora quadrigemina causes immediate blindness. 



449. Functions of the Optic Thalami. The ganglia of the 

 optic thalami also have something to do with sight, as 

 serious injury to them invariably results in disturbance 

 or destruction of vision. There is also evidence that 

 they play a subordinate part in connection with sensa- 

 tion, and, with the corpora striata and conducting fibers, 

 establish a shorter afferent-efferent circle which does not 

 involve the higher centers of the brain, but whose action 

 results in consciousness and volition. 



450. Functions of the Cerebrum. If a frog be entirely 

 deprived of the cerebral hemispheres and left quite undis- 

 turbed, it will sit quietly in the same spot forever. Though 

 it breathes, it will be an insensible, immovable lump of 

 matter. It may be surrounded by food, but it will die 

 of starvation. No spontaneous movement is possible to 

 it. But let the frog be touched, and it will move. If 

 gently stroked, it will croak. If its foot be pinched, it will 

 hop. If thrown into water, it will swim to land. A sud- 

 den movement close to its eyes will cause it to draw back. 



