ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 459 



her that by delegating the control over these complicated 

 motions to the cerebellum the cerebrum is enabled to turn 

 its attention to higher functions. 



THE PHYSIOLOGY OF THE MID -BRAIN. 



In giving the functions of the mid-brain it is desirable 

 to include the optic thalami, for although these bodies are 

 anatomically classed with the cerebrum they belong physi- 

 ologically to the mid-brain. The crura cerebri of the mid- 

 brain are, of course, mere bands of fibers connecting spinal 

 cord and brain, so that we have to do here only with the 

 corpora quadrigemina of the dorsal side. 



The difficulty of experimenting on these structures, 

 since they are hard to reach without injuring other parts of 

 the brain, makes our knowledge somewhat fragmentary. 

 Enough evidence is, however, at hand to show that the 

 corpora quadrigemina, especially, are great reflex centers 

 between visual impressions and the motor impulses which 

 govern the movements of the eyeballs. Here, without the 

 intervention of consciousness, the visual sensations produce 

 reflexes, by means of which the eyeballs are turned as oc- 

 casion or necessity requires. Every one is aware that he 

 keeps his eyeballs in constant motion turning hither and 

 thither as one object after another arrests his attention, and 

 yet does all of this without any real conscious intervention. 

 It is only when special points come up demanding special 

 scrutiny that we become consciously aware and in a volun- 

 tary way direct the muscular movements of the eye. That 

 visual sensations figure so prominently in all our actions, 

 shows the necessity, or at least the great desirability, of 

 having a reflex center where these many visual sensations 

 may be properly interpreted and reflected in purposeful 

 motor impulses. 



But not only do the motor impulses of the eye originate 

 here but the optic thalami and mid-brain seem also to be 

 materially concerned in receiving the visual sensations and 

 reflecting them in purposeful locomotary impulses. It is 



