524 ELEMENTARY PHYSIOLOGY 



15. The Functions of the Cerebral Hemispheres.— 



The Hemispheres the Seat of Intelligence and W^ill. — 



The functions of most of the parts of the brain wliich lie 

 in front of the spinal bulb are, at present, very ill under- 

 stood ; but it is certain that extensive injury, or removal, 

 of the cerebral hemispheres puts an end to intelligence 

 and voluntai-y movement, and leaves the animal in the 

 condition of a machine, working by the reflex action of 

 the remainder of the cerebro-spinal axis. 



We have seen that in the frog the movements of the 

 body which the spinal cord alone, in the absence of the 

 whole of the brain, including the bulb, is capable of 

 executing, are of themselves strikingly complex and 

 varied. But none of these movements arise from changes 

 originating within the organism, they are not what are 

 called voluntary or spontaneous movements ; tliey never 

 occur unless the animal be stimulated from without. 

 Removal of the cerebral hemispheres is alone sufficient to 

 deprive the frog of all spontaneous or voluntary move- 

 ments ; but the presence of the bulb and other parts of 

 the brain (such as the corpora quadrigemina, or what 

 corresponds to them in the frog, and the cerebellum) 

 renders the animal master of movements of a far higher 

 nature than when the spinal cord only is left. In the 

 latter case the animal does not breathe when left to itself, 

 lies flat on the table with its fore-limbs beneath it in an 

 unnatural position ; when irritated kicks out its legs, and 

 may be thrown into actual convulsions, but never jumps 

 from place to place ; when thrown into a basin of water 

 falls to the bottom like a lump of lead, and when placed 

 on its back will remain so, without making any effort to 

 turn over. In the former case the animal sits on the 

 table, resting on its front limbs, in the position natural to 

 a frog ; breathes quite naturally ; when pricked behind 

 jumps away, often getting over a considerable distance ; 

 when thrown into water begins at once to swim, and con- 

 tinues swimming until it finds some object on which it 



