xvn THE NERVOUS SYSTKM 193 



or loss of voluntary movement, and loss of sensation on the 

 opposite side of the body. 



Functions of the Cerebral Hemispheres. The 

 cerebral hemispheres are the seat of the perceptions, of the 

 intelligence, and of the will. A frog which has been deprived 

 of the cerebral hemispheres only, the other parts of the brain 

 having been left, is like a frog deprived of its whole brain in 

 so far as it feels nothing and performs no voluntary movements, 

 but the two differ in important respects. The frog, the whole 

 of the brain of which has been removed, lies flat and flaccid in 

 whatever position it is placed and does not breathe. The frog 

 deprived of its cerebral hemispheres only sits up in the attitude 

 of an ordinary frog and goes on breathing. A slight touch 

 causes it to move, so also does a flash of light, provided that 

 the optic nerves have not been injured, and even placing it on 

 its back or in any unusual attitude is a sufficient stimulus to 

 make it move to its natural sitting posture. If it is put into 

 water, it will start swimming, and go on till it gets out of the 

 water, if this is possible, and then it will come to rest and 

 remain at rest till disturbed again. From these observations 

 it is clear that while the cerebral hemispheres are necessary 

 for originating voluntary movements, the other parts of the 

 brain can give rise to complicated and well-balanced move- 

 ments, movements apparently as perfect as those the ordinary 

 animal can perform, provided that afferent impulses, even 

 slight impulses, are received. 



The Sympathetic Nervous System. The spinal 

 nerves on each side of the body, soon after they pass outside the 

 vertebral column, give off a small short branch which goes 

 to a row of ganglia lying on each side of the front of the 

 vertebral column. Each ganglion is connected by nerve fibres 

 to the ganglia above and below it, and so a chain of ganglia, 

 called the sympathetic chain, is formed on each side, ex- 

 tending all the way from the base of the skull to the coccyx. 

 There is in the thoracic and lumbar regions a ganglion of 

 each chain corresponding with great regularity to each spinal 

 nerve, but in the cervical region many of them appear to be 

 missing. From the sympathetic chain on each side nerve 

 fibres pass in great numbers to the viscera of the abdomen 

 and thorax. From them nerves are also given off which pass 



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