THE CONQUEST OF NATURE 



is a centre in the brain scarcely larger than the head 

 of a pin, the destruction of which will cause the heart 

 instantly to cease beating forever. From this centre, 

 then, and from the other centres of the brain, impulses 

 are constantly sent to the involuntary muscles, which 

 determine the rate of activity. Nor are these centres 

 absolutely independent of the seat of consciousness, as 

 anyone will admit who recalls the varied changes in 

 the heart's action under stress of varying emotions. 



That the voluntary muscles are controlled by the 

 central nervous mechanism needs no proof beyond 

 the appeal to our personal experiences of every moment. 

 You desire some object that lies on the table in front of 

 you, and immediately your hand, thanks to the elaborate 

 muscular mechanism, reaches out and grasps it. And 

 this act is but typical of the thousand activities that 

 make up our e very-day life. Everyone is aware that 

 the channel of communication between the brain and 

 the muscular system is found in a system of nerves, 

 which it is natural now-a-days to liken to a system of 

 telegraph wires. We speak of the impulse generated 

 in the brain as being transmitted along the nerves to 

 the muscle, causing that to contract. We are even able 

 to measure the speed of transfer of such an impulse. 

 It is found to move with relative slowness, compassing 

 only about one hundred and twelve feet per second, 

 being in this regard very unlike the electric current with 

 which it is so often compared. But the precise nature 

 of this impulse is unknown. Its effect, however, is 

 made tangible in the muscular contraction which it 

 is its sole purpose to produce. The essential influence 



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