30 THE MECHANISM OF LIFE 



the environment acts upon a receptor organ by stimulating the 

 proximal dendrite of a nerve cell, and thus setting up a nervous 

 impulse which travels up the axon of this cell into a nerve centre, 

 or nucleus, or ganglion, in the brain or spinal cord. The axon 

 ends in the centre as a series of distal dendrites, which form a 

 synapse with the proximal dendrites of another neurone. The 

 axon of the latter leaves the brain or spinal cord in an efferent 

 nerve, which goes to the muscles concerned. The impulse 

 travelling out is distributed to the muscle fibres by the distal 

 dendrites of the final neurone, and, entering the muscle, it releases 

 energy and causes the latter to contract or relax, thus causing 

 the movement. 



This, it must be remembered, is only a scheme of the mechan- 

 ism, and we elaborate it in Chapters VI. to VIII. Meanwhile, it 

 will suffice to give the reader a preliminary notion of what are 

 the essential activities of the sensori-motor system. 



