16 



Conduction and Coordination— 

 Nervous System 



All living matter is in a unique way excitable and responsive to changes 

 that go on inside and outside of it. Much has been learned about its aware- 

 ness and response, but a great amount remains to be discovered. Facts con- 

 cerning it that are clearly shown in laboratory observations and experiments 

 are hard to admit when they are met in the courtroom and the church. 



Through our nervous systems and sense organs we stand on the earth 

 and explore the universe, The light of the stars produces chemical changes 

 in the sensory cells of the eye; these start changes in the nerves and brain, 

 and we have ideas about the stars. We know a good deal about those chemical 

 processes, but of the making of the ideas we know almost nothing. 



Response and Conduction 



Touch an ameba at one point and a wave of motion sweeps over the animal 

 as it gradually draws away. But watch a smart dog pick up the sound of a 

 footstep, the scent of a rabbit! In the ameba the changes spread slowly 

 through generalized protoplasm; in the dog they are received, conducted, and 

 interpreted with great speed through the consummate performance of the 

 nervous system. 



Response. It would take an extraordinary light to excite the nerve cells 

 whose fibers compose the optic nerve. On the other hand, an unbelievably 

 faint one will stimulate the rod cells in the retina of a dark-adapted eye be- 

 cause they are specialized receptors of light. The light changes them and the 

 changes are communicated to the nerve cells. 



A receptor is a group of cells, one cell, or part of a cell that is particularly 

 sensitive to certain stimuli (Fig. 16.1). External receptors receive stimuli 

 from an animal's surroundings, temperature, light, sound, touch and others. 



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