A STUDY OF THE NERVOUS SYSTEM 265 



ical or mechanical action. We must therefore admit our 

 ignorance of the real nature of the nervous impulse that 

 passes along the axis cylinders ; nor do we know the real 

 nature of the changes that take place in the nerve cells 

 after receiving the so-called message. 



Reflex Action. To get an idea of the action of our own 

 complicated system of nerve cells and fibers, let us consider 

 the common experience of burning one's finger. If I acci- 

 dentally touch a hot stove, my hand is withdrawn instantly, 

 and afterward I feel the pain of the burn. This uncon- 

 scious and automatic withdrawal of the hand is called a 

 reflex action. We will now try to explain this 'action from 

 what we have learned of the structure of the spinal cord 

 and its nerves (see Fig. 117). 



By following the afferent fibers outward from the spinal 

 cord, we find that some of them terminate in the dermis 

 of the hand. When I touch the hot stove, these fiber termi- 

 nations in my hand are roused by the stimulus of the heat 

 into some kind of activity. The impulse thereby aroused 

 is conducted up my arm along the axis cylinders of the 

 afferent fibers, and soon reaches the cells in the ganglia of 

 the dorsal roots; thence it passes along the second axis 

 cylinder from each of these ganglion cells, and finally 

 reaches the terminal brushes in the gray matter of the 

 cord. From this region a stimulus is transmitted in two 

 directions. In the first place the cells in the ventral horn 

 are at once aroused, and a message is sent out along the 

 efferent fibers of the ventral roots to the muscles of my 

 arm. The muscles contract, and my hand is pulled away 

 from the hot stove. All the events we have been describ- 

 ing occur almost instantly and are carried on without any 

 action on the part of the brain. 



While the nerve impulse is being rushed from the cord 

 out to the muscles, a second message is hurrying through the 

 white matter of my spinal cord toward my brain. These 

 fibers finally terminate in the so-called sen'so-ry cells of the 



