44 JNU1BITIO\. 



ably very soon, to the left half of the cord, and thence up on 

 that side to the left half of the brain, rousing certain cells of 

 the cerebral convolutions to an activity that we call a sensa- 

 tion of touch. 



Voluntary Interference with Reflex Actions. We 

 have seen that the jerking of the hand away from a hot 

 object is due to reflex action of the spinal cord. But it is 

 possible, by act of .will, to prevent, to a certain extent, the 

 execution of reflex acts. One might, by a powerful effort of 

 the will, keep the hand on an object that is hot enough to 

 burn the skin. But we frequently see examples that are not 

 so extreme as this. One may command the foot to remain 

 quiet when it is tickled ; he may keep himself from scratch- 

 ing a part of the skin which is irritated ; but as soon as the 

 person is asleep the same stimulations would be followed by 

 the reflex actions such as we have considered. 



In these cases of interference it is understood that the 

 brain sends a nerve impulse down to the centers of the 

 reflex action, and stops or diminishes their operation. This 

 retarding influence of a group of cells is called Inhibition. 

 It is not always due to voluntary interference, but may be 

 due to reflex interference, as we may see later. 



The Nature of a Nervous Impulse. Of the nature of 

 a nerve impulse we know but little. It is convenient to com- 

 pare the nervous system, with its conducting fibers and cen- 

 tral ganglia, to a telegraph system. And electricity is the 

 most convenient stimulus for exciting nerve impulses. Yet 

 a nerve impulse is very different from an electric current. 

 A nerve fiber is a poor conductor of electricity. An electric 

 current may travel along a copper wire at the rate of between 

 100,000 and 200,000 miles a second, while a nerve impulse in 

 a motor nerve travels only 170 feet in a second. 



When a motor fiber is stimulated in the middle of its 



