NERVOUS IMPULSES 227 



rons. It also contains still other neurons which are 

 distributed up and down its length. The latter serve to 

 connect the more distant sensory and motor neurons 

 with those which lie in the upper portion of the spinal 

 cord and in the brain. In this way, both sensory and 

 motor nervous impulses are carried to and from the 

 great centre of nervous activity, the brain. 



Motor impulses from sensory. The spinal cord 

 forms a distinct portion of the nervous system which 

 is responsible for certain activities. If, for example, 

 we accidentally touch a hot object, the irritation of the 

 ends of the nerves in the skin starts nervous impulses, 

 which pass inward along the sensory fibres, and thence 

 along the dorsal root of the spinal cord into the branches 

 of the neurons which lie in the cord itself. They then 

 pass along these branches to their many-branched tips, 

 where they come into close relation with many motor 

 cells, which they arouse to activity. The impulses started 

 in the motor cells are then transmitted outward along 

 the motor fibres to the groups of muscle cells controlled 

 by them. As a result, the muscles are caused to contract 

 and the hand is snatched away from the hot object. 



Reflex action. This kind of nerve activity is called 

 reflex action, and takes place when the sensory impulses 

 are so strong that on their way to the brain they excite 

 the motor nerves in the spinal cord which lie within 

 reach of their branches. Reflex action is involuntary, 

 for the reason that it takes place before the brain has 

 time to know about it and to aid or interfere with it. 

 It is so rapid that the time which elapses between the 

 touching of the skin with a hot object, for example, and 

 its withdrawal is only about six- or seven-hundredths 

 of a second. If touching the object had not been pain- 

 ful and the -reaction had not been reflex, the time taken 



