228 



THE NERVOUS SYSTEM 



to withdraw the hand under the direction of the brain, 

 that is voluntarily, would have been from twelve- to 

 eighteen-hundredths of a second. 



Motor nerve cells 

 of anterior horn 



Sensory cell 



of ganglion 



of dorsal root 



Connecting 

 branches of 

 sensory cell 



Posterior horn 



Branches of 



sensory cell 

 Intermediate cell 



msory nerve 

 cell (ganglion 

 dorsal root) 

 Branches of 

 intermediate cl 



FIG. 122. Diagram of the spinal cord, showing the mechanism of reflex action : A, 

 shows how a sensory cell, by means of its branches, may influence many motor 

 cells directly; B, how an intermediate cell, may serve to connect a sensory cell to 

 motor cells, thus making possible more complicated reactions. (After Ko'lliker.) 



Its characteristics. As we study reflex action in its 

 control of muscles, we find that the movements are in 

 general planned to take the part of the body threatened 

 away from danger. For example, the hand is never 

 thrust against the hot object, but always definitely and 

 accurately removed from it. This is true regardless of 

 whether the object is under, above, or on either side 

 of the hand. This means that each sensory nerve is 

 connected by its branches with the particular motor 

 nerves which will cause the proper protective movement 

 and with no others. Thus there is never an opportu- 

 nity for an irritation to start the wrong movement, but 



