416 THE NERVOUS SYSTEM 



impulse is as follows: End organ to cord, cord to brain 

 centers by connecting neura, brain centers to other brain 

 centers by other connecting neura, and finally back from 

 the last brain center to the motor cell in the cord and over 

 its axon as a motor impulse to the muscles. In such a cir- 

 cuit it often happens that the cord motor cell is so near the 

 sensory fiber brush that a division of the discharge is made, 

 part of the impulse passing over the long circuit noted above, 

 while the other part passes directly over to the motor cell 

 and thus establishes a short-circuited reflex. In traveling 

 the long circuit to the brain no motor discharge can take 

 place until the impulse is retm-ned from that region. Such 

 a circuit is characteristic of all deliberate actions. In the 

 case of the division of the impulse, while part of the im- 

 pulse may continue on to the brain and produce conscious- 

 ness, the short circuited impulse is able to produce the 

 action perhaps even before consciousness is aroused. In 

 the case of the burned finger the importance of this latter 

 arrangement is evident in regard to speed of action. This 

 short circuiting also illustrates how a voluntary action may 

 become reflex, for if gradually the bulk of the discharge 

 follows the shorter circuit the less necessary becomes the 

 long circuits, and it may ultimately be abandoned entirely. 

 In general, then, a voluntary circuit differs from a reflex 

 circuit only in the length and in the number of neura 

 involved. The tracing of all these connecting neura and 

 their distribution in the central nervous system forms a 

 part of the study of the nervous system in which much 

 work still remains to be done. Almost nothing is known 

 as yet about the character of the changes which take place 

 in the brain cells and the manner in which past experiences 

 can be stored and modify the nervous discharges. By 



