414 



THE NERVOUS SYSTEM 



contact with the terminal brush of the sensory fiber. The 

 axon of this nerve cell passes out through the anterior root 

 and beconaes one of the motor fibers of the spinal nerve. 

 It terminates in a muscle. 



If, now, the sensory end organ of such a system is stimu- 

 lated, as when the finger is brought in contact with a hot 

 stove, the impulse passes up over the axon or fiber to the 

 ganglion cell and through that is transmitted to the ter- 

 minal brush of the other axon in the cord. Prom this 

 brush it passes into the dendrites of the motor cell, and 

 this cell in consequence discharges an impulse over its axon 

 to the muscle and causes that to contract. 



In this simplest form of reflex action only two neura are 

 involved; one neuron conveying the sensory impulse and 

 the other the motor impulse. Most of our reflexes are 



more complex than 

 this, but their com- 

 plexity consists in the 

 fact that they involve 

 more than two neura 

 and not in any essen- 

 tial difference in ac- 

 tion. The manner in 

 which a single sensory 

 impiilse may stimulate 

 several motor fibers 

 is seen in Figure 195. 

 Here the sensory axon 

 splits up in the cord into collaterals, and each collateral dis- 

 charges through a different motor cell. Such an arrange- 

 ment explains how several muscles can be reflexly thrown 

 into action by a single sensory impulse. It must be noted, 



Fig. 195 — Diagram showing how one sensory 

 nerre may stimulate sereral motor nerves in 

 a simple reflex. 



