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ADVANCED PHYSIOLOGY 



-J_-. C 



jump from one of these fibres to the other through their 



tuft-like endings and then go to the brain. 



But there is another direction which the impulse from the 



skin may take after its arrival in 

 the cord. The branch in the 

 cord marked a in Figure 164 also 

 has side branches ending in arbo- 

 rations e. If the ingoing impulse 

 passes out into these, it may jump 

 across into the fibrils of another 

 neuron whose fibre,/, does not lead 

 to the brain at all, but passes out 

 through the anterior motor root 

 of the spinal nerve to some muscle. 

 The result will be movement at 

 the end of the motor fibres. From 

 the course which the impulse in 

 this last instance took, it is evi- 

 dent that the resulting motion 

 must have occurred without the 

 mediation of any conscious centers 

 in the brain, since only the cells of 

 the gray matter of the spinal cord 

 were concerned in the process. 

 This production of movement in 

 a muscle without the "consent" 

 of any conscious centers of the 

 brain is called a reflex action. In- 

 voluntary movements of both vol- 

 untary and involuntary muscle 

 occur very frequently in the body 



and play an important part in life processes. They will be 



considered at greater length a little later. 



From an examination of Figure 164 it is easy to see that 



two results might follow the arrival in the cord of an impulse 



FIG. 164. DIAGRAM 



Illustrating the course of messages 

 to and from the brain through 

 the spinal cord. 



