REFLEX ACTION. 253 



ence in popular language, we may say that when the attention 

 of a segment of the cord, or its extension in the brain is taken 

 up by some other stimulus, a reflex already in action, or about 

 to act, is depressed. Pain, such for example as toothache, may 

 likewise be lessened by applying counter-irritation such as a 

 blister to some neighboring skin area. (3) By means of certain 

 drugs known as anesthetics, which depress the excitability of the 

 nerve cells. (4) By fatigue. 



The reflex movement may be increased: (1) by applying a 

 second stimulus to some other area of skin of the same hind leg 

 or by applying electrical stimulation to the central end of one 

 of its sensory nerves; (2) by raising the excitability of the 

 nerve centers by certain drugs, such as strychnine; (3) by first 

 of all causing the movement to disappear, though the stimulation 

 causing it is maintained, by exciting some other part of the 

 body (see above). When the reflex reappears it is much more 

 pronounced than formerly. 



Muscular Tone and Reciprocal Action of Muscles. Having 

 learned some of the general characeristics of the reflex move- 

 ments, we may now proceed to inquire into the method by which 

 the spinal cord is enabled, by itself, so to direct the afferent im- 

 pulses which enter it, that the nerve cells of the anterior horn 

 discharge suitable impulses to bring about such complicated 

 movements as have just been described. When a motor nerve 

 or an anterior spinal root is stimulated, the muscles which con- 

 tract are not grouped in such a way as to cause any purposeful 

 or co-ordinated movement. Contractors, extensors, adductors 

 and abductors are quite likely all to contract at once and by 

 thus opposing one another to effect no definite movement. When 

 such stimulation is extensive (e.g., involves a considerable num- 

 ber of motor fibers), it is common to find that the extensor 

 muscles predominate over the others, so that the limb becomes 

 extended. Such is the case when some poisonous substance 

 causes irritation of the nerve centers in the spinal cord. 



To cause a co-ordinated movement it is necessary that one 

 group of muscles should become relaxed whilst their antagonistic 

 group is undergoing contraction. Now, it might at first sight be 



