46 TIIK XKHVOUS SYSTEM AND ITS COXSKKVATInX 



pulses of a certain intensity may under sonic circumstances 

 he blocked in the gray matter, while under other condi- 

 tion-^ it ma\ emerge to reach the el't'ectors more or less 

 -trikingly. This property of the gray matter is spoken of 

 as variable resistance. Often it is denominated M/m//;//c 

 resistance, on the basis of the assumption that it is the 

 synapses as previously defined which so greatly change 

 their conductivity from time to time. 



A little reflection will convince 1 one that resistance to 

 the passage of impulses through the gray matter must 

 have everything to do with the behavior of the organism. 

 If it is temporarily lowered to a certain extent, reactions 

 will be readily produced by stimuli of slight intensity. 

 If it is lowered still further, the responses will become 

 exaggerated, confused, and exhausting. Increase of re- 

 sistance, on the other hand, will render it more and more 

 difficult to evoke reactions. Life itself has been defined 

 approximately as a continued "adjustment of internal 

 to external relations," and the biologic term, "irritability." 

 stands for the capacity to make such adjustments. Evi- 

 dently the resistance of the central nervous system must 

 be maintained between limits not too widely separated if 

 the good of the organism is to be served. 



Not only does the resistance of the central gray mat lei- 

 vary under the influence of drugs and of the drug-like 

 product^ of the body's own activity, but, what is of yet 

 greater interest, it is modified by the prolonged use of the 

 structure in question. In general, repealed reactions are 

 -ecured with more and more ease. This is a fact of the 

 utmost import: it will be foreseen that it mu-t be the 

 starting-point for any discussion of habit formation and 

 even of education. \Ve say that paths of easy transmis- 

 sion tend to become otablished in the nervous system ill 

 connection with its employment in a routine. Tln-e 

 path- are the "nits" of everyday speech. The evidence 

 that they mme to e\M i> found ill physiology rather than 

 anatomy, that is, in observed behavior rather than in 

 visible Structural features of the gray matter, but we 



