520 SCIENCE PROGRESS. 



sage along the new nerve path which it has entered. It 

 travels along the line thus afforded until it reaches the next 

 gap, when it has to struggle through this. What are the 

 conditions which will be favourable to the forcing of gaps 

 by such impulses ? They may be described broadly as of 

 two kinds. One set of conditions is obvious enough : it is 

 associated with the intensity of the impulse itself; the way 

 to produce reflex movements in response to nerve-stimula- 

 tion is to have such stimulation strono- enoup'h. The other 



& o 



set of conditions is more subtle in its causation but equally 

 simple to realise ; it is the alteration in the resistance which 

 the gap itself offers to the struggling impulse. 



The whole of modern physiology is inexplicable, except 

 on the supposition that the gaps are susceptible of altera- 

 tion. What this alteration is we do not know ; the gap 

 consists of living tissue, and, like all living structures, is 

 constantlv undergoing molecular change. It is such ebb 

 and flow which is the essential feature comprised by the 

 term living. It is possible that the alteration is an actual 

 approximation or separation of the terminal cell processes on 

 each side of the gap. But although the nature of the mole- 

 cular changes causing the alteration are at present doubtful, 

 the physiological agencies by which this can be produced 

 are more definitely known. The resistance of the gap is at 

 the beck and call of nervous impulses proceeding to it along 

 fibres from other portions of the whole nervous system. 

 This is especially the case with nerve impulses which de- 

 scend to the neighbourhood of the gap from the higher 

 nerve centres. The arrival of such impulses may augment 

 the stability of the molecular arrangements, increase the 

 resistance and thus inhibit or check by this added resistance 

 the passage of the gap by other nerve impulses. It has 

 also been found that the arrival of nerve impulses along 

 other lines or from other higher regions, may, but to a much 

 less decree, decrease the resistance and thus auo;ment the 

 facilities for the passage. Such play, technically termed 

 inhibition and auo-mentation, or reinforcement, is alwavs 

 going on ; the higher centres are always discharging im- 

 pulses which act in these two ways upon the lower, and the 



