OPENING ADDKESS. 15 



in this region, at the base of the hair, it is enormously 

 in the ascendant. Structure is no guide at all to this 

 ascendancy, because the former remaining apparently the 

 same, the latter alters. This is the case with most of the 

 so-called excitatory phenomena. The whole of physiology 

 simply bristles with instances ; thus, a muscle is acted 

 upon by a certain salt — sulphate of veratria ; — its micro- 

 scopic structure is apparently unaltered, but its activity is 

 changed so enormously, that when aroused by a single 

 stimulus, instead of shortening and resuming its length 

 again in yg-", it now contracts and remains unable to relax 

 for almost 1 minute. 



Again, a nerve, if exposed to weak ether vapour, suffers 

 no perceptible alteration in structure ; but as long as the 

 vapour is present it ceases to carry out that ascendant 

 phase of its activity for which it is really framed ; it cannot 

 pass on the mysterious awakening which, flashing down 

 through each successive part, is called a nervous impulse. 

 On removing the ether, back comes this conducting 

 property so that the vapour has not permanently injured 

 the nerve. All the phenomena of anaesthesia are exten- 

 sions of such a case as this. 



I might multiply examples almost indefinitely ; I should 

 simply be stating the functions of the excitable tissues and 

 the circumstances which modify these, for in very few 

 instances are such modifying circumstances sufficiently 

 gross to cause any perceptible alteration in structure pari 

 passu with the profound alteration they produce in 

 function. 



Fourth class of instances ; inferring Structure from 

 Function. 



This is an inference of precisely opposite character to 

 those hitherto considered and is eminently the property of 



