Keith Lv^cas. 



xli 



excitability after an effective stimulus is due to the passage of the propagated 

 disturbance itself. This refractory period is, in fact, independent of whether 

 the two stimuli fall on the same point or on different points. 



A peculiar phenomenon, known as Wedensky's inhibition, also found its 

 explanation in Keith Lucas' work. It was shown to be due to a resistance to 

 Conduction in the nerve greater than normal, and the possibility of similar 

 conditions in the nerve centres was discussed as a probable basis of some 

 inhibitory phenomena. ^ ' 



A further step was taken, in conjunction with Adrian, towards the 

 analysis of the process of stimulation, in that this was shown to consist of 

 two stages, a purely local effect and a propagated effect. The former may be 

 present, although insufficient to set in motion a propagated disturbance, and 

 the fact that it does not immediately disappear renders possible summation 

 of stimuli, each in itself an inadequate one. An important method was 

 developed for the estimation of the magnitude of a propagated disturbance, 

 as referred to above. This consisted in determining the distance travelled 

 through a region in which it is progressively diminished in magnitude, that 

 is, a region of decrement, such as is produced by anaesthetics. This method 

 was afterwards applied by Keith Lucas himself to the decision of the 

 important and disputed question as to the possibility of distinguishing 

 between conductibility and excitability in nerve, a distinction which, it had 

 been stated, could be made out by the use of alcohol as a narcotic. The two 

 factors were shown in reality to disappear together, and this disappearance 

 to be due to the same cause, namely^ increased difficulty in setting up a 

 propagated disturbance. It was also found that, when such a nerve impulse 

 is set up by a strong stimulus in the " relative " refractory period, it is 

 smaller in magnitude than the normal one, produced by a stimulus outside 

 the refractory period. 



The Croonian Lecture was given by Keith Lucas in 1912, and was 

 devoted in part to a discussion of the work referred to in the preceding 

 pages, and to the investigations of other workers on questions related to it. 

 The Lecture concluded with a more detailed description of the modified form 

 of Nernst's theory of excitation, and with criticism and suggestions 

 concerning it. 



In a valuable paper on " Summation in the Claw of the Crayfish," 

 published in the 'Journal of Physiology' after the author's death, some of 

 the facts previously discovered are made use of to elucidate the complex 

 " problems of this interesting neuro-muscular mechanism. By application of 

 the law governing the relation of current strength to the time of closure 

 required to stimulate, it was found that there are two sets of nerve fibres in 

 the nerve to the claw. One of these is responsible for a slow prolonged 

 form of contraction, the other for a brief quick twitch. It was also found 

 that the type of summation described by Eichet was due to the fact that the 

 first stimulus, although sending an impulse along the fibre, fails to cause 

 contraction because it has been reduced in intensity by having to pass through 



