46 Dr. D. F. Harris. On the Occurrence of — [Feb. 15, 
the differences in molecular mobility between the gastrocnemius and the 
hyoglossus muscles. 
Similarly, a poison like curare, which diminishes protoplasmic mobility, 
brings down the periodicity of the post-tetanic tremor from the six per second 
of frog’s normal gastrocnemius to 2°5 to 3 per second in the curarised 
gastrocnemius (cf. fig. 9). 
In other words, the physiological insusceptibility is now about three times 
as great as in the normal frog muscle; or, again, the curarised gastrocnemius 
becomes reduced, as regards molecular mobility, to the level of the less mobile 
hyoglossus. 
(4) The post-tetanic tremor is characteristically irregular in intensity from 
moment to moment. : 
Previously to my publishing the first note on the periodicity of this tremor, 
Dr. Bayliss was good enough to suggest, in a private communication, that the 
irregularity in the record of the tremor was due to irregular variations in the 
intensity of the various members of the series of stimuli given by Neet’s 
hammer, that is to say, that the stimuli varied from submaximal to maximal 
and vice versd. I had eliminated this possibility, by noticing that post-tetanic 
tremor could be produced at all distances between the primary and secondary 
coils, both when the stimuli were submaximal and when they were maximal. 
As I had already used the Helmholtz side-wire equaliser, and did not 
consider that the vibrating reed gave stimuli of sufficiently uniform intensity, 
I decided to test the point by (1) using a 100 a second electro-magnetic 
tuning-fork as interruptor in the primary, and (2) by using as interruptor, 
in the primary circuit, a metallic rotating wheel provided with teeth which 
dipped into a pool of mercury, and was driven at varying speeds by a small 
air-engine. 
By neither of these methods was the irregularity abolished; and in the 
case of certain muscles was not in any obvious manner diminished (notably 
fig. 2, muscle of kitten, where the 100 a second fork was used). 
While freely admitting it possible that a series of stimuli of absolutely 
equal physical intensity would give the least irregular form of post-tetanic 
tremor, yet I am compelled, from a study of a large number of these tremors 
through long periods during which the stimuli were as constant in energy as 
possible, to believe that the irregularity of the post-tetanic tremor is 
inherently characteristic of it. 
It seems to me that the irregularities can be explained by :— 
1. Non-simultaneous onset of fatigue in the several fasciculi or fibres of the 
muscle ; so that when we use even physically equal stimuli, a stimulus which is 
effective for one fibre or fasciculus may not be so for another, or if effective 
