50 
to those obtained by Zoethont in experiments on the gastroc- 
nemius, "^ but, in reality, these results are due to the action 
of KCl and CaClg in lowering and raising the stimulation 
etficiency of NaCl respectively. Pure CaCL2 or KCl 
applied to a heart strip throws it into strong tone, | as might 
be expected from the high stimulation efficiency of both, 
since the frequency of contraction is greater the greater the 
difference between the ionic velocities. Hence the two sets of 
results are, by this theory, brought into entire harmony. 
With regard to the influence of the threshold number in 
lowering the rate of rhythm, it is obvious that the height of 
contraction depends upon the magnitude of the threshold 
number, for the greater the potential of the wave of negativity 
the greater is the maximum P.D. produced between the 
hyaloplasm and spongioplasm ; hence we should expect that 
the slower the rhythm the greater the height of contraction, 
other things being equal. This has been experimentally proved 
for smooth muscle by Woodworth. j As /8 grows greater in 
excitable tissues we find that the "refractory period"' grows 
greater. During this period the tissue will not respond to 
stimuli, and it is greater in cardiac than in striated 
skeletal muscle. § The reason is that, /8 being greater, a 
greater time must be allowed for the amount of ion-proteid 
corresponding to /? to become unstable ; of course, the moment 
at which the kations at the point of initiation are sufficient 
to cause a contraction will coincide with the moment at which 
the ion-proteid is in a certain minimal state of instability. 
This is the same as saying that immediately after a wave of 
negativity has passed a point, ^ is great at that point, and 
the amount of decomposable material small ; the amount of 
decomposable material grows, and (3 diminishes until a cer- 
tain point is reached at which excitation by a given stimulus 
is possible. Thus the slowing of a wave of negativity travel- 
ling too soon after a contraction is due to the greater magni- 
tude of the threshold number. || Since the frequency of 
contraction is greater the greater the difference between the 
ionic velocities, any solution in which the ions move at very 
different rates will cause tonic contraction. Hence alkalies 
cause tonic contraction.^ 
* Vide section 12. 
t C. W. Greene: American Journal of Physdology, 1899, 
vol. ii., page 101. 
+ R. S. Woodworth: American Journal of Physiology. 1899. 
§ G-askell : Schafer's Textbook of Physiology, vol. ii., page 
189. 
II Ihid., vol. ii., page 195. 
H Ihid., vol. ii., page 195. 
