50 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
ever delicate our means of observation may be. It appears to me probable that every 
effectual excitation—every excitation which is marked by electrical disturbance—is also 
followed by diminution of the water-charge of the excited protoplasm. At first, this 
may either expend itself entirely or produce only such small effects as, in the experi¬ 
ment last related, presented themselves in excitations 3 to 7. 
In our previous paper the phenomena described above were referred to as evidences 
of summation. It will be better in future to limit the application of this word to the 
still more remarkable phenomenon which has been described under the heading 
<c Summation” in Part V., p. 47 ; for those phenomena are strictly analogous to the 
effects to which the term “Summation of stimuli” has been applied by Stirling.* 
The term “Summation” is in itself as applicable to the process now under considera¬ 
tion as to the other, but it differs from it in this respect—that it is rather a summation 
of effects than a summation of excitations. The process with which it is most com¬ 
parable in the physiology of animal excitable tissues is that which is known as the 
Treppe or staircase.! By this term is designated the fact that when the ventricle of 
the heart of the frog is subjected, after a period of rest, to a series of effectual 
excitations, the contractions become stronger and stronger, although the excitations 
are all of equal intensity. In both processes the mechanical effect of the first excita¬ 
tion is much smaller than that of the second, the second smaller than that of the 
third, and so on, but in the Treppe the difference which is large at first rapidly 
diminishes, whereas in the leaf it as rapidly increases. Further, the total mechanical 
effect of the first excitation is so small as to be barely perceptible, whereas in the heart 
the first contraction (i.e., the weakest) is at least half as vigorous as the strongest. 
But in proceeding with our comparison we find that in both, the change of form is 
opposed by a resistance which has its seat in the structure of the organ, and is removed 
step by step when it is subjected to repeated excitations. In the heart this resistance 
diminishes rapidly at first, more slowly afterwards : in the lamina of the leaf it does 
not begin to yield until after several excitations, provided that they are only just 
strong enough to provoke an electrical response. This difference is unimportant, and 
becomes almost imperceptible if somewhat stronger excitations are substituted for the 
minimal ones. In this case the effect of the very first excitation may be large, that 
of the second larger, the third still larger, and so on, but with decreasing, not increas¬ 
ing increments, just as in the Treppe. The most fundamental difference is that which 
concerns the relation between the resistance and the mechanism of contraction in the 
two cases. In the ventricle only a fraction of the mechanical effect, even of the first 
contraction, is due to abolition of resistance. At each repetition of the excitation this 
fraction diminishes, until after a dozen or so of excitations the successive mechanical 
effects become sensibly equal, From the first, therefore, the change of form is due 
* Stirling, “ Ueber die Summation electriscber Hautreize.” Ludavig’s Arbeiten, 1874, p. 223. 
f See Boavditch, “ Ueber die Reizbarkeit der Muskelfasern des Herzens.” Ludavig’s Arbeiten, 1871, 
p. 156. 
