31 
tedly raise the potential at each point in the muscle and un- 
interruptedly let it fall again. The effect of this will be (just 
as in the capillary electrometer when the potential on one 
side of the meniscus is raised) to diminish the surface tension 
at the contact surface of the spongioplasm and hyalopla.';m 
owing to the increase in the P.D. between them. 
Now, if we suppose the walls of the sarcous element to be 
elastic — the effect of the si;rface tension of the hyaloplas.'n 
will be to exert a pull inwards upon the wall — and therefore 
the walls are pulled in. To this pull the 
wall will offer a resistance owing to its elasticity. If 
these two forces are in equilibrium, increasing the surface 
tension will narrow the tube, while «'liminishing the surface 
tension will widen it. But widening this elastic tube must 
shorten it, just as an indiarubber tube when stretched longi- 
tudinally grows narrower, and when stretched laterally grovvs 
shorter. The sarcous element, in shortening, must exert a 
pull on the fine fibrils which, it is conjectured, attach them to 
Krause's membrane : herice, the two membranes of Krause 
are pulled together and the muscle contracts. Hence, sinc3 
the "wave of negativity" must diminish the surface tension — 
not by deposition of ions, for in that case it would undergo 
excessive decrement, which it does not^ — but by simply rais- 
ing the P.D. between the hyaloplasm and spongioplasm it 
must give rise to a contraction. 
6. — On the Propagation of Excitation in Nerve and 
Muscle. 
We have seen that the hyaloplasm of striated muscle con- 
tains a kation-proteid owing to the presence and metabolism 
of which the surface of contact between the spongioplasmic 
sarcous elements and the hyaloplasm is always positively 
charged on the nyaloplasmic side, or, in physiological termi- 
nology, the surface of the hyaloplasm is always "negative'' to 
that of the spongioplasm. "When any breaking up of the 
kation-proteid takes place, kations must therefore be set free. 
Now, I have previously pointed out that the fundamental 
property pf ion-proteid is that it is very unstable in the pres- 
ence of ions, tending to form new ion-proteid compounds with 
any ions which may be present in excess: and, indeed, it is 
upon this property of the ion-proteid that the phenomena of 
contraction and irritability in living tissues depend. I may 
now throw this assertion into a more definite form, and state 
that when a certain minimal number of free ions (the numbe;'' 
varying in different tissues) is present at any point in an excit- 
* Biedennann : Electro-physiology : Trans, by F. A. WeibYj 
vol. i., page 395. 
