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IOWA ACADEMY OF SCIENCE Vol. XXVIII, 1921 
lism. That the characteristic semi-permeability depends upon this 
latter peculiarity is seen in the fact that the death-process, how- 
ever induced, is always associated with a marked increase in gen- 
eral permeability and electrical conductivity of the cells. In 
other words, the normal semi-permeable condition — involving 
as it must a certain constancy in the composition and physical 
state of the surface film — is maintained only so long as the cell 
remains alive. This fact shows that semi-permeability, and the 
electrical polarization which is associated with this, are not simply 
static properties of the plasma-membrane, but are functions of 
specific metabolic activity — including probably oxidations in 
most cases — which maintains constant the physico-chemical char- 
acteristics of the surface-layer* of protoplasm. In the irritable 
element this metabolism seems to be altered in a definite manner 
by changes in the electrical polarization of the membrane; and 
along with these chemical alterations go alterations of permeabil- 
ity, and secondarily of electrical polarization. These latter in- 
volve the production of local electrical circuits which traverse and 
hence stimulate the adjoining inactive portions of the irritable ele- 
ment. In this manner the state of excitation spreads and the 
whole element is stimulated. But this is the case provided only 
that the membrane retains its normal sensitivity to changes of 
electrical polarization; if it has previously been rendered resis- 
tant by an anaesthetic, no such effect follows ; the element as a 
whole then shows itself irresponsive to stimulation. 
With this brief discussion, we may proceed to the consideration 
of the mode of experimentation described. 
METHODS AND APPARATUS 
The experiments about to be described were performed inter- 
mittently between April and December of the past year, 1920, 
the frogs being obtained in five dozen lots from a supply house 
in Chicago, the batches consisting for the most part of the com- 
mon or leopard frog, Rana pipiens Schreber, sometimes called 
Rana virescens Kalm. After making all due mechanical arrange- 
ments, the specimen was killed by pithing, the brain and spinal 
chord were destroyed, and the gastrochnemius muscle was re- 
moved and placed immediately in the glass cylinder of a Harvard 
type muscle warmer (J, Fig. 30), arranged and mounted in such 
a way as to allow easy manipulation in pouring solutions of de- 
sired strengths, and so connected to the inductorium (E) and the 
