52 SCIENCE 
point of stimulation and activates the whole. 
The characteristic functional manifestation 
then appears—contraction in a muscle cell, 
motor reaction in a protozoon, cell division 
and development in a resting egg cell, etc. 
Conduction is in fact a widely general if not 
universal cell process. Excitation may thus be 
transmitted not only between different regions 
of the same cell or cellular element but also 
between different cells or elements which are 
in contact with one another; the transmission 
between neurones in the central nervous sys- 
tem and from a nerve to its muscle or other 
terminal organ illustrates this type of conduc- 
tion. It is thus possible to distinguish be- 
tween intracellular and intercellular conduc- 
tion, although there is probably no essential 
difference between the two types. 
Physiological transmission of the kind de- 
scribed seems to have in it something myste- 
rious and specifically vital; in fact the problem 
of the essential physico-chemical nature of 
nerve conduction—the type phenomenon of 
this class—is still regarded by most physiol- 
ogists as unsolved, and apparently by many as 
insoluble. The difficulty of the problem has 
been accentuated by the apparent lack of any 
close analogies with known inorganic proc- 
esses. Comparisons with the electric current, 
with the transmission of mechanical influences 
such as elastic strain or vibration, and with 
the propagation of explosive waves or of germ- 
effects like crystallization in supersaturated 
solution, have all proved inadequate and often 
highly misleading. Yet it would seem that 
any phenomenon which is so universal in or- 
ganisms and upon which many of their most 
characteristic activities directly depend—es- 
pecially in animals—must have some general 
physico-chemical basis present also in inor- 
ganic nature. The problem is to find some 
simple and readily reproducible inorganic 
process, involving transmission of chemical in- 
fluence, which is similar in its most general 
features to the conduction process in living 
cells, initiated under similar conditions, and 
dependent upon the same fundamental factors. 
What is to be looked for is not complete or 
detailed identity of the physiological process 
[N. S. Vou. XLVIII. No. 1229 
with its inorganic model, but rather a class 
resemblance of a definite and unmistakable 
kind; the inorganic process should exhibit 
peculiarities which stamp it clearly as a phe- 
nomenon of the same essential kind as the 
physiological process. If the comparison is a 
true one, the transmission of chemical in- 
fluence to a distance in cells or nerve fibers 
and the transmission of similar influence in 
the inorganic model should take place at 
similar rates, be influenced similarly by ex- 
ternal conditions, be initiated by the same 
means, have the same external manifestations, 
and be dependent upon the same underlying 
physico-chemical factors. Just as the passage 
of the pulse wave in an artery and that of a 
distension wave in a simple elastic tube are 
both determined by general physical factors 
common to both objects, so the transmission of 
chemical or metabolic influence along a living 
conducting element like a nerve should—in 
the case of a valid comparison—depend upon 
certain fundamental features of physico-chem- 
ical constitution present also in the inorganic 
model. Is there in fact any known general 
class of non-vital physico-chemical phenomena 
to which we can thus assign the phenomenon 
of protoplasmic conduction ? 
In the stimulation of an irritable living 
structure by an external agent, the primary or 
releasing event is undoubtedly a surface proc- 
ess of some kind; the characteristic activation 
or “response” of the whole irritable element 
follows automatically upon this surface change. 
Yn most irritable cells any local mechanical or 
chemical alteration of the protoplasmic surface 
layer (or “plasma membrane”), or a slight 
change in its electrical polarization due to an 
electrical current, may cause excitation. There 
is little doubt, however, that the essential deter- 
mining factors in any form of stimulation are 
electrical; and that mechanical and chemical 
stimuli excite the cell indirectly by means of 
the local electrical effects which they produce. 
The stimulating agent alters locally the struc- 
ture or composition of the surface film; the 
state of electrical surface polarization is there 
changed; and the bioelectrie circuit arising be- 
tween altered and adjoining unaltered regions 
