Philosophical Conceptions of Life. 247 
through the nerve-fibres ; and I suppose that most physiologists 
at the present time think of the nervous current in some such 
way as this. Even those who attach most importance to the 
electrical phenomena will, I take it, agree with Michael 
Foster that these “ are in reality tokens of molecular changes 
in the tissue much more complex than those necessary for the 
propagation of a mere electrical current ” *, 
We do not, however, as yet possess any sufficient founda- 
tion of facts on which to build a reasonable hypothesis as to 
the nature of the molecular disturbances that accompany a 
nervous impulse. The labours of the physiological chemists 
have taught us nothing with regard to the changes that go on, 
except that the axis-cylinder, which in the inactive living 
nerve is alkaline, becomes acid after long-continued activity 
or after death +. We can measure the velocity with which 
the impulse travels, we can study the conditions under which 
it arises, we can believe, as I certainly do, that it will ulti- 
mately receive a chemico-physical explanation; but its real 
nature we do not yet know. 
So far as we can ascertain, the phenomena of the conduc- 
tion of nervous impulses by the sensitive nerves are so similar 
to those of the conduction of motor impulses that any explana- 
tion ultimately adopted for the one will probably apply to the 
other also. When, however, we ascend to the study of the 
nervous centres, by which sensitive and motor nerves are con- 
nected together, and attempt the interpretation of the complex 
functions of nerve-cell, ganglion, spinal cord, and brain, we 
find that none of the hypotheses hitherto brought forward to 
explain the observed phenomena repose on any defensible 
chemico-physical basis. 
I cannot of course undertake to give to-night even the most 
meagre outline of the wondrous mechanism which physiolo- 
gical experiments show must exist. That reflex actions, 
co-ordinated muscular movements, and all the complex phe- 
nomena of this class do depend upon a wonderfully complex 
mechanism, and occur in strict accordance with the ordinary 
chemical and physical laws, I do not for a moment doubt, and 
I cordially invite the cooperation of the chemists and physicists 
to aid the physiologists in the explanation of this mechanism, 
for we stand only upon the threshold as yet. 
If now we turn from the more general discussion of mus- 
cular contraction and nervous action to the consideration of 
the several functions carried on in animals by means of 
special arrangements of the muscular and nervous systems, 
* Foster, p. 79, op. cit. supra. 
+ A Gamgee, p. 447, op. cit, supra. 
