192 ANIMAL PHYSIOLOGY. 
over, the result may be obtained by other than electrical 
stimuli. : 
The explanation of these phenomena of the “rheoscopic 
frog” (physiological rheoscope) is simply that the electrical 
condition of B has been suddenly changed by the passage of 
the current into the nerve, and that this difference of electrical 
condition (potential) between the muscle of B and A’s nerve 
suffices to stimulate the muscle of A (one is in fact + and the 
other —); hence the stimulus and the contraction, the nature 
of which in A is the same as that in B—i.e., a single twitch 
in B gives rise to the same in A, and a tetanic contraction to a 
tetanic contraction. Plainly the contraction of A must be due 
to a current in B, hence the proof that a current actually exists 
during the contraction of a muscle. It may be noted that a 
mere prick of B will arouse in it a contraction which is fol- 
lowed by the same result as before in A, so that in this we can 
exclude the original stimulating current altogether as a pos- 
sible source of fallacy, as stated above. But one of the most 
striking proofs that there is a current of action (or negative 
variation), is obtained by placing the nerve of such a prepara- 
tion as that represented in B on a contracting mammalian heart ; 
with each systole there is a spasm of the frog’s leg. 
It is important to note that the electric current of muscle, 
however viewed, is an event of the latent period. It is asso- 
ciated with the chemical and all the other molecular changes 
of which the actual contraction is but the outward and visible 
sign; and since the currents of rest have an appreciable dura- 
tion, wane with the vitality of the tissue, and wholly disappear 
at death, they must be associated with the fundamental facts 
of organic life; for it is to be remembered that electrical cur- 
rents are not confined to muscle, but have been detected in the 
developing embryo, and even in vegetable protoplasm. Though 
the evidence is not yet complete, it seems likely that electrical 
phenomena may prove to be associated with (we designedly 
avoid any more definite expression) all vital phenomena. 
Chemical Changes in Muscle—In an animal, at a variable 
period after death, the muscles become rigid, producing that 
stiffness (rigor mortis) so characteristic of a recent cadaver. 
The subject can be studied in some of its aspects to great 
advantage in an isolated individual muscle. 
Three changes in a muscle that has passed into death rigor 
are constant and pronounced, The living muscle, either alka- 
line or neutral in reaction, has become decidedly acid; an 
