196 ANIMAL PHYSIOLOGY. 
No machine known to us resembles muscle except super- 
ficially. The steam-engine changes fuel into heat and mechani- 
cal motion, but there the resemblance ends. Muscle changes 
its food, or fuel, not directly into either heat or motion, but into 
itself; yet as a machine it is more effective than the steam- 
engine, for more work and less heat are the outcome of its 
activity than is the case with the steam-engine. 
THE PHYSIOLOGY OF NERVE. 
Muscle and nerve are constantly associated functionally, and 
have so much in common that it becomes desirable to study 
them together. Much that has been established for muscle 
holds equally well for nerve; and the latter, though apparently 
wholly different in structure at first sight, is really not so. 
Nerve has its: protoplasmic part (axis-cylinder), which is the 
essential structure, its protective sheaths, and its nuclei (nerve- 
corpuscles) % 
As already indicated, a nerve possesses irritability, and, 
since a muscle does not respond to an electric current sent 
through a nerve except when there is a 
sudden change in the strength of the 
current, it becomes interesting to learn 
why this should be the case. 
Experimental.—In Fig. 190 are shown 
diagrammatically two muscle-nerve prep- 
arations, and the apparatus necessary 
for applying a constant current and a 
(momentary) induced current by single 
shocks to the nerve. 
A strength of current sufficient to 
cause a (sub-maximal) contraction by an 
induction shock is determined, and the 
inductorium left at this graduation. A 
Fic. 190.—Diagrammatic rep. COnStant current of moderate strength is 
cee otihe excitapity allowed to pass into the nerves of the 
of thenerveinelectrotonus preparation. It is found that, in the one 
(Landois). Positive poles 
marked +, negative, —; 7 ta 7 
marked +, negative, —,; case, the muscle contraction is increased, 
cated by arrows. R, Ry, i imini % 
e i, ate paints at which es in the other diminished or absent, 
exqitadi! 0 e nerve 1s 
Sar aterel when the same strength of induction 
shock is sent into the nerve at the points 
below the entrance of the constant current—that is to say, 
the irritability of the nerve has been increased or diminished. 
A 
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