170 
ANIMAL PHYSIOLOGY. 
contraction ensues; the same happening if the nerve be thus 
treated instead of the muscle. 
The changes in the muscle and 
the nerve will be seen later to have much in common; the mus- 
cle alone, however, contracts, undergoes a visible change of form. 
Now, the agent causing this is a stimulus, and, as we have 
seen, may be mechanical, chemical, thermal, electrical, or nerv- 
Fia. 162.—Mode_ of termination of the motor nerves 
(Flint, after Rouget). A. Primitive fasciculus of the 
thyro-hyoid muscle of the human subject, and its 
nerve-tube: 1,1, primitive muscular fasciculus ; 2, 
nerve-tube ; 3, medullary substance of the tube, 
which is seen extending to the terminal plate, where 
it disappears ; 4, terminal plate situated beneath the 
sarcolemma —that is to say, between it and the ele- 
mentary fibrille ; 5,5, sarcolemma. B., Primitive 
fasciculus of the intercostal muscle of the lizard, in 
which a nerve-tube terminates: 1, 1, sheath of the 
nerve-tube; 2, nucleus of the sheath ; 3, 3, sarco- 
lemma becoming continuous with the sheath; 4, 
medullary substance of the nerve-tube, ceasing 
eeopy at the site of the terminal plate ; 5, 5, ter- 
minal plate ; 6, 6, nuclei of the plate ; 7, 7, granular 
substance which forms the principal element of the 
terminal plate and which is continuous with the 
axis-cylinder ; 8, 8, undulations of the sarcolemma 
reproducing those of the fibrillas ; 9, 9, nuclei of the 
sarcolemma. 
ous, As both nerve and 
muscle are capable of 
being functionally af- 
fected by a stimulus, 
they are said to be irrita- 
ble; and, since muscle 
does not contract with- 
out a stimulus, it is said 
to be non-automatic. 
Now, since muscle is 
supplied with nerves as 
well as _ blood - vessels, 
which end in a peculiar 
way beneath the muscle- 
covering (sarcolemma) 
in the very substance of 
the protoplasm (end- 
plates), it might be that 
when muscle seemed to 
be stimulated, as above 
indicated, the responsive 
contraction was really 
due to the excited nerve 
terminals; and thus has 
arisen the question, Is muscle of itself really irritable ? 
What has been said as to the origin of muscular tissue 
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Fiq@. 163,—Intrafibrillar terminations of the motor nerve in striated muscle, stained with gold 
chloride (Landois). 
points very strongly to an affirmative answer, though it does 
not follow that a property once possessed in the lower forms of 
