MUSCULAR MOTION. 
the contraction has the appearance of being 
permanent. In the latter case, certain parts of 
the fibre (as its broken extremities) are fixed 
more or less firmly, so as to offer a resist- 
ance to the contraction that takes place, this - 
resistance enabling the contractile force ad- 
vancing to new parts to obliterate the traces 
of contraction in the parts in which it is sub- 
siding, by stretching them. The ends usually 
become fixed in consequence of their being 
the first to thicken from contraction and from 
their thus receiving the pressure of the la- 
mina of mica or glass with which it is requi- 
site to cover the object, and they are the first to 
contract, because irritated both by being broken 
and also by the water, which is absorbed soonest 
where the sheath is deficient. This fixing of 
the ends brings the fibres in question nearly 
into the condition under which they exist 
in the living body, where it has already been 
explained that there is always a resistance to be 
Overcome in active contraction. This particular 
variety of the phenomenon, therefore, deserves 
special study. Those animals whose muscles 
are most tenacious of their contractility are the 
best suited for examination, and among these 
the young crab or lobster may be most easily 
obtained. In an elementary fibre from the 
claw, laid out on glass, and then covered with 
a wet lamina of mica, the following phenomena 
are always to be observed. ‘The ends become 
_ first contracted and fixed. Then contractions 
_ commence at isolated spots along the margin of 
the fibre, which they cause to bulge. At first 
they only engage a very limited amount of the 
Mass, spreading into its interior equally in all 
directions, and being marked by a close approx- 
imation of the transverse stripes. These contrac- 
tions pull upon the remainder of the fibre only 
in the direction of its length, so that along its edge 
the transverse stripes in the intervals are very 
much widened and distorted. These contrac- 
tions are never stationary, but oscillate from 
end to end, relinquishing on the one hand what 
they gain on the other. When they are nume- 
rous along the same margin they interfere most 
irregularly with one another, dragging one ano- 
ther as though striving for the mastery, the 
larger ones continually overcoming the smaller, 
then subsiding as though spent, stretched again 
by new spots of contraction, and again, after a 
short period of repose, engaged in their turn 
by some advancing wave: this is the first stage 
of the phenomenon. ( Fig. 302.) The con- 
tractions increase in number and extent, and 
gradually engage the whole substance of the 
fibre. There is still the same struggle, the same 
alternate action and repose in individual parts, 
but as the contractions by degrees predominate, 
the ends of the fibre are drawn more and more 
near, (intermediate portions by their contrac- 
tion receiving some of the pressure.) until at 
last the whole fragment is reduced to a third of 
its original length, and stiffened with the rigor 
mortis. 
The muscular tissue in these animals is very 
tough, but where it is more fragile, as in the 
Frog, it may give way in the intervals between 
spots of contraction, and become ruptured and 
525 
Border of an elementary fibre of a young Crab, shew~- 
ing a spot of contraction (b) and the sarcolemma 
elevated in the form of bulle by the expressed 
water (a). Magnified 300 diam, 
disorganized in various degrees.* In fishes I 
have seen a succession of phenomena similar to 
what has been described in the Crab; waves of 
contraction advancing and receding, but gradu- 
ally augmenting in bulk, till the whole fibre 
was finally contracted. (Fig. 303.) 
Fig. 303. 
Stages of contraction seen on one in an el: 
tary fibre of the Skate. The uppermost state is that 
previous to the commencement of contraction. 
a, a, a@, successive ‘ waves’ of*contraction seen 
moving along one margin of the fibre, marked by a 
bulging of the margin, by an approximation of the 
tranverse stripes, and by a consequent darkening 
of the spots. 
b, b, b, similar ‘ waves” still moving along the 
fibre, bnt engaging its whole thickness. 
In all these examples, as long as the ends of 
the fragment are fixed, and will not yield to 
the convellent force, that force is seen to be 
exerted in a momentary manner in successive 
portions of the mass. In proportion as they 
yield to it, the resistance which enabled the 
contraction of new parts to stretch those from 
which it was receding is removed, and the ap- 
pearances of contraction remain. A distinction 
is required between the contractile force and 
the contraction resulting from its exercise. The 
latter will be permanent, if no force from with- 
out be exerted to obliterate it by stretching, for 
a contracted muscle has no power of extending 
itself; there is no repellent force between its 
molecules. From these phenomena, therefore, 
it is possible to eliminate the appearances re- 
sulting from a subsided force, and to judge of 
* Phil. Trans. 1840, p. 490, pl. xix. -fig. 75. 
