524 
Independently of the immense disadvantage 
at which the hypothesis in question supposes 
the force to act, (viz. either’ between the par- 
ticles at the retiring angles only of the zig-zags, 
or between the distant angles themselves,) it 
seems quite inconsistent with the able experi- 
ments of Schwann, which show that the power 
of a muscle diminishes in a direct ratio with 
the degree of its contraction. With these 
experiments, indeed, any hypothesis is at 
variance which is based on the idea ofan 
attraction between isolated and separate points 
or particles, as, for example, the sarcous ele- 
ments, for it cannot be conceived but that such 
an attractive force would augment in a multi- 
plying ratio with the proximity of the points 
attracted. 
2. On passive and active contraction. Pas- 
sive contraction.—Passive contraction is that 
which every muscle is continually prone to 
undergo, independently of stimuli, and by the 
mere quality of its tissue. The muscles are 
ever kept on the stretch by the nature of their 
position and attachments, and cannot have their 
ends so approximated by attitude or other- 
wise, as that their tendency to shorten them- 
selves shall cease. If, for example, the rectus 
muscle of the thigh have its extremities brought 
as near together as can be effected artificially 
by posture, they would yet be found to ap- 
proach still nearer on being freed from their 
attachment to the bones. This tendency to 
contract has been distinguished by the term 
retractility, from its being manifested by the 
retraction that occurs when the belly of a mus- 
cle is cut across. But, in this instance, the 
retraction would appear to be in part caused by 
an active contraction excited by the stimulus of 
the injury. It has also been styled tonicity. 
The ive contraction of muscles continually 
opposes their elongation by the action of anta- 
gonists, and restores them when that action 
ceases. It is that which accommodates them 
to an attitude artificially given, when no mus- 
cular effort is required to maintain it. When 
no active contraction is present in a limb, the 
passive contraction remains, and being brought 
to a state of equilibrium in all the muscles by 
their mutual antagonism, the limb is said to be 
at rest. This is the general condition during 
sleep. The passive contractility of muscles, 
therefore, is being ever exerted, without being 
attended by fatigue; there seems no good rea- 
son for supposing it to be a property different 
from active contractility ; it is rather the neces- 
sary condition of that property, in its passive 
or unstimulated state. Passive contraction is a 
vital act, for it ceases with the rigor mortis. 
Active contraction.—This is the form of con- 
traction which is attended with those manifes- 
tations of power or force that specially charac- 
terize muscle. It is always excited by a sti- 
mulus, and is always exerted in opposition to 
another force within the body, which it is able 
more or less completely to master. The op 
sing force is generally the passive covtmatity 
of antagonist muscles, but it may be the elasti- 
city of parts, or, in the case of hollow muscles, 
the resistance of their own contents. Active 
MUSCULAR MOTION, 
contraction is ial in extent and duration, 
It requires uitereelassb-ey being attended with 
exhaustion of the power which produces it, 
which exhaustion in the voluntary muscles is 
attended with the sensation termed muscular 
Satigue. bd 
3. Of the differences between the minute — 
movements of muscle in passive and active con- 
traction. In passive contraction.—It is, per- 
haps, impossible in the higher animals to ob- 
serve the nature of the microscopic movements 
occurring during the passive shortening of a — 
muscle; but in the lower and smaller forms of 
life ite may omgres accompa It 
may always a oubtful, however, whether, 
any contraction that may be one witnessed be 
entirely of the ive kind, and consequently — 
the noceianuntins noticed are not worthy of 
implicit reliance. But it is more easy and quite 
as satisfactory to bring a muscle under i 
tion, which is still in situ and in equilibrium 
with its antagonists ; in such, contractile force 
is still present, though its effects are neutralized. 
This may be done in various small animals; per-_ 
haps the tail of small fish or of the tadpole of the 
common frog is the best adapted for the 
In the latter, when deprived of its thick } 
ment, I have succeeded in gaining such a ae 
and have found the contraction to be quite uni- 4 
form throughout, the transverse stripes bei 
tionary oat equidistant. This is nothing mo 
than might have been expected on @ priori 
grounds. The contraction being the effect of the - 
pee exercise of the property shared equally — 
y all parts of the tissue, would be equal in equal - 
masses, and as the elementary fibres are of pre-— 
cisely equal width and substance from end to 
end, no part of them could predominate in ac-— 
tion, as long as no special stimulus was applied: 
It may be concluded, therefore, that passis 
coutraction is attended by a movement abso 
lutely uniform throughout the whole mass of 
an elementary fibre or of a muscle. ie? 
In active contraction.—The case is far othe 
wise in active contraction, as may now be cor 
sidered proved by a considerable body of e 
dence. ‘y 
It might be argued, prior to direct pre 
that active contraction, being the answer to 
stimulus, must be partial, at least at its 
mencement, since no stimulus can be ap 
at the same instant to every particle of 
¥ 
muscle. i. 
Certain features of the contractions 
in fragments removed from the body, and 
amined in water under the microscope, hav 
close bearing on the present question. Ti 
been already said that such contractions 
uniformly partial; but they t two fur 
varieties, either remaining in the part wi 
they first occur, or leaving it as they en 
others in the neighbourhood. The accidt 
circumstances under which the fragmé 
placed explain these varieties. In the forn 
case the fragments are free to move; their ent 
approach in proportion to the amount of o 
traction, and as there is no force to extend th 
again when the contractile force ceases to” 
manifested in them, and advances to fresh par 
. 
an 
“ 
<9 
J ha 
