Every muscular motion is either simple or compound. Let us now speak of
the first; by it we shall understand the second.
_Simple Motion._
It must be considered, 1st, in the muscles with a straight direction;
2d, in those in a reflected one; 3d, in those in a circular one.
In the first, as in those of the extremities, the trunk, &c. if they
are of an elongated form, and as they terminate by a tendon, each
fibre contracting draws this tendon from its place; whence it follows
that all act together to bring it towards the centre of the muscle, but
at the same time each of them tends to give it another direction, and
in this respect they are antagonists. The common motion remains; the
opposite is destroyed.
Every effort of contraction in the long muscles is concentrated upon
a single point, the tendon. In most of the broad muscles, on the
contrary, the attachments being made at two sides by different points,
all the fibres do not contribute to the same end. Thus the different
parts of the same muscle can have very different and even opposite
uses; thus the inferior portion of the great serratus does not act like
the superior; often even the different portions of the same muscle do
not contract at the same time. In a long muscle, on the contrary, as
all the fibres contribute to produce the same effect, they always act
simultaneously.
To estimate the effect which a muscle in the straight direction
produces upon the bones in which it is inserted, different means
are employed. A very simple one appears to be that, which I believe
has never been mentioned. It consists in examining the direction
of the muscle from its fixed to its moveable point, and in taking
the inverse of this direction; this last is always the direction of
the motion. Do you wish to know how the anterior radial acts upon
the wrist; take it at its insertion at the condyle, then follow its
direction downwards and outwards; you will see that it carries the
hand upwards and inwards, that it bends it and places it a little in
adduction. The tibialis anticus directed downwards and inwards raises
the foot and carries it outwards. The anterior rectus of the thigh
going straight from the pelvis to the patella, raises the leg directly
up. All the other muscles will exhibit this arrangement. Whatever
may be the attachment of their fixed or moveable point, they always
act inversely to the supposed line of direction going from the first
point; and as each attachment can be alternately moveable or fixed, the
two bones which serve them are carried in an opposite direction; the
coraco-brachialis directed downwards and outwards from the shoulder
towards the arm, carries this last upwards and inwards; directed
from below upwards and from without inwards from the arm towards the
shoulder, it moves this downwards and outwards. By this general rule,
it is sufficient to see a muscle in a dead body, to pronounce upon its
uses.
When the whole of a broad muscle is united at a common point, as the
deltoid which having many points of attachment above, is fixed below in
a single tendon, the middle line of direction of all its fibres should
be taken to estimate its uses.
When a muscle is attached by its two extremities at many points, and
consequently the fibres that compose it, form many fasciculi with
different directions and insulated motions, the line of direction of
each fasciculus must be examined in order to estimate the action of
the muscle. It is thus that we should study that of the trapezius, the
great serratus, the rhomboid, &c.
In the muscles with reflected direction, as the great oblique of the
eye, the lateral peronei, the circumflexus, &c. the action of the
muscle should only be estimated by the point of reflection; thus the
great oblique carries the eye inwards, though its fleshy portion
contracts so as to carry the moveable point backwards.
The orbicular muscles, those placed around the lips, the eyes, the
anus, &c. have in general no fixed or moveable points; they are not
designed to approximate two parts to each other, but only to contract
the opening around which they are situated. The anus is shut by its
sphincter, when the excrements do not dilate it. The mouth remains
closed, when the depressors, the elevators or the abductors of the
lips are inactive. The eye is shut, when the elevator of the superior
eye-lid is relaxed. I would remark upon this subject that the inferior
eye-lid having no depressor, it is principally the other which
contributes to shut or open the eye; and as its muscle cannot be in
permanent contraction, the alterations of its relaxations produce those
continual winkings which take place when the eye is open; they are to
the eye what the alternate change of the weight of the body from one
leg to the other is in long standing without motion. At every instant
the muscle relaxes; the sphincter acts immediately; then it contracts
and distends the sphincter; winking then is a continual struggle
between the elevator of the eye-lid and the orbicularis. In sleep, it
is not by the contraction of this that the eye is shut; it is relaxed
like all the muscles; it is because the elevator is inactive, that the
eye-lid falls by its own weight upon the eye; it communicates as it
were the motion to the orbicularis that it shuts up, whilst, during
the day, it is the orbicularis on the contrary that communicates this
motion to it.
_Compound Motions._
There are but few motions in the economy that are simple, but few
muscles that can contract separately. Almost every sort of contraction
supposes another, and for this reason; the two points to which a muscle
is ordinarily attached are both capable of being moved; if one of
them was not fixed, both would then be put in motion when the muscle
contracted; thus in the contraction of its extensors, the leg if it
was not fixed would approach the foot as much as the foot approached
the leg; now it could not be fixed but by the muscles which act in an
opposite direction to the effect which the extensors tend to produce
upon it; then whenever the two attachments of a muscle are moveable,
the insulated motion of one of them supposes the contraction of
different muscles to fix the other.
It is only those muscles that are attached on one side to a fixed point
and on the other to a moveable one, like those of the eye, and most of
those of the face, that can move in an insulated manner, and without
requiring a motion in the other muscles. It should be remarked however
that in general the contractions destined to fix the point which should
be immoveable in the ordinary motions, are less than they at first
seem to be. In fact, in these ordinary motions, the point which moves
is always the most moveable, that which remains without motion is the
least so; for example, it requires a much greater effort in flexors
to bend the arm upon the fore-arm, than to bend the phalanges upon
the fore-arm, or the fore-arm upon the arm. By supposing their two
attachments moveable, the gemelli would act much more powerfully on
the foot than on the femur, &c. In the extremities, the superior point
is always more moveable than the inferior, now it is this which almost
always moves, the other being fixed; then as it offers more resistance
by its position, it requires less effort of the muscular powers to
retain it. It is only in violent motions, that the previous contraction
of the muscles destined to fix one of the points of insertion Is very
painful. This takes place on the chest when the trapezius, the great
serratus and the great pectoral contract powerfully; then all the other
muscles of this cavity contract strongly to dilate it, and thus offer
a broader and more fixed attachment to those muscles, which move the
shoulder in the support of burdens, or in any other analogous effort.
The diaphragm contracts also; hence hernias, the descents which take
place from a concussion in those motions which, at first view, have no
analogy with the abdominal cavity. When in a horizontal position of the
body we raise the head, the rectimuscles of the abdomen contract to
fix the chest, and present a solid point to the sterno-mastoideus, &c.
We call especially a compound motion that which two or more muscles,
acting upon the same point, contribute simultaneously to produce. In
this case, the moveable point follows the direction of neither muscle,
if there are two of them, but takes the diagonal of their direction.
It is thus that the eye is moved outwards and upwards, outwards and
downwards, &c.; that the head is depressed, that it is carried to
one side, and that the arm is applied to the trunk, &c. In general
nature has distributed muscles only in some principal directions
around a moveable point, for example around the eye, in those of
elevation, depression, adduction and abduction; the combination of
these simple motions produces the compound ones. If the adductor and
depressor contract equally, the eye will be carried exactly in a middle
direction; if one acts with more force than the other, it will be
carried a little nearer the other; so that the four muscles, by moving
separately, or two by two in an equal manner, carry the eye in eight
different directions. In all the intermediate directions, there is also
a simultaneous action of two muscles, but always a superiority in the
action of one of them. Thus almost all the motions of circumduction
operate.
When two opposite muscles contract, the moveable part is not moved;
they are perfect antagonists. When two muscles which contract at
the same time are placed in the same direction, there is no loss
of power; this is what takes place when the genio-hyoideus and the
mylo-hyoideus depress the jaw or elevate the os hyoides; these muscles
act completely together. But when two muscles are in part opposed and
in part in the same direction, as the sterno-mastoidei, one portion
of the forces is destroyed and the other remains. The action by which
the sterno-mastoidei tend to carry the head to the right or the left,
is nothing; that alone by which they direct it downwards produces its
effect which is double, considering the action of the two muscles,
which are thus at the same time acting together and antagonists.
Hence we see that this applies not only to the motion produced by the
contractility of texture, but also very often to those which the animal
contractility occasions.