The force of the contraction of the muscles of animal life varies
much, according as it is brought into action by stimulants, or by the
cerebral action.
Every irritation made upon a muscle laid bare produces only a brisk,
rapid motion, but generally not very powerful. I have frequently
satisfied myself in my experiments that it is impossible to approximate
even at a great distance by this means, the great energy which
the brain communicates to the muscles of animal life. The organic
muscular system which stimuli directly applied put principally in
motion, never has exacerbations of force corresponding to those
which the animal contractility exhibits in so great a degree under
certain circumstances. It is then especially when the muscles move
in virtue of this last property, that the force of their contraction
must be considered. Now this contraction can, as we have seen, be
produced, 1st, by stimulating the brain in experiments; 2d, when
its excitement takes place in the natural state by the will, or by
sympathy. In the first case, the force of the contraction is never
very powerful, whatever may be the stimulant employed, either upon
the brain, or the nerves laid bare. I have uniformly observed a very
rapid convulsive motion, analogous to that produced by exciting the
muscles themselves, but never as strong as that which is the result of
vital action. Notwithstanding what some physiologists have written, we
can never by irritating the nerves of the flexors impart to them an
energy comparable to that which the will can give them. Irritate for
example the sciatic nerve in an inferior extremity which has just been
amputated, the toes will never bend with the force which they do in
certain cases in the natural state. I have twice made this experiment
in amputations performed by Desault. Unacquainted then with physiology,
I was much struck with this phenomenon.
In the cerebral excitement and in that of the spinal marrow, we cannot
so well appreciate the force of the contractions which result from
it, as when we stimulate an insulated nerve; in fact, all the system
entering then into convulsive action, the extensors destroy in part
the effort of the flexors and vice versa. The muscles simultaneously
in action, counterbalance, interfere with and injure each other. The
stimulant which gives the greatest force to the contractions, has
always appeared to me to be galvanism.
In the living state, the force of muscular contraction depends on two
causes; 1st, on the muscle; 2d, on the brain. These two causes are in a
variable proportion; it is necessary to consider them separately.
Under an equal cerebral influence, a muscle well nourished, which
appears distinctly through the integuments, and has very large fibres,
will contract much more strongly than that which is delicate, slender,
with loose, pale, small fibres, and which makes but a slight prominence
under the integuments. In our ordinary manner of considering muscular
force, it is to this state of the muscles that we especially attend.
The statues which exhibit strength and vigour, have always as an
attribute a powerful development of the muscular forms. When the brain
acts upon these muscles with energy they are capable of extraordinary
motions. I shall not relate examples of the astonishing efforts of
which they are susceptible. Haller and others have cited many of them,
either in the muscles of the back in carrying burdens, or in the
muscles of the superior extremities in raising great weights, or in the
inferior extremities in leaping or in order to preserve attitudes which
suppose enormous resistances to be overcome.
It is especially the cerebral influence that increases much the force
of muscular contraction. The will can raise this force very high; but
the different excitements that are foreign to it, raise it infinitely
more. We know the force that a man acquires in anger, that of maniacs,
of persons in the cerebral excitement of a fever, &c. In all these
cases the impulse communicated by the brain, is sometimes such, that
the most delicate muscles of the feeblest woman surpass in energy those
of the strongest man in the ordinary state.
The force of muscular contraction is then in a ratio compounded of the
force of the organization of the texture of the muscles, and of the
force of the cerebral excitement. If both are slight, the motions are
almost nothing; if both are at the highest degree, it is difficult to
conceive how far the effects may go which result from them; a maniac
with thick and strong muscles is capable of efforts that we should
in vain attempt to calculate. If the nervous force is very powerful,
and the muscular texture feeble, or if an inverse state exists, the
phenomena of contraction are less. In general nature has almost always
united these two things in this last manner. Women and children who
have a weak fleshy texture, have a very great nervous mobility; men
on the contrary, those especially with athletic forms, whose nervous
systems are less easily excited, receive more rarely the causes of a
strong influence upon their muscles.
Whatever may be the point of view in which we consider the force of
the contractions of the muscular system of animal life, it is always
very great in proportion to the effect which results from these
contractions. Nature in the economy follows a law the reverse of that
of the motion of our common machines, the great advantage of which
is to increase the moving powers, to produce a great effect with a
small force. Here there is always a great expenditure of force for
a small effect, which is owing to the numerous causes that tend to
destroy the effect of this force. 1st. The muscles act almost always
upon a very unfavourable lever, upon that in which the power they
represent is nearer the point of support than the resistance. 2d. All
in contracting have to overcome the resistance of the antagonists. 3d.
As in each motion there is always a fixed point, the effort which,
after contraction, is carried upon this fixed point, is entirely lost.
4th. Various frictions injure also the motion. 5th. The obliquity
of the insertion of the muscles upon the bones, an obliquity that
approaches nearer a horizontal than a perpendicular direction, an
obliquity not less remarkable for the fleshy attachments upon the
tendon or aponeuroses, offers a double cause of weakness. All these and
many other reasons which we might with Borelli, who was the first to
make these important remarks upon muscular motion, add to them, prove
that the absolute or real force of the muscles is infinitely superior
to their effective force. Yet all are not so unfavourably arranged;
in some, as the solæus, the insertion is perpendicular to the bone;
in others, as the muscles which act upon the head, we observe that
they are powers of a lever of the first kind. In general, in order to
estimate the force of a separate muscle, the deltoid, for example, it
is necessary especially to have regard to the distance of its insertion
at the point of support, to the degree of opening of the angles formed
by the fleshy fibres upon the tendon, and afterwards by the tendon
upon the bone, and to the division of the forces between the fixed and
moveable points.
Some advantages seem to compensate in a slight degree in certain
muscles for their bad arrangement as to the power of motion; such are,
1st, the sesamoids, the patella, the different eminences of insertion,
the enlargement of the large bones at their extremities, &c. which
remove the fibres to a distance from the moveable points; 2d, the
intermuscular fat, that which is in the neighbourhood of the muscles,
the fluid of synovial sheaths, which facilitate motions by lubricating
the surfaces that execute them; 3d, the aponeurotic expansions that
confine down the motions on the extremities; 4th, these motions
themselves, those of flexion for example, which, as they take place,
diminish the obliquity of the insertion of the flexors, and render it
even perpendicular, as has been well observed by a modern author.
Many calculations have been made upon the waste of muscular motion,
upon the effort of a muscle which contracts, compared with the effect
that results from it. They can never be precise because the vital
forces vary to an infinite degree, because they are not the same in
two individuals and because the cerebral influence and the force of
muscular organization are never in constant proportion in the same
subject. It is a peculiarity of the vital phenomena to escape all
calculations, and to exhibit, like the forces from which they emanate,
a character of irregularity which distinguishes them essentially from
the physical phenomena. Let us conclude only from the preceding
observations, that the muscular effort carried to the highest point by
cerebral excitement, can produce astonishing effects, which suppose a
force of contraction hardly conceivable; such is the rupture of the
strong tendons, of the patella, the olecranon, &c.; such is also the
resistance often opposed to the enormous distensions that are used in
luxations, fractures, &c.