If we look at a muscular filament as fine as a hair under the microscope, we see that it consists of nearly a hundred extremely fine fibrillæ lying close to each other in the form of bundles. It is these which together form those minute threads seen with the naked eye in fibrous meat. Each fibril, seen under a powerful microscope magnifying three or four hundred times, appears formed of a series of muscular elements, or, let us say, of so many little boxes, piled one on the top of the other like a battery and about two-thousandths of a millimetre in thickness. Each little box is prismatic in form and terminates in two flat ends. It was an English physiologist who described these little boxes for the first time; they are therefore known in science as Bowman’s muscular elements. The resemblance of each fibril to a voltaic pile is so great that some have tried, but in vain, to establish an analogy between their functions.
The nerves which go to the muscles send their branches to every fibre, like the fine cords of a match which serve the purpose of firing the powder in the mines at a distance, or, to make use of another simile nearer the truth, although still very far from reality, like the thin wire which conducts the electric spark to a cartouch of dynamite. When the nerve discharges its influence into the muscles, a very rapid molecular change takes place in the substance contained in the cartouches or muscular boxes, which contract, pressing their ends close together. Scarcely has the action of the nerves ceased than they relax and resume their previous form. It is well known, that when a muscle contracts, it becomes thicker and shorter; it suffices to take hold of the arm a little above the elbow and then to bend it in order to feel how the biceps muscle swells and hardens. Wherever a muscular contraction is produced, we may always imagine it as the transverse thickening of an innumerable series of little prismatic boxes, which at the same time shorten in the direction of the fibres.
The blood, which circulates in all the most remote corners of the organism, brings, so to speak, fresh explosive materials to charge the muscles again, cleansing it also from the soot and scoriæ. The movement of the blood in the cells and fibres of the body is like a brook flowing through a village, from which every house may draw water for its needs, and into which any useless articles may be thrown and so carried away.
If we stop our ears with our fingers, we hear a dull noise, like the distant roar of cannon or a prolonged peal of thunder. This thunder is produced by the contraction of the muscles, for the nerves do not exercise a constant action on the fibres, but develop their influence in very rapid, irregular shocks, like the rattle of musketry in a battle.
It is seldom that a nervous discharge takes place all at once, like a volley of shot, in order to produce an instantaneous contraction, or, as physicians would say, a clonic contraction. Generally the discharges in muscular exertion begin in a few fibres; when these become weak, others come to reinforce the contraction; these cease, and others are charged; those are exhausted and others take up their work; thus a continuous tension of the muscle may be maintained. We must, therefore, consider the contraction of a muscle as an extremely rapid trembling of its most minute parts. When we become weaker through illness or from any other cause, we tremble because the contractions are so drawn out and distended as to show the elements composing them. If we poison a frog with some substance which diminishes the vitality of the nerves, there is a trembling of the legs at every exertion which the animal makes to move. In the violent and fatal contractions of tetanus, one can hear the muscular noise even at a distance. Those poor dogs which we see in the street cruelly poisoned with strychnine send forth during their convulsions, if laid on a sounding-board, a characteristic sound which arises from the extremely rapid vibrations of their muscles. It is the sound of tetanus.
IV
Trembling may be produced by two opposite causes, either by an excessive development of nervous tension, or by weakness. 'Les tremblements ont deux diverses causes: l’une est qu’il vient quelquefois trop peu d’esprits du cerveau dans les nerfs, et l’autre qu’il y en vient quelquefois trop.’ Descartes told us this two hundred years ago.
If we bend the forearm forcibly against the upper arm, as though to touch the shoulder with the clenched fist, we notice at once that our hand trembles, because the discharges, by means of which the contractions are produced and regulated, do not exactly answer their purpose. If we press the butt of the musket too firmly against our shoulder, or shoot with a heavy gun, we hit the mark less easily because of the trembling of our arms. We may, however, in a great measure correct these physiological imperfections; thus it is that a few months of practice in drawing will enable anyone to draw straight lines and to outline with a firm touch.
In order to understand the whole mechanism of trembling, we must remember that in grasping an object, we not only make use of those muscles which bend the fingers but also of those which serve to open the hand. The work of the muscles which oppose a movement, and which are therefore called antagonistic, is extremely efficacious, and is indeed indispensable in order to graduate and regulate muscular actions with accuracy. When we wish to move our eyes, all the muscles enter into tension, but one prevails and guides them to the desired point. When we take hold of the pen to write, we do not only bend the flexors of the fingers but also involuntarily contract the extensors. Without this it would be impossible suddenly to arrest the hand, the eyes, or any other part of the body in rapid motion.
Exhaustion or over-excitement of the nerve-centres destroys the harmony of aim of muscular contractions. The hand trembles, because the tension of the flexors and extensors is no longer evenly and firmly, but jerkily maintained. If we endeavour to keep the arm stretched out, we find we are not able to regulate the nervous discharges in such a manner as to preserve the equilibrium of the muscles during work, they relax and contract alternately on one side or the other; scarcely do the flexors give way, than the antagonist muscles succeed in bending the arm in their direction, then these shorten in their turn, rapidly resuming their previous position; but no sooner has an effort been made by the muscles of the other side, than they are again overcome by the antagonists. In this way a perpetual wavering is brought about, and the organs of the body sway, waver, or tremble according to the rapidity with which the muscles relax, without the will being able to control them.