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.
In joy and intense pain there is a degree of emotion in which the intonation of the voice is changed, because the nerves which move the muscles of the larynx no longer regularly adjust the vocal cords. This is the origin of the tremolo which serves to heighten the pathetic expression in singing. Many are scarcely able to speak, but stammer under the influence of an emotion. It is difficult to pitch a loud note and sustain it with expanded chest without the voice trembling; in the same way one cannot scream for any length of time without the voice turning shrill and harsh, because the muscles tire and the movements of the larynx can only be imperfectly regulated; similarly, when we write after running or violent exercise, certain unusual flourishes appear which make the characters unrecognisable.
I have noticed a curious tremor during inspiration in suffering men and animals. I have found it in a less degree also in healthy animals, particularly in dogs. At every inspiration there is a very noticeable tremor of the limbs and of nearly the whole muscular system. The excitement arising in the nerve-centres to produce a contraction of the diaphragm and of the muscles of the thorax seems to have become so strong, that it goes beyond the limits of the respiratory centre and diffuses itself over a great part of the nerves. In anger, fear, and mental perturbations, when the stormy winds of passion rage, waves flow to all parts of the nervous system, then break, revealing themselves in the agitation of the muscles.
Trembling often has a peripheral origin, and may be due to both heat and cold. It is sufficient to hold the arm in water heated up to 48° or 50° in order to produce a visible tremor. This fact, which I observed repeatedly on my brother, corresponds to the chattering of the teeth when a cold stream of air strikes our face.