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.