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.