on paper, it is a wave. This wave travels in warm-blooded animals with the rapidity of 35 metres in a second. When it reaches a muscle, its rate—that is to say, the rate at which the wave of contraction invades the muscle—is 6 metres in a second. The time during which any particular level in the muscle remains contracted in a single spasm, under the influence of an artificial stimulus, is about 0·05 second. The length of the wave is 300 to 400 millimetres. These measurements give us a very clear idea of the events which occur in a nerve-muscle. An impulse picked up by a motor cell in the spinal cord runs down its axon—termed later a nerve-fibre—with great rapidity. Even the most distant muscle is reached in less than one-thirtieth of a second. From the end-plate of the nerve it travels in both directions along the muscle-fibre—or group of fibres, since each nerve divides into branchets for thirty to forty muscle-fibres—with reduced velocity. Every particle of each fibre rises and falls; but, seeing that the wave of contraction is much longer than the fibre, the whole fibre is in a state of contraction at the same time, although not with equal vigour throughout its whole length. We cannot dismiss the further consideration of the electric phenomena of nerves and muscles without some inquiry into their meaning. It is evident that they are intimately related to the molecular changes which constitute an impulse. But at present the physics of the phenomena are beyond our grasp. We may speak in a general way of dissociation of ions; but we do not really know what is happening at the spot which is in a state of impulse. We cannot bring the transformation which it is undergoing into line with chemical and physical transformations which we understand. Probably the electrical phenomena which mark it are not peculiar to muscle and nerve. All living changes of state are of the same nature. Cellular activity, or protoplasmic activity, to use a better term, wherever it occurs, is accompanied by electrical change. But it so happens that nerve-substance and muscle-substance have a definite orientation which gives to the electric force a cumulative effect. In a liver-cell it is dispersed in all directions. In a muscle the change of potential at one particle is added to the change at the next, until the sum of all these changes, transmitted along the length of the fibre, is sufficiently large to deflect the needle of a galvanometer. Owing to its summation it attracts our attention.