When an impulse is generated artificially by an induction shock, a single spasm or twitch is the result; but in Nature contraction is never limited to a single twitch. Impulses descending from a motor nerve-cell to a muscle are always rhythmic. They follow at the rate of eight or ten a second in human nerves; and since in our muscles contraction and relaxation take longer than in a frog, a second impulse reaches the muscle before the effect of the first has passed away. The muscle has not had time to relax, when it is again called upon to contract. Hence a summation of contractions. The muscle continues to shorten until the maximum of contraction is reached. This condition is termed “tetanus,” to distinguish it from a single spasm. In fullest contraction the length of a muscle may be diminished by one-half, or even by two-thirds.
It would be impossible to treat of the =electrical phenomena= displayed by nerves and muscles without presupposing some acquaintance with the methods and laws of physics. As this is contrary to our understanding with our readers, we must be content with the statement of a few salient facts. At the moment when an impulse is passing along a nerve, or a wave of contraction along a muscle, the electric potential of the active part of the structure, whether nerve or muscle, is different from that of the not-acting parts on either side of it. A battery in its commonest form is a glass vessel containing sulphuric acid in which a plate of zinc and a plate of copper are immersed. The zinc is electro-positive as regards the copper. In a muscle the contracted portion is electro-positive as regards the parts uncontracted. The degree of positivity can be measured by connecting the muscle at two spots with the two wires of a galvanometer. When one wire makes contact with the contracted portion, and the other with a part which is not contracted, a current passes through the galvanometer, causing its needle to swing; and since the wave of contraction is not stationary, but passes down the muscle, the current is subsequently reversed. The wave, as it were, first tilts up one end, and then, passing on, tilts up the other, letting down the first. The contracted spot is electro-positive to the spot not contracted, and then the latter, contracting, becomes electro-positive to the former, which has relaxed. The needle of the galvanometer swings first to the left, then to the right. The importance of this method of investigation lies in the fact that the electric variation exactly represents, both in time and in intensity, the change which is occurring in nerve and in muscle. By following it, we can ascertain the rate at which an impulse travels down a nerve. We can determine its length and its “form.” Represented 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.