between stimulus and response. He can vary the strength of the stimulus and vary the weight which the muscle has to lift. He can observe the progressive onset of fatigue, and otherwise gain much information regarding the behaviour of muscle as an isolated piece of apparatus. It is the ambition of the expert to obtain absolutely correct records of the time-phases and of the changes in electric potential of nerve and muscle under varied experimental conditions. For this purpose he needs the finest apparatus which instrument-makers can furnish, and the knowledge and dexterity requisite for its employment. Consider, for example, the record of the change of form. A nerve-muscle preparation, obtained by the method already described, is arranged so that the point of the lever scratches on a rapidly travelling blackened surface. As the muscle contracts it makes a “tracing.” A tuning-fork vibrating at the rate of, say, 400 times a second also scratches a tracing on the same travelling-plate. It is easy to time the several phases of contraction and relaxation by comparing them with the undulations made by the tuning-fork. By means of an induction shock a single impulse is generated in the nerve and a single spasm evoked in the muscle. Our tracing shows that the spasm lasts about one-tenth of a second, and that about half this time is occupied by contraction, and half by relaxation. But the ascending curve is usually a little steeper than the descending curve, and the apex a little nearer to the commencement of ascent than to the termination of descent. An electric signal marked the instant at which the current was sent into the nerve. The time taken by the impulse in travelling from the spot where the electric current entered the nerve to its junction with the muscle can therefore be estimated. The contraction begins so much sooner or later, according as the shock is delivered nearer to, or farther from, the muscle. By shifting the electrodes up and down the nerve, the rate at which the impulse travels is directly measured. After the time that the impulse took in reaching the muscle has been allowed for, there still seems to be an interval before the muscle begins to shorten. This was termed the “latent period,” under the impression that some time is actually lost in turning the nerve-impulse into a muscle impulse. The impulse was supposed to be latent in the end-plates of the nerve. Various hypotheses were formulated as to the nature of the transformation. The progressive improvements in apparatus and methods is testified by the diminution in this latent period as given in the text-books of successive decades. It is now put at ¹/₄₀₀ second, and is regarded by most physiologists as a delay due to the inertia of the muscle. Owing to its elasticity, the molecular change in muscle does not immediately affect its shape. When the latent period appears to be longer—say ¹/₁₀₀ second—the balance is due to the inertia of the recording apparatus.