The way in which voluntary muscle is studied is very simple. A frog is killed by thrusting a probe into the brain and down the spinal cord, and a muscle is then dissected out and attached to a piece of apparatus (see Diagram 21) in such a way that on its contracting it raises a lever, and draws a line on a moving surface. The rate at which the surface is moving is ascertained, so that the nature of the curve, which is a graphic record of the contraction, can be analyzed. (See Diagram 22.) For instance, when an electric shock is used to make the muscle contract, we find that a slight shock causes a small contraction, as shown by a low curve, while a stronger one, up to a certain point, causes an increase. [Illustration: DIAGRAM 22.—GRAPHIC RECORD OF A RESPONSE TO A SINGLE STIMULUS APPLIED AT A. Lower line = tuning-fork records of ⅟₁₀₀″.] But having described how muscle is studied, it is only necessary to state a few facts concerning it; to discuss muscle, fully describing the experiments by which its more obscure properties have been elucidated, and the devices by which causes of error have been eliminated, would fill volumes. [Illustration: DIAGRAM 23.—CONTRACTIONS WITH TWO STIMULI AT DIFFERENT INTERVALS OF TIME.] Muscle is thrown into a state of contraction by an impulse reaching it from a nerve, but it contracts quite as readily if excited directly by a mechanical or electrical shock. A second shock causes a second contraction, or, if the muscle is still in a state of contraction owing to the first, causes it to contract still more. (See Diagram 23.) If a number of stimuli are applied to a muscle in such rapid succession that the effect of the preceding one has not passed off by the time the next arrives, it will contract as far as possible, and remain contracted—a state known as tetanus. (See Diagram 24.) A muscle is therefore kept in a state of contraction by a continuous nervous effort, not arranged and then left contracted. [Illustration: DIAGRAM 24.—TETANUS.] [Illustration: DIAGRAM 25.—FATIGUE CURVES. Fast drum: a, point of stimulation. Every tenth contraction recorded.] [Illustration: DIAGRAM 26.—EFFECT OF FATIGUE ON MUSCULAR CONTRACTION. Slow drum. Every contraction recorded.] Various conditions alter the character of a muscular response. With repeated stimuli at short intervals a muscle fatigues, and each contraction becomes smaller in extent and longer in duration. (See Diagrams 25 and 26.) If the muscle has to lift a load it has a certain check on its contraction, and its relaxation time is shortened. Temperature also affects muscular contraction, moderate increase causing a sharper, and moderate cooling a slower, rise and fall of the lever on stimulation. (See Diagram 27.) Lastly, we have drugs which exert an influence, but the only one of these which it is necessary to mention here is veratria, which makes the slowly contracting fibrils continue their activity after the quick ones have subsided. (See Diagram 28.)