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.])
Diagram 27.—Effect of Temperature.
Diagram 28.—Veratria Curve.
Finally, there are the electrical changes in muscle. These, again, may be passed over briefly, since they are not easily understood or described. To put the facts in a nutshell, the part of a muscle which is in activity is negative to all other parts. Thus, if a muscle be dissected out and cut across, the activity at the seat of the injury, while it lasts, causes a current to pass through a galvanometer from uninjured parts to the wounded. ([See Diagram 29.]) Again, if a muscle be dissected out without injury, connected at two points with a galvanometer, and then stimulated at one end, as the wave of contraction passes along it, first one, then the other, contact becomes negative. ([See Diagram 30.]) S, Stimulating electrodes; N, contraction which marks the wave of excitation passing along the muscle; G, galvanometer which shows that the seat of activity (N) is negative to the rest of the muscle.
Diagram 29.—Injury Current: Cross-section of Muscle Negative to Rest.
