II.
Diagram 21.—Apparatus for recording a Muscular Contraction.
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.
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.
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.
Diagram 24.—Tetanus.
Diagram 25.—Fatigue Curves.
Fast drum: a, point of stimulation. Every tenth contraction recorded.
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.])
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.
Diagram 30.—Action Current.
In passing, it may be mentioned that, as the heart is a muscle slung obliquely across the body, and waves of contraction are continually passing down its long axis, the whole body is affected by continual electrical changes. By very delicate instruments it can be demonstrated that with each beat the two hands alternately become electrically positive and negative to each other.
Whilst dealing with the electrical phenomena of muscle, it may be as well to state that nerve fibres, which are studied with very much the same apparatus, show the same electrical changes, the point of injury or of the greatest activity being negative to all the rest. Single cells are less easily investigated, but in glands it is possible to show that the same rule holds.
Undoubtedly the most curious fact about the generation of electricity by protoplasm is that, by a modification of muscle and nerve, which causes them to lose their ordinary properties, they are converted into a special organ for giving electric shocks. Armed with powerful batteries of this description, an otherwise rather helpless class of fish are enabled to defend themselves from their enemies, and deal unexpected death to their more agile prey.
Having now run over a few of the physical properties of protoplasm, we may pass on to a brief investigation of the movements we find in the body of man.