The circumstances which call forth muscle contraction may be enumerated thus:

1. Mechanical Stimulation

Any sudden blow, pinch, etc., of a living muscle causes a momentary contraction, which rapidly passes off when the irritation is removed.

2. Thermic Stimulation

If a frog's muscle be warmed to over 300 C. it will begin to contract, and before it reaches 40 C. it will pass into a condition known as heat rigor, which will be mentioned presently. If the temperature of a muscle be reduced below o° C. it shortens before it becomes frozen.

3. Chemical Stimulation

A number of chemical compounds act as stimuli when they are applied to the transverse section of a divided muscle. Among these may be named: (1) the mineral acids (HC1,.1 per cent.) and many organic acids; (2) salts ot iron, zinc, silver, copper and lead; (3) the neutral salts of the alkalies of a certain strength; (4) weak glycerine and weak lactic acid; these substances only excite nerves when concentrated; (5) bile is also said to stimulate muscle in much weaker solutions than it will nerve fibres.

4. Electric Stimulation

Electricity is the most convenient form of stimulation, because we can accurately regulate the force of the stimulus. The occurrence of variation in the intensity of an electric current passing through a muscle causes it to contract. The sudden increase or decrease in the strength of a current acts as a stimulus, but a current of exactly even intensity may pass through a muscle without further exciting it, after the initial contraction has ceased. A common method of producing such a variation is that of opening or closing an electric circuit of which the muscle forms a part, so as to make or break the current; and thus a variation of intensity equal to the entire strength of the current takes place in the muscle, and acts as a stimulus.

Du Bois Reymond's Inductorium with Magnetic Interrupter.

Fig. 183. Du Bois-Reymond's Inductorium with Magnetic Interrupter.

c. Primary coil through which the primary, inducing; current passes, on its way to the electro-magnet (b). i. Secondary coil, which can be moved nearer to or further from the primary coil (c), thereby allowing a stronger or weaker current to be induced in it. This induced current is the stimulus. b. Electro-magnet, which on receiving the current breaks the contact in the circuit of the primary coil by pulling down the iron hammer (//), and separating the spring from the screw of e. When using Helmholtz's modification (g'), e is screwed up, and the current brings the spring in contact with the point of the pillar (a), and so demagnetizes (b) by "short circuiting " the battery. When tetanus is to be produced, the wires from the battery are to be connected with g and a.

When a single contraction is required, the magnetic interrupter is cut out by shifting the wire from a to the binding screw to the right of f.

The direct current from a battery (continuous current) is used to stimulate a muscle in certain cases, but a current induced in a secondary coil by the entrance or cessation of a current in a primary coil of wire {induced current) is more commonly employed on account of the greater efficacy of its action. The instrument used for this purpose in physiological laboratories is Du Bois-Reymond's inductorium, in which the strength of the stimulus can be reduced by removal of the secondary coil from the primary. It is supplied with a magnetic interrupter, by means of which repeated stimuli may be given by rapidly making and breaking the primary current (interrupted current) (Fig. 183).

The irritability of muscle substance is not so great as that of the motor nerves; that is to say, a less stimulus applied to the nerve of a nerve-muscle preparation* will cause contraction than if applied to the muscle directly. In experimenting on the contraction of muscle, as already stated, the nerve is commonly used to convey the stimulus, because, when an electric current is applied to the nerve, the stimulus is the more safely and completely distributed throughout the muscle fibres than when it is applied directly.