In order to ascertain the irritability of muscle itself or the readiness with which it responds to various stimuli independently of the nerves within it, the muscle is first poisoned by curare, and then exposed to various conditions, or to the action of drugs. The muscle thus poisoned by curare, woorara, woorali, or urari (for the poison has all these names), is much less sensitive to the action of fara-daic currents. The readiest way of testing its excitability is by the making and breaking of a constant current, the strength of which can be estimated very exactly by using du Bois Rey-mond's rheochord. The excitability of muscles is increased by heat and diminished by cold. It is increased by physostigmine and diminished by most poisons which paralyse muscle.2

Contraction

When the ends of the muscle are not kept apart by force too great for it to overcome, and it is stimulated by heat, mechanical injury, chemical irritants, or electricity, it contracts and then relaxes.

The form of this contraction varies according to the species of animal, and the particular muscle tested.

In cold-blooded animals, as a rule, the contraction is slower than in warm-blooded animals. It is not alike in all the muscles of the body of mammals. Thus in the rabbit there are two kinds of muscles - red and white; the white muscles contract more quickly and relax more quickly than the red ones. The muscle usually employed in experiments is the gastrocnemius of the frog, freshly prepared, with the nerve and end of the femur attached to it.

1 Brunton and Cash, Phil. Trans., 1884, p. 197.

2 Harnack and Witkowski, Arch. f. exp. Path. u. Pharm. v. 1876, p. 402.

The femur is fixed in a clamp, and the lower end of the muscle is attached to a writing lever usually loaded with a weight (Fig. 37). The end of this lever writes upon a revolving cylinder (Fig. 38), which is made to rotate with greater or less rapidity. The rate of revolution is usually ascertained by marking the time upon it by means of an electro-magnet (Fig. 39) communicating with a clock or metronome, or, when the revolution is quick, with a large tuning-fork vibrating 100 times or more per second. When the cylinder is not in motion each contraction of the lever makes a straight line upon it (Figs. 40 a and 46); when the cylinder is moving, the lever describes a curve which is more or less elongated, according to the rapidity of the cylinder's rotation (Figs. 40 and 41).

Fig. 37.   Apparatus for registering muscular contraction. It consists of an uprightstand on which two horizontal bars may be moved by a rack and pinion.

Fig. 37. - Apparatus for registering muscular contraction. It consists of an uprightstand on which two horizontal bars may be moved by a rack and pinion. The upper bar ends in a clamp, the lower carries a delicate lever, the part near the hinge being of metal, and the part beyond of light wood tipped with quill or tinfoil. a, a, wires for exciting muscle; b, muscle; c, writing lever. In the figure no arrangement is shown for exciting the nerve, and for the sake of simplicity the weight is shown directly under the muscle. In actual experiment, however, the weight should be applied close to the axle, or on it, so as to lessen oscillation due to the inertia of the lever.