We may distinguish several groups of muscular poisons, but at present the classification is difficult, and the division into six groups based on that of Robert, which I have adopted, although it possesses some advantages, is far from satisfactory, and can only be regarded as temporary.

Group I

Leaves the irritability of the muscle unaffected, but diminishes the total amount of work it is able to do.

Group II

Diminishes the excitability of the muscle as well as-its capacity for work.

Group III

Diminishes the capacity for work, and produces marked irregularity in its excitability.

Group IV

Alters the form of the muscular curve.

Group V

Increases the excitability.

Group VI

Increases the capacity for work.

Group VI 78

Fig. 46. - Tracings showing the gradual loss of contractile power from fatigue in a normal muscle, o, and in one poisoned by carbolic acid, b. Each section, 0'-1', etc, shows the contractions in one minute. (After Gies.)

The poisons in Group I. do not alter the muscle curve, so that if the action of the poison were tested by a single contraction only, it would be supposed that the muscle was unaffected; they lessen, however, the amount of work which the muscle can yield.

The amount of work is estimated by the weight which a muscle raises multiplied into the number of times it is lifted and the height it is raised each time. These are ascertained by registering the contractions on a slowly revolving drum, as in Fig. 46, which shows the rapid exhaustion of a muscle poisoned by carbolic acid as compared with a normal one. The rapid exhaustion of muscles may also be observed in the form of the tetanus curve which, under the influence of such poisons, falls much more rapidly in height than that of the normal muscle.

1 Wedenskii, Archiv f. Anat. u. Physiol. Phys. Abthlg. 1883, p. 325.

This group contains a number of drugs having an emetic action.1 These are: apomorphine, asclepiadine, cyclamine, del-phinine, sanguinarine, and saponine, copper, zinc, and cadmium. Antimony has a somewhat similar action, but only in large doses, and after a great length of time. Arsenic, platinum, and probably mercury, act in the same way as antimony.2 Tin, nickel,3 cobalt,3 manganese,2 aluminium, and magnesium, have little or no action on muscle. Large doses of iron are nearly as powerful as arsenic, but in small doses it rather increases the amount of work the muscle can do.

Carbonic oxide at the atmospheric pressure does not affect muscular contractility, but abolishes it at a pressure of five atmospheres.

Perhaps we may take as a subdivision of this group those poisons which lessen the contractile power of the muscle without altering its irritability. When a muscle poisoned by one of these is stimulated, it may contract quite as readily as a normal muscle, provided the weight that it has to raise is but slight, but it cannot raise such a heavy weight as a normal muscle. This is tested by loading it with a given weight, and the slightest contraction is ascertained by adjusting the lever of the myograph in such a way that if raised in the very least it breaks a connection in an electrical current and causes a bell to ring. By this means contractions quite imperceptible to the eye are readily appreciated. Digitalis has an action of this sort, as I found in some experiments carried on under the direction of Professor J. Rosenthal in 1868, but not published.

Group VI 79

Fig. 47. - (After Harnack.) Shows the action of lead on muscle. a shows the contraction of a normal muscle after eighty stimulations; b, the irregular contractions of a muscle poisoned by lead after ten to fifty stimulations; c shows the slow relaxation of the muscle after contraction in a muscle poisoned by lead after numerous stimulations.

Group II. contains salts of potassium, lithium, ammonium,

1 Harnack, Archiv f. exp. Path. u. Pharm., Bd. ii. p. 299, and iii. p. 44.

2 Kobert, Arch. f. exp. Path. u. Pharm., Bd. xv. p. 22, and xvi. p. 361.

3 Anderson Stuart, Journ. of Anat. and Physiol., vol. xvii. p. 89.

quinine, cinchonine, oil of mace, alcohol in large doses, chloroform, etc.

Chloral, chloroform, and ether also belong to this group, but they might also be reckoned as belonging to Group IV., for they slow the ascent, lessen the height, and prolong the descent of the curve. Curare has a similar action.

It is usually stated that curare, while it paralyses motor nerves, leaves the excitability of the muscles unaffected, but this appears not to be quite correct, for, when very weak currents are employed, the muscle loses its excitability by them before the nerve, and the contractions of the muscle at the same time become unequal. It is perhaps not yet perfectly certain how far these appearances are due to the curare, and how far to the gradual death of the muscle.1

Group III. contains poisons of which lead is a typical example. These poisons cause the muscular contractions to become very unequal, although the stimuli are equal and regular. Emetine and cocaine have a similar action to lead. This action is probably due only to the gradual death of the muscle. It is produced also by ptomaines, and it may occur in muscles which are simply dying without being poisoned at all.2

Group IV. contains poisons which alter the form of the curve to a marked extent.

The action of veratrine is very peculiar: it does not lessen the rapidity of contraction, and even increases the height of the curve, but it prolongs the descent to an enormous extent.

Fig. 48.   Tracing of the contraction curve of a muscle poisoned by veratrine, showing enormous prolongation of the contraction, the recording cylinder making many complete revolutions before the muscle is completely relaxed.