"While you and Murchie were absent the first day that I went out to your stable, I succeeded in drawing off some of the blood of Lady Lee," Craig resumed, talking to Broadhurst. "Here, in my laboratory, I have studied it. Lady Lee, that day, had had no more than the ordinary amount of exercise, yet she was completely fagged."
By this time the little guinea-pig had become more and more listless and was now curled up in a corner sound asleep.
"I have had to work very hurriedly this morning," Craig continued, "but it has only been covering ground over which I have already gone. I was already studying a peculiar toxin. And from the fluid I obtained from Murchie's body, I have been able to calculate that a deadly dose of that same powerful poison killed him."
Kennedy plunged directly from this startling revelation into his proof.
"Perhaps you have heard of the famous German scientist, Weichardt, of Berlin," he resumed, "and his remarkable investigations into the toxin of fatigue. Scientists define fatigue as the more or less complete loss of the power of muscles to respond to stimulation due to their normal activity. An interval of rest is usually enough to bring about their return to some degree of power. But for complete return to normal condition, a long interval may be necessary.
"As the result of chemical changes which occur in a muscle from contraction, certain substances are formed which depress or inhibit the power of contraction. Extracts made from the fatigued muscles of one frog, for instance, when injected into the circulation of another frog bring on an appearance of fatigue in the latter. Extracts from unfatigued muscles give no such results. More than that, the production of this toxin of fatigue by the exercise of one set of muscles, such as those of the legs in walking, greatly diminishes the amount of work obtainable from other unused muscles, such as those of the arms."
Kennedy went on, looking at the sleeping guinea-pig rather than at us:
"Weichardt has isolated from fatigued muscles a true toxin of a chemical and physical nature, like the bacterial toxins, which, when introduced into the blood, gives rise to the phenomena of fatigue. This is the toxin of fatigue—kenotoxin. Those who have studied the subject have found at least three fatigue substances—free sarcolactic acid, carbon dioxide, and monopotassium phosphate, which is so powerful that, after the injection of one-fifteenth of a gram, the poisoned muscle shows signs of fatigue and is scarcely able to lift a weight easily lifted in normal conditions. Other fatigue products may be discovered; but, if present in large quantity or in small quantity for a long time, each of the substances I have named will cause depression or fatigue of muscles.
"Further than that," continued Kennedy, "the depressing influence of these substances on what is known as striated muscle—heart muscle—is well known. The physician at the Idlewild might very well have mistaken the cause of the relaxation of Murchie's heart. For German investigators have also found that the toxin of fatigue, when injected into the circulation of a fresh animal, may not only bring on fatigue but may even cause death—as it did finally here." Kennedy paused. "Lady Lee," he said, looking from one to the other of his audience keenly, "Lady Lee was the first victim of the fiendish cunning of this—"
A shrill voice interrupted.