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