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the other has been tetanised at intervals by an arrangement I need not at present describe. You observe that the mercury has risen in each case. I add a little lime-water to the tube containing the muscle that has been working hard, and you see how muddy it Fig. 61.— Muscle in tube of oxygen over mercury. a, platinum wire fused into end of tube and connected with small hook, from which a frog's limb is suspended; c, toe of limb ; b, trough containing mercury. A small amount of mercury is on the side of the tube between mercury in b and toe at c, so that induction shocks sent in by x and y readily tetanise the limb. The limb receives tetanising shocks at intervals of thirty seconds, and the experiment may go on for sixty or eighty minutes. at once becomes. The same experiment with the resting muscle does not show the same degree of muddiness, indicating that the resting muscle has not produced so much carbonic acid as the working muscle. Still it is interesting to observe that even