As evacuating the blood from a living animal is rather a severe operation, and might have occasioned some fallacy in the last experiment, by subjecting the frog, in which this was done, to a greater degree of pain, and consequently of exhaustion, than what the other was subjected to, I crushed the brains of two other frogs before I proceeded, as in the former experiment, to withdraw the blood from one of them. Instead of forty, I injected no more than thirteen drops of the strong solution of opium, into each of the hearts of these frogs. The instant the injection had entered, both hearts became white, and ceased from contracting. Forty eight hours after the injection of the opium, the contractions excited by the metals in the frog, deprived of its blood, had become very slight, particularly in the limb whose vein and artery had been opened. The other frog still continued to contract with so much vigour, as to raise its body from the plate of zinc, upon which it was laid. Seventy two hours after the injection, no contractions could be excited in the frog, from which the blood had been withdrawn, except some very slight ones in the leg, whose artery and vein had not been opened. The contractions in the legs of the other frog, continued still so vigorous as to raise its body from the plate, and some were produced even by mechanical irritation. Ninety six hours after the opium had been injected, (both the frogs having lain out of water all night,) that without blood was found quite putrid. In the other, the contractions, produced by exciting the legs, were sufficiently strong to move the feet: as the body, however, had become putrid and offensive, it was thrown away. EXPERIMENT III. The heads of two other full grown and lively frogs, having been crushed, their hearts were laid bare, and the blood was evacuated from one of them, as in the former experiments. A small portion of the skull of each then being removed, eight drops of the strong solution of opium was injected upon their brains. At least half the quantity seemed to return from the wound. Both frogs became instantaneously motionless after the injection, but, in about an hour, were considerably recovered. Spontaneous motions continued during more than fifty hours, in the legs of that from which the blood had not been drawn, and contractions were excitable by the metals, upwards of 24 hours after they had ceased to be so, in that from which the blood had been drawn. The following experiments may be deemed still more satisfactory, than the preceding, from the circumstance of the comparison having been instituted, between the effects of opium, upon different, but similar parts of the same frog, differently circumstanced. EXPERIMENT IV.