EXPERIMENT IX.
Having laid equally bare both the sciatic nerves of a frog, at the upper part of its thighs, I passed a ligature round one of them, and drew it as tight as it was well possible, without dividing the nerve. I then removed a portion of its skull, and with a small brush, kept it constantly wet with laudanum during several hours. The frog soon became convulsed; and, during ten or twelve hours, continued in that state of exquisite sensibility, which opium never fails to produce in these animals. It may here be worth remarking, that, while they are in this state, the slightest touch of a feather, or even breathing upon them, excites instantaneous convulsions. The leg whose nerve was tied, remained paralytic during this time, but when it was laid upon zinc and excited with silver, it contracted as strongly as the other. After forty three hours, the contractions were very feeble in the leg whose nerve was not tied, but still vigorous in the other. After fifty three hours, no contractions could be excited in any part of the frog, except in the leg whose nerve was tied. In this they were sufficiently strong to move the foot, and continued so for more than an hour longer.
One of the crural nerves of another frog having been tied in a similar manner, eight drops of the strong solution of opium were injected upon its brain. The animal instantly became motionless, but, in less than an hour afterwards, was considerably recovered.
The contractions, excited by the metals, in the leg whose nerve was free, soon became more feeble than those excited in the leg, whose nerve had been tied. This disproportion, between them, continued increasing during ninety six hours, after the opium had been injected, when contractions could no longer be excited in the leg whose nerve remained free. In that, in which the nerve had been tied, they continued upwards of 4 hours afterwards.
EXPERIMENT XI.
Immediately after having divided the sciatic nerve, in one thigh only, of three other frogs, I injected as much of the strong solution of opium underneath their skulls, as could possibly be retained. The legs, in which the nerves had been divided, continued contractile several hours after the others had ceased to be so.
Hence, then, we see no reason for suspecting that the more speedy cessation of contractions in those legs, in which the crural arteries were tied, than in those on which no operation was performed, was owing to the pain occasioned by such operation, since even the more painful operations of tying or dividing the sciatic nerves, were attended with no such effect.
Upon the whole, therefore, it appears, that the conclusion which M. Fontana draws from his numerous experiments with opium, ‘That the circulation of the blood and humours in the animal machine, is the vehicle for opium, and that, without this circulation, it would have no action on the living body,’ is the very reverse of that which I am warranted to draw from the experiments I have just related; since the parts, most affected by the action of opium, were not those in which the circulation remained most entire, but those in which it had been almost altogether interrupted; and since in two parts where the circulation remained equal, and entire, the action of opium was rendered unequal, by interrupting the communication of one of them, by means of nerves, with the parts to which the opium was applied.
The existence, consequently, of any such principle in the blood, as that supposed by M. Fontana to exist there, is rendered far too problematical, even to allow me to expect that it can ever be proved: far less that it may turn out to be the same with that discovered by M. Galvani, or with that, whatever it may be, upon which the phenomena of nerves and of muscles may depend.