[13]. Essays Physical and Literary.
[14]. This solution, which is the same that I employed in all my subsequent experiments, was of the same strength with that used by Dr Alexander in the greater number of his, viz. an ounce of crude opium mixed in a mortar with two ounces and a half of water, and filtered through paper, after having remained twelve hours, in a close corked bottle, near a chamber fire.
APPENDIX.
I was unwilling to interrupt the narration of the preceding experiments, by the mention of the following facts, which they afforded me an opportunity of observing, as they were not immediately connected with the objects on account of which the experiments were instituted; and I have yet some few to relate, which, from the haste with which these sheets were prepared for the press, I had omitted to insert in their proper places.
1. In one of my first experiments, in which I had occasion to suffer a frog to remain tolerably entire, so long as contractions could be excited in any part of its body, I was surprised to find, on removing its sternum, that its heart had ceased to contract, nor could be roused by the application of any stimulus whatever, notwithstanding the contractions in its hind legs, excited by the metals, were still vigorous, and continued so for several hours afterwards. On paying particular attention to this circumstance in another frog, upon whose brain opium had been injected, I found that its legs continued excitable, upwards of forty hours longer than its heart. This discovery of the continuance of the contractile power, in the muscles of the posterior extremities, so long after its disappearance in the heart, is so contradictory to the opinion generally received upon this subject, and long established among physiologists, that I can scarcely expect it should be credited, by those who may not themselves have opportunities of observing it. It is a fact, however, of which, in the course of these experiments, I have had the most satisfactory and uniform proofs, both in such frogs as have, and in such as have not, been under the influence of opium. If a different opinion has hitherto been held by experimentalists upon this subject, it should be recollected, that, till the discovery made by Galvani, we had no means of ascertaining the presence of the contractile power of muscles, which had not, at the same time that they indicated its continuance, a tendency to destroy it, and consequently to render it impossible for us to trace its natural progress to extinction.
I have, more than once, observed the same circumstance in both cats and rabbits.
2. Dr Alexander, in his excellent Thesis already quoted, tells us, that the contractility of all the voluntary muscles of frogs was destroyed in the course of a very few minutes, by injecting eight drops of a strong solution of opium in water, (similar to that which I employed) upon the surfaces of their brains. But that the contractions of their hearts did not appear to be much, if at all, affected by this treatment. In all the similar experiments, which I have made, the event has been very different. I have not found it possible by any quantity, either of aqueous or of spirituous solution of opium, injected upon the brains of frogs, to produce that rapid extinction of the contractility of their voluntary muscles, of which Dr Alexander speaks. They commonly recovered in less than an hour, from the complete insensibility and paralysis, first occasioned by the injection of the opium, and after that time, their spontaneous motions almost always continued for several hours longer, and, by the application of the metals, contractions were excitable even for days. Their hearts, as I have already said, uniformly lost their susceptibility of the action of stimuli, long before their posterior extremities.