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fibres takes place, which is frequently fo great as to occafion a fecond over-exertion from even the ordinary ftimuli, and thus an unequal balance of the excitability and natural ftimuli may continue for two or more days.

But where the dose first administered is great, its fedative effects are almost immediately perceived. By a folution of opium, injected into the stomach of a frog, while the tranfparent membrane of its toes was under a good microscope, the dose being small, there was at first an increase, and afterwards a diminution of the blood's velocity. By a second and larger dose, given an hour after the first, the blood was feen to move immediately flower, and its velocity gradually decreafing, it ftagnated at length, and the animal expired.

A folution of opium injected into the intestines of a dog, brought on palfy of his posterior extremities, attended with convulfions and ftupor.

Some days after, when the dog was recovered, the like folution was injected, by a perforation through the integuments, into the abdomen of the fame dog: he became paralytic inftantaneously, and died in a few

minutes.

We are now to enquire, by what channel does opium at?-Seeing the many erroneous opinions that have prevailed,

vailed, we fhould be careful how we fuffer ourselves to be led away by great authorities, and fhould bow our affent only to facts, and fuch conclufions as naturally refult from them. It was proved before, that irritability was diftinct from fenfibility, and how much these properties depend upon a due oxygenation of the blood, was also clearly explained *.

We are to enquire, "whether opium acts by means of the BLOOD on the irritable fibre," as was before proved with the Ticunas, the American vegetable poison, or "has its operation folely on the NERVES."

Notwithstanding it appears certain, that opium, even when fimply diffolved in water, whether it is introduced into the stomach, or into the inteftines; whether it is injected beneath the fkin, or into the abdomen; whether it be applied to the heart or muscles; equally acts on the animal body: yet a doubt still remains whether its action and energy are wrought on the nerves, or whether it needs the vehicle of the blood, and the circulation, to give it activity.

We have seen that the venom of the viper acts in no other way than by the medium of the blood; and the

* Vide Vol. I. Sect. XXXII. p. 340, and Sect. XXXIV. p. 359.

vegetable

vegetable poifon of the ticunas appears to act in the fame

manner

*

It is certain that all poifons, as well as

opium, kill when swallowed; but this does not prove that their action is wrought immediately on the nerves†, and that they do not employ the medium of the blood. There are we know in nature principles attractive and deftructive of each other, and may not the fubtle and active particles of a poifon, penetrate from this law of affinity, and introduce themselves into the blood? We are under the neceffity of admitting unknown powers, of whofe principles and mechanifin we are ignorant. We allow that iron is attracted by the magnet, though we are wholly ignorant of magnetifm. Thus the difficulty which arifes from the mortal effects of opium when taken internally, does not prove that it acts iin

* Vide Vol. II. p. 389.

Ift. FONTANA divided the nerves going to the leg of a rabbit, it was rendered infenfible and paralytic. He then applied the venom to the leg, and though the nervous influence was intercepted, it communicated all the fymp. toms of the poifon of the viper.

zd. On the contrary, when the veins and arteries going to, and returning from, the leg where intercepted, and the poison inserted, it did not communicate the disease of the venom of the viper.

And 3d. When the nerves were feparated from the body, and furrounded with venom, it produced no fymptoms of this difenfe, Vide Vol. II.

page 396.

VOL. III.

4 M

mediately

mediately on the nerves; and befides it has been demonta ftrated, that the venom of the viper and ticunas have no immediate action on the nerves themselves*.

To be enabled to make fome very probable affertion on this difficult matter, an experiment must be imagined in which opium may act freely against the nerves, without the smallest introduction of it into the blood, or rather, without its touching the blood-veffels. Such an experiment, confidering the dexterity and precision it requires, is not one of the easiest to make, and can be only well tried on very small animals, and on a very few of the nerves. To obtain certain confequences, and fuch as do not proceed from deceitful and variable experiments, it was neceffary to make a great many trials, to exclude all the refults that accidental circumftances might have rendered imperfect, to compare the different confequences with each other, and to weigh them in each cafe, with those of the experiments intended to ferve as comparative ones.

I deftined, fays FONTANA, 300 frogs for these experiments, and by means of pincers and fciffars, I laid bare the crural nerves in fuch a manner as they were

* Vide Vol. II. p. 396.

entirely

entirely free of every other part, and obtained about eight or ten lines of nerve totally clear, and in fome very large frogs even more. I then let fall the nerves of each thigh into a small hollow glass, which receives them in fuch a way, that I can fill each glass with a fluid of any kind without its touching the adjacent inufcles. I usually have been able to put into these glaffes fuch a proportion of whatever I wish to try on the nerves, as to cover the greater part of them with it, without its being poffible for any of the liquor to find its way to the thighs, and mix with the blood. In this way I can make a comparifon betwixt the nerves that are envenomed, and thofe that are not, compute the time that they continue to contract the muscles, and judge of the vivacity of the motions.

At the end of the first ten minutes, I ftimulated the medicated nerves; I fhall diftinguith in this way those to which I applied the opium, and those which were not medicated, and found that the two extremities, the right as well as left, contracted with the fame force and vivacity.

At the end of twenty minutes, I tried the ftimulation, and could perceive no fenfible difference betwixt the mo4 M 2

tions

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