abernethy's experiments on the muscles in frogs, etc.
"there are more things in heaven and earth than are dreamt of in your philosophy, horatio," is a sentiment which, in some form or other, occurs to the most uninformed peasant, and to the most profound philosopher.
the very small difference between the acquisitions of the two, however marvellous when viewed abstractedly, sinks into nothing when compared to the secrets of nature which yet remain unexplored. this comparison is the true source of that humility which, while it adds dignity to the acquirements of intellect, is the foundation on which we may most securely rest the hope of increasing possessions.
the intellectual vision of the wisest man confines him to a very small area, when compared with the boundless realms of nature. there are, indeed, a number of objects within the range of his perceptions whose nature and relations he has the power of examining; but there are also a multitude of others which, from their dimly sketched outline, he feels to be beyond the bounds assigned to his limited faculties.
one of the most curious things in animals is the rigidity or stiffness of their muscles after death. it is, as it were, the last effort of the living principle. this phenomenon may be indefinitely modified by particular states, by lightning, by poison, and other peculiar conditions, induced by the manner and the period at which the death may have occurred; and in all cases it continues but for a short time. it is the last exercise of that power which resides in muscles or flesh, of contracting, and thus moving94 the various parts to which it is attached. in a very large sense, this power is under the dominion of the will, and enables animals to move as their instincts or their wants suggest.
now it is a curious thing to think that this power can be excited after death, by placing the parts between two pieces of metal, or galvanizing them; so called after the name of the discoverer, galvani.
it is difficult, at this day, to imagine the astonishment of the wife of galvani, or his pupil, when first they observed the leg of a dead frog thrown into convulsions on being touched by a piece of metal. such, however, was the apparently simple origin of a long series of wonderful discoveries. it has been well observed, however, that "discoveries, apparently the result of accident, always imply the exercise of profound thought." and this was no less the case in respect to galvanism. a fact, which, but for the mention of it to galvani by his wife, might have passed unobserved, was, by the scarcely less than creative power of mind, improved into a most important branch of human science.
ignorant as men still remain of the intrinsic nature of the principle or power which gives rise to the phenomenon, the observation and study of its laws and operations have led to discoveries which, in their value, their importance, and their surprising character, yield to no other yet achieved.
abernethy, who, at this laborious period of his life, had his observation directed everywhere, made some experiments on this power (galvanism), in its relations to the muscles of frogs.
his object seems to have been as follows: fontana (a celebrated physiologist, born in the tyrol about 1734) had showed that a muscle which could no longer be excited to contract under water, might be excited anew, if taken out of the water, and exposed for some time to air. this observation had suggested the idea that air was in some way or other conducive to this "irritability," as it was termed. dr. girtanner had also endeavoured to prove that the irritability depended on the oxygen taken into the blood during respiration; and further, that it was in a direct ratio to the quantity of oxygen respired—"an opinion which some writers in this country seem disposed to adopt."
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abernethy doubted the soundness of such a view, and he accordingly instituted some experiments, in the hope that if he could not absolutely determine the question, he might throw some light on it. his experiments were very numerous, but he published only a few of them. we will give one or two. "having killed a frog (for he properly objected to experiments on living animals), he experimented on the muscles of two legs; one was put into a bottle containing oxygen gas procured from manganese, and which was very pure; the other into a bottle containing atmospheric air; the quantity in each bottle was about six ounces by measure; the limbs were supported in the gases, and wholly surrounded by them. after five hours, the muscles had nearly ceased to act in both limbs; those, however, of the thigh belonging to that limb inclosed in the common air acted more vividly than the others, but in a little time even these could no longer be excited. upon comparing the limbs afterwards, the muscles of that limb which had been exposed to the oxygen gas were evidently the most flabby. several other trials were made with a similar result;" whence he observes: "i am disposed to conclude that oxygenous gas has no greater power of supporting the irritability of parts separated from the animal than the common atmosphere."
in some of his experiments the limbs continued to be excitable after eighteen hours, but with little difference in the two gases.
he next made several experiments, by placing the limbs of frogs in nitrogen and hydrogen: the limbs in nitrogen lost their irritability in about two hours and a half; those in hydrogen, in about four hours.
experiments then follow which consisted in placing other limbs in carbonic acid and nitrous gases respectively; when he found that in both cases the muscles ceased to act in an hour and a half.
he also placed limbs in carburetted hydrogen, and found that they ceased to act after the same period. in other experiments, he found the correctness of fontana's results; viz. that limbs placed under water, and which had lost their irritability, had for a time recovered it by exposure to air and moisture.
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perhaps the most interesting of the whole series are those in which he compared the results obtained in vacuo and atmospheric air. he says: "i put one prepared limb of a frog under the exhausted receiver of an air-pump; it lay on a plate of glass, supported by a cup; zinc was placed beneath the thigh, and gold under the leg; and, by means of a probe passing through a collar of leather, i could touch both metals, so as to excite the muscles to contraction. this i did occasionally, and found the limb capable of excitement for twenty-two hours. the corresponding limb, which was left exposed to the atmosphere, also contracted at the end of that time; so that it was doubtful which of them retained their powers in the greater degree. the same experiment was repeated several times, with results so nearly alike, that i am inclined to believe irritability continues very little longer in common air than it does in the exhausted receiver of an air-pump.
"i have frequently produced numerous contractions in the limbs of frogs inclosed in azotic, hydrogenous, and other gases; which likewise tend to show that the cause of irritability does not depend on oxygen for its power of action."
he then remarks that, notwithstanding the great importance of oxygen, he thinks it has been overrated; for, says he, "different tribes of animals partake of it in different degrees; and those who have the least of it are far from being the least vivacious."
he here reasons on premises which were then universally admitted, and which form at present a portion of many very questionable impressions in relation to respiration.
we mention one: "that fish, frogs, &c., breathe less oxygen than warm-blooded animals." but whilst, in respect to the frog, there are many conditions relating to the skin to be considered before we can admit this proposition, we hold it to be demonstrable that fish breathe more oxygen than most other animals; due attention not having been paid to the enormous proportion of oxygen in the air found in water; being in fact, about, one-third. in his concluding remarks, he says, that as regards nitrogen, hydrogen, and carbonic acid, it only shows what we knew before:97 that they are injurious to life, and that oxygen is not more beneficial than common air. the experiments "showing the long continuance of life and action in muscles in an exhausted receiver, he considers worthy of notice, as tending to show that the cause of irritability in muscles, when once formed, does not require the assistance of external matter."
lastly, he gives an experiment on the blood (which shows how he was working in every direction), in aid of the opinion that the blood derives its scarlet colour from the action of oxygen. "i took the coagulum of venous blood left in a basin after bleeding, and, turning it bottom upwards, waited till its surface had become of a scarlet colour. i then took slices of this surface, and similar slices of the interior part of the coagulum, which had a very dark appearance, and exposed them repeatedly to azotic and nitrous gases. the scarlet colour gradually faded upon such exposure; and the azotic gas being afterwards examined, was found to contain oxygen, while nitrous gas was much diminished, doubtless by combining with the same principle. the gases to which the dark-coloured blood was exposed underwent no change in this experiment. that blood takes oxygen from the air, when it becomes florid, will not, i suppose, be denied, and the experiment i have related shows that it will again part with it, though slowly, without any alteration in its temperature."
the principal interest, as we think, of this paper on "irritability," is the evidence it affords of his determination to keep his mind free from preconceived notions on a subject which was at that time calculated to mislead him; especially as he then participated in the general impression that the oxygen was "the great source of animal heat;" a view which he afterwards, and as we think for excellent reasons, mistrusted.
this view has been revived, but, as far as we know, in no very philosophical spirit. whilst we would respect the opinions of men, we can only reason on the paramount authority of nature; and we see increasing ground to believe that he who would leave out of physiological inquiries so large a portion of the necessary induction as the phenomena of disease, no matter what be his98 authority, will only add to the number of those who have shown that, the moment we neglect the most comprehensive search for facts of which our knowledge admits, we fall into error. mr. hunter has recorded his opinion of the impossibility of obtaining a knowledge of functions without considering the phenomena of disease; and all experience hitherto has tended to give this observation the validity of an axiom.
