Digitale Ausgabe

M. Humboldt is one of the philosophers who has made themost numerous observations on the phenomenon discovered by Gal-vani, concerning the irritability produced by the contact of differentmetals with the parts of animals in which the principle of life is ap-parently extinguished. As long ago as the year 1795, he observedthat the animal irritability was augmented by the oxygenated muri-atic acid. Not having discontinued his attention to this object, theperusal of the physiological writings of Reil, and his correspondencewith Scarpa and Volta, afforded him indications for new enquiries,of which he has occasionally had the courage to make himself thesubject. “In a conversation,” says he, “with M. Scarpa, at Pavia, on theeffects which galvanism produced upon myself, nothing surprisedhim more than the appearance of a lymphatic and serous humour onmy back. ‘What can be the nature,’ said he, ‘of this stimulant,which in a few instants changes the nature of the vessels to such adegree as to cause them to prepare humours, which, the instant theytouch epidermis, excite inflammation, and mark their courseby a redness which lasts for whole hours?’” M. Humboldt pro-mised to repeat the experiment; and the account he gives of thefacts constitutes one of the most interesting articles in his letter. For this purpose he caused two blistering plasters to be appliedon the deltoid muscle of both shoulders. When the left blister wasopened, a liquor flowed out which left no other appearance on the

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* From Gren’s “Journal of Natural Philosophy.”|523| skin than a slight varnish, which disappeared by washing. Thewound was afterwards left to dry up: this precaution was necessary,in order that the acrid humour which the galvanic irritation wouldproduce might not be attributed to the idiosyncrisis of the vessels.This painful operation was scarcely commenced on the wound, bythe application of zinc and silver, before the serous humour wasdischarged in abundance: its colour became visibly dark in a fewseconds, and left on the parts of the skin where it passed, traces ofa brown inflamed red. This humour having descended towards thepit of the stomach, and stopped there, caused a redness of morethan an inch in surface. The humour, when traced along theepidermis, left stains, which after having been washed, appeared ofa blueish red. The inflamed places having been imprudentlywashed with cold water, increased so much in colour and extent,that M. Humboldt, as well as his physician Dr. Schallern , who as-sisted at these experiments, entertained some apprehension for theconsequences. M. Humboldt has not undertaken to determine the nature of thefluid which produces such astonishing effects; but he applies him-self to circumscribe the phenomena in the real circumstances whichproduce them. He judiciously varies the preparations, and carefullynotes all the results; being persuaded that the cause of galvanismcannot be explored with success, but by observing the proportionsin which the chain of metals either irritates or has no effect: andto extend still more this vast field of observation, he employs variousmeans to raise or diminish the irritable capacity of the animalorgans. What is the sensation which the galvanic irritation produces?M. Humboldt has discussed this question. “No one,” says he,“can speak more decidedly on this subject than myself, having madeseveral experiments on my own person, the seat of which, in someinstances, was the socket of a tooth which I had caused to be ex-tracted; in others, certain wounds which I made in my hand; andin others, the excoriations produced by four blistering plasters.”The following is his answer:— The galvanic irritation is always painful, and the more so in pro-portion as the irritated part is more injured, and the time of irritationmore prolonged. The first strokes are felt but slightly; the fiveor six following are much more sensible, and even scarcely to beendured, until the irritated nerve becomes insensible from continuedstimulus. The sensation does not at all resemble that which iscaused by the electric commotion and the electric bath; it is a pe-culiar kind of pain, which is neither sharp, pungent, penetrating,nor by intermissions, like that which is caused by the electric fluid.We may distinguish a violent stroke, a regular pressure, accom-panied by an unintermitting glow, which is incomparably moreactive when the wound is covered with a plate of silver, and irritatedby a rod of zinc, than when the plate of zinc is placed on the wound,and the silver pincers are used to establish the communication. |524| When the communication is made by the contact of the epider-mis, it produces no effect; it appears to insulate like glass, wheninterposed between the wound and the metal; but if the skin beremoved, by making two wounds at eight inches distance, and aplate of zinc be placed on one of them, and on the other a leg of afrog prepared, this last is seen to contract itself when it communi-cates with the zinc by the silver wire, which proves that the gal-vanic fluid there passes beneath the epidermis. This fluid produced in some circumstances a very sensible acidtaste. The two wounds of M. Humboldt having been covered, onewith silver, the other with zinc, an iron wire of several feet inlength, attached to the zinc, was conveyed between his upper lipand the spongy substance of the teeth, and thence to the tongue ofanother person. When the iron wire was made to touch the silver,a strong contraction of the scapular muscle took place, and at thesame instant the person whose tongue formed part of the chain ofcommunication perceived the sensation of acidity. There are alsocases in which the fluid acts on the organs of taste without pro-ducing any sensible effect on the organs of motion: such is thatwhere the epidermis serves as the conductor from zinc to the frog;for there is not then any contraction, but merely an acid taste onthe tongue. The author, having learned from M. Volta that he employed thesolution of potash (oleum tartari per deliquium), in order to augmentthe conducting power, availed himself successfully of this meansto raise the capacity of the animal organs. He moistened one ofhis wounds with this liquor, which produced little pain; but the galvanic irritation was more violent, and accompanied with moreheat; sparks appeared and disappeared before his eyes; the tonguemoistened with the same, distinctly perceived the acid sensation,although the communication was established only between zinc andzinc. The thigh of the frog, moistened with the alkaline solutionand laid upon a plate of glass, without touching either metal or car-bonic matter, fell of itself into violent convulsions, the antagonistmuscles of the legs and toes being incessantly agitated. Irritabilityhas been re-established by this application in the animal parts, whereit had been extinguished by warm solutions of the oxide of arsenic.Lastly, the irritation (which does not commonly take place when thenerve and the muscle are armed with the same metal, the differentmetals being between the coatings), becomes manifest after thispreparation; which seems to indicate that the alkali not only irri-tates the nerve, but likewise adds to its irritability. The author applied this method to amphibious animals, which heroused from their winter sleep, and in which he perceived a pecu-liar symptom of irritability. These observations led him to distinguish two states of the animalorgan. The first, of irritability naturally or artificially raised orexcited; the second, irritability in a less degree. These two states, |525| which he calls positive and negative, are merely, as he remarks, dif-ferent degrees, and not phenomena absolutely distinct from eachother. In individuals naturally sensible, the effects produced by alkalinesolutions, by the oxygenated muriatic acid, by the solution of oxideof arsenic, are very rarely of the same intensity. In the case of increased irritability, muscular motions are observedwithout metal or carbonic matter. They may be obtained withmetals, though without communication between the nerve and themuscle; that is to say, without the regular connection or chain.They may be also obtained by forming the chain of similar metals. Let the crural nerve of an animal naturally tenacious of life beplaced upon glass. Let a small piece of fresh muscular flesh befixed on a stick of sealing-wax, and then brought into contact withthe crural muscle. The result will be a violent convulsion at theinstant when the chain of communication is completed. The samething happens if, instead of the small piece of muscular flesh, a de-tached piece of the crural nerve be fixed on the stick of sealing-wax.The connection is therefore formed of two things, nerve and mus-cular fibre. How in this simple case can the fluid which passes fromthe nerve into the muscle cause it to be contracted? M. Humboldtthinks that it becomes stimulant, merely because it returns from thenerve into the nerve by a foreign animal matter; that is to say, notorganically connected with the nerve. The disparity of the metals forming the chain has hitherto ap-peared as a necessary condition to produce galvanic irritation. Thishypothesis, however, is overturned by the experiments of M. Hum-boldt. If it be true that, in the state of less irritability, there isvery rarely contraction with similar metals (as Volta affirms, con-trary to Aldini), this circumstance becomes indifferent in the case ofincreased irritability. M. Humboldt put into a china cup somemercury exactly purified; he placed the whole near a warm stove,in order that the entire mass might assume an equal temperature:the surface was clear, without the appearance of oxidation, humidity,or dust. A thigh of a frog, prepared in such a manner that a cruralnerve and a bundle of muscular fibres of the same length hungdown separately, was suspended by two silken threads above themercury. When the nerve alone touched the surface of the metal,no irritation was manifested; but as soon as the muscular bundleand the nerve touched the mercury together, they fell into convul-sions so brisk that the skin was extended as in an attack of tetanus. We ought not to be surprised at the precaution here taken by M. Humboldt to heat the mercury. This is required in consequence ofthe opinion which he announces, that the parity of the metals doesnot depend on the homogeneity of their chemical constituent parts,but of their heat, polish, hardness, and force. Gold, placed between two armatures of zinc, produces irritationonly when the gold is moistened by some volatile fluid, or by themoisture of respiration. |526| Lastly, M. Humboldt has attempted to include all the cases inthe following formula:—