Digitale Ausgabe

MR. Humboldt is one of the philoſophers who has made the moſt numerous obſerva-tions on the phenomenon diſcovered by Galvani concerning the irritability produced by thecontact of different metals with the parts of animals in which the principle of life is appa-rently extinguiſhed. As long ago as the year 1795 he obſerved that the animal irritabilitywas augmented by the oxygenated muriatic acid. Not having diſcontinued his attentionto this object, the peruſal of the phyſiological writings of Reil, and his correſpondence with Scarpa and Volta, afforded him indications for new enquiries, of which he has occaſionallyhad the courage to make himſelf the ſubject.

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* This Letter forms part of Gren’s (German) Journal of Natural Philoſophy for the month of October laſt.The extract was read to the Inſtitute of France, by Citizen Guyton.|257| “In a converſation,” ſays he , “with M. Scarpa, at Pavia, on the effects which Galvaniſmproduced upon myſelf, nothing ſurpriſed him more than the appearance of a lymphatic andſerous humour on my back. ‘What can be the nature,’ ſaid he, ‘of this ſtimulant, whichin a few inſtants changes the nature of the veſſels to ſuch a degree as to cauſe them toprepare humours, which, the inſtant they touch the epidermis, excite inflammation, and marktheir courſe by a redneſs which laſts for whole hours?’” M. Humboldt promiſed to repeatthe experiment; and the account he gives of the facts conſtitutes one of the moſt intereſt-ing articles in his letter. For this purpoſe he cauſed two bliſtering plaſters to be applied on the deltoid muſcle ofboth ſhoulders. When the left bliſter was opened, a liquor flowed out which left no otherappearance on the ſkin than a flight varniſh, which diſappeared by waſhing. The woundwas afterwards left to dry up: this precaution was neceſſary, in order that the acrid humourwhich the Galvanic irritation would produce might not be attributed to the idioſyncriſis ofthe veſſels. This painful operation was ſcarcely commenced on the wound, by the applica-tion of zinc and ſilver, before the ſerous humour was diſcharged in abundance: its colourbecame viſibly dark in a few ſeconds, and left on the parts of the ſkin where it paſſedtraces of a brown inflamed red. This humour having deſcended towards the pit of theſtomach, and ſtopped there, cauſed a redneſs of more than an inch in ſurface. The hu-mour, when traced along the epidermis, left ſtains, which after having been waſhed ap-peared of a blueiſh red. The inflamed places having been imprudently waſhed with coldwater, increaſed ſo much in colour and extent, that Mr. Humboldt, as well as his phyſicianDr. Schallern , who aſſiſted at theſe experiments, entertained ſome apprehenſion for theconſequences. Mr. Humboldt has not undertaken to determine the nature of the fluid which producesſuch aſtoniſhing effects; but he applies himſelf to circumſcribe the phenomena in the realcircumſtances which produce them. He judiciouſly varies the preparations, and carefullynotes all the reſults; being perſuaded that the cauſe of Galvaniſm cannot be explored withſucceſs, but by obſerving the proportions in which the chain of metals either irritates or hasno effect: and to extend ſtill more this vaſt field of obſervation, he employs various meansto raiſe or diminiſh the irritable capacity of the animal organs. What is the ſenſation which the Galvanic irritation produces? Mr. Humboldt has diſcuſſedthis queſtion. “No one (ſays he) can ſpeak more decidedly on this ſubject than myſelf,having made ſeveral experiments on my own perſon, the ſeat of which, in ſome inſtances,was the ſocket of a tooth which I had cauſed to be extracted; in others, certain woundswhich I made in my hand; and in others, the excoriations produced by four bliſteringplaſters.” The following is his anſwer: The Galvanic irritation is always painful, and the more ſo in proportion as the irritatedpart is more injured, and the time of irritation more prolonged. The firſt ſtrokes are feltbut ſlightly; the five or ſix following are much more ſenſible, and even ſcarcely to be endured,until the irritated nerve becomes inſenſible from continued ſtimulus. The ſenſation does notat all reſemble that which is cauſed by the electric commotion and the electric bath; it is apeculiar 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 regularpreſſure, accompanied by an unintermitting glow, which is incomparably more active when |258| the wound is covered with a plate of ſilver, and irritated by a rod of zinc, than when theplate of zinc is placed on the wound, and the ſilver pincers are uſed to eſtabliſh the com-munication. When the communication is made by the contact of the epidermis, it produces no effect;it appears to inſulate like glaſs, when interpoſed between the wound and the metal: but ifthe ſkin be removed, by making two wounds at eight inches diſtance, and a plate of zinc beplaced on one of them, and on the other a leg of a frog prepared, this laſt is ſeen to contractitſelf when it communicates with the zinc by the ſilver wire; which proves that the Galvanicfluid then paſſes beneath the epidermis. This fluid produced in ſome circumſtances a very ſenſible acid taſte. The two wounds ofMr. Humboldt having been covered, one with ſilver, the other with zinc, an iron wire of ſeveralfeet in length, attached to the zinc, was conveyed between his upper lip and the ſpongy ſubſtanceof the teeth, and thence to the tongue of another perſon. When the iron wire was made to touchthe ſilver, a ſtrong contraction of the ſcapular muſcle took place, and at the ſame inſtant theperſon whoſe tongue formed part of the chain of communication perceived the ſenſation ofacidity. There are alſo caſes in which the fluid acts on the organs of taſte without producingany ſenſible effect on the organs of motion: ſuch is that where the epidermis ſerves as theconductor from zinc to the frog; for there is not then any contraction, but merely an acidtaſte on the tongue. The author, having learned from Mr. Volta that he employed the ſolution of pot-aſh (oleumtartari per deliquium) in order to augment the conducting power, availed himſelf ſucceſsfullyof this means to raiſe the capacity of the animal organs. He moiſtened one of his woundswith this liquor, which produced little pain; but the Galvanic irritation was more violent,and accompanied with more heat; ſparks appeared and diſappeared before his eyes; thetongue moiſtened with the ſame diſtinctly perceived the acid ſenſation, although the com-munication was eſtabliſhed only between zinc and zinc. The thigh of the frog, moiſtenedwith the alkaline ſolution and laid upon a plate of glaſs, without touching either metal orcarbonic matter, fell of itſelf into violent convulſions, the antagoniſt muſcles of the legs andtoes being inceſſantly agitated. Irritability has been re-eſtabliſhed by this application in theanimal parts, where it had been extinguiſhed by warm ſolutions of the oxide of arſenic.Laſtly, the irritation (which does not commonly take place when the nerve and the muſcleare armed with the ſame metal, the different metals being between the coatings) becomesmanifeſt after this preparation; which ſeems to indicate that the alkali not only irritates thenerve, but likewiſe adds to its irritability. The author applied this method to amphibious animals, which he rouſed from their winter’sſleep, and in which he perceived a peculiar ſymptom of irritability. Theſe obſervations led him to diſtinguiſh two ſtates of the animal organ. The firſt, ofirritability naturally or artificially raiſed or excited; the ſecond, irritability in a leſs degree.Theſe two ſtates, which he calls poſitive and negative, are merely, as he remarks, differentdegrees, and not phenomena abſolutely diſtinct from each other. In individuals naturally ſenſible, the effects produced by alkaline ſolutions, by the oxy-genated muriatic acid, by the ſolution of oxide of arſenic, are very rarely of the ſame in-tenſity. In the caſe of increaſed irritability, muſcular motions are obſerved without metal or |259| carbonic matter. They may be obtained with metals, though without communication be-tween the nerve and the muſcle; that is to ſay, without the regular connection or chain. Theymay be alſo obtained by forming the chain of ſimilar metals. Let the crural nerve of an animal naturally tenacious of life be placed upon glaſs. Let aſmall piece of freſh muſcular fleſh be fixed on a ſtick of ſealing-wax, and then brought intocontact with the crural muſcle. The reſult will be a violent convulſion at the inſtant whenthe chain of communication is completed. The ſame thing happens if, inſtead of the ſmallpiece of muſcular fleſh, a detached piece of the crural nerve be fixed on the ſtick of ſealing-wax. The connection is therefore formed of two things, nerve and muſcular fibre. Howin this ſimple caſe can the fluid which paſſes from the nerve into the muſcle cauſe it to becontracted? Mr. Humboldt thinks that it becomes ſtimulant, merely becauſe it returns fromthe nerve into the nerve by a foreign animal matter; that is to ſay, not organically connectedwith the nerve. The diſparity of the metals forming the chain has hitherto appeared as a neceſſary con-dition to produce Galvanic irritation. This hypotheſis, however, is overturned by the ex-periments of Mr. Humboldt. If it be true that, in the ſtate of leſs irritability, there is veryrarely contraction with ſimilar metals (as Volta affirms, contrary to Aldini), this circumſtancebecomes indifferent in the caſe of increaſed irritability. Mr. Humboldt put into a china cupſome mercury exactly puriſied; he placed the whole near a warm ſtove, in order that theentire maſs might aſſume an equal temperature: the ſurface was clear, without the appear-ance of oxidation, humidity, or duſt. A thigh of a frog, prepared in ſuch a manner that acrural nerve and a bundle of muſcular fibres of the ſame length hung down ſeparately, wasſuſpended by two ſilken threads above the mercury. When the nerve alone touched theſurface of the metal, no irritation was manifeſted; but as ſoon as the muſcular bundle andthe nerve touched the mercury together, they fell into convulſions ſo briſk that the ſkin wasextended as in an attack of tetanus. We ought not to be ſurpriſed at the precaution here taken by Mr. Humboldt to heat themercury. This is required in conſequence of the opinion which he announces, that theparity of the metals does not depend on the homogeneity of their chemical conſtituent parts,but of their heat, poliſh, hardneſs, and form. Gold, placed between two armatures of zinc, produces irritation only when the gold ismoiſtened by ſome volatile fluid, or by the moiſture of reſpiration. Laſtly, Mr. Humboldt has attempted to include all the caſes in the following formulæ: