When a frog was so placed as to form part of the elec tric circuit, it was found that an extremely minute quanti ty of electricity produced contractions in the muscles. if the hind legs were dissected from the body, the connection being kept up by the crural nerves only, and the electric fluid was passed through it in this state, a still more minute quantity was rendered visible ; so that a frog, prepared in this manner, was capable of exhibiting very decisive marks of electricity, where none could be detected by Bennet's gold-leaf electrometer.
After employing the electric fluid, as disengaged from the common machine, he next tried the atmospherical electricity ; and it was in pursuance of this object, that he was first led to observe the effects of galvanism, properly so called. Having suspended a number of frogs by metal lic hooks to an iron railing, he found that the limbs were frequently thrown into convulsions, when it did not appear that there was any electricity in the atmosphere. Having duly considered this phenomenon, he discovered that it did not originate from an extraneous electricity, but that it depended upon the position of the animal, with respect to certain metallic bodies.
It appeared, that when the muscle and nerve of a frog were each in contact with metallic bodies, and these were al so connected by a metal, the contractions were always pro duced. The effect was considerably increased by arming the nerve with a metallic coating, by which means a larger portion of it was brought into contact with the metal. But the most important of Galvani's discoveries was the effect produced by the combination of two metals. Of these com binations the most powerful was that of zinc and silver, and the most violent convulsions ensued when the nerve was coated with one of these metals, the muscle placed in contact with the other, and the two metals connected by a conductor of electricity. (PLATE CCLXIII. Fig. 19.) Galvani's general conclusion from his experiments was, that the animal body possesses an inherent electricity of a specific kind, which is connected with the nervous system, He afterwards made some curious observations upon the effect of galvanism on animals not furnished with distinct limbs, such as worms of various kinds. These animals could not be made to contract, yet by the nature of their motions, they seemed to be sensible to the impression of the two metals, when they were placed, partly on one, and partly on the other.
Dr Fowler afterwards enters upon an interesting inquiry respecting the effect of galvanism on the different parts of the body. " What are the relations which subsist between the influence discovered by Galvani, and the muscles, the nervous, and the vascular systems of animals ?" He found that the contractions were very readily excited in all the muscles which are subservient to the will, but that they were with great difficulty produced in the heart, and that they could not be rendered perceptible in the stomach and intestines. He observed, that when a part is in a state of inflammation it acquires an additional sensibility to the gal vanic stimulus, and he made the curious discovery of the flash of light, which is produced by placing the two metals in contact with the ball of the eye, and then causing them to communicate with each other. Sultzer, a German me
taphysical writer, had mentioned several years before, the effect produced on the organ of taste, by applying two me tals, one above and the other below the tongue, and then bringing them into contact ; but the sensation was ascribed to a peculiar vibration excited by the metals, and conveyed to the tongue. The experiments with the two metals, upon the eye and the tongue, were varied in different ways by Professor Robinson, an account of which is published at the end of Dr Fowler's Essay. Professor Robinson mentions the sensation of taste which is excited, when the tongue is applied to the edges of a number of plates of zinc and sil ver, placed alternately upon each other ; a construction which may be regarded as an approximation to the appara tus afterwards discovered by Volta.
In the same year in which Fowler's Essay was publish ed, a very important communication appeared in the Phi losophical Transactions of London, from the pen of Volta, in the form of letters to Cavallo. He gives a luminous ac count of Galvani's discovery, and adds many curious ex periments and valuable remarks of his own. He attempt ed, and with complete success, to overthrow Galvani's opinion, that the animal body bears an analogy to the Ley den phial, the nerve and the muscle being in opposite states of electricity. He found that for the production of the ef fect it was essential to have two different metals, and hence he arrived at the important conclusion, which may be re garded as leading to all his future discoveries, that the muscular contractions are produced by small portions of electricity, that are liberated by the action of the metals upon each other. Another point which Volta established was, that the nerve is the organ on which the galvanic in fluence immediately acts ; but he found that if a part of a muscle be laid upon two different metals, and these be made to communicate, a contraction is produced. This probably depends upon the nervous matter that is dispers ed through the muscles, and also upon the moisture that is al ways present, and which serves to conduct the electricity to all parts of the body. Volta performed:many experiments in order to discover what circumstances are favourable to the excitation of the contractions, and upon the parts of the body which are the most easily called into action. His ob servations agree, in many respects, with Fowler's, although it is certain that their experiments must have been made without concert or communication. He found that snails and worms could not be made to contract, but that many of the insects, as butterflies and beetles, were subject to the influence of the metals. It appeared from his numerous trials, that those animals alone were sensible to galvanism, who are furnished with distinct limbs, having flexor and ex tensor muscles. In the animals of this description, it ap peared that it was the voluntary muscles alone which are capable of being made to contract. Although the heart is a muscle which is easily thrown into powerful action by chemical or mechanical stimuli, yet he could never pro duce any effect upon it by the action of the two metals. Volta made some of the same observations upon the effect of the two metals on the organs of sense, as have beer mentioned in our abstract of Fowler's Essay.