Page:The American Cyclopædia (1879) Volume I.djvu/550

 518 ANIMAL ELECTRICITY only developed at one period of the year, and their term of existence is very short. This is rendered probable by a very curious observa- tion made by Mr. Dalrymple. He found in one genus male individuals, that possessed neither mandibles, nor alimentary canal, nor glands. The only apparatus that was fully de- veloped was the generative. The animal was in fact a mere male genital system, endowed with power of independent existence, though that existence must have been of very short duration. The transparency of the tissues en- ables us to trace very satisfactorily the forma- tion and progress of the ova. Their growth is very rapid, and they are in some genera ex- truded from the ovary two or three hours after their germ is first detected, and hatch in less than half a day. In other families the eggs remain in the ovary or cloaca, and are there hatched, the young being born alive. From the transparency of all the tissues, it is often possible to trace the form, and to a certain extent make out the details of the structure of the young animal while it is yet in the body of the parent. ANIMAL ELECTRICITY, electricity produced in the bodies of animals. Of this electricity there are two kinds, the dynamical or galvanic and the statical. I. The production of dynamic elec- tricity. Few discoveries in science have more importance than the almost accidental obser- vation made by Luigi Galvani in 1786. After having examined the influence of the shock produced by a spark of the electrical machine on a frog's leg, Galvani observed a new and very curious phenomenon. He had skinned a frog, taking away its two legs with a part of the spine, and attached the whole to a copper hook which he had hung upon an iron railing near his laboratory. He stood watching to see if the electricity of the atmosphere would produce upon these legs the same effect as an electrical machine. After some time, having observed no sign of electrical influence,, he decided to remove the frog's limbs, and while doing so he perceived the very muscular con- traction which he had been vainly expecting to see produced by atmospheric electricity. He soon discovered the condition of this con- traction, which was the contact of the moist limbs of the frog with the iron rail. Having substituted for the copper hook and iron rail a metallic arc composed of pieces of these two metals, he found that he could produce the con- traction at will. For the production of sud- den muscular contraction and of a movement of the limb, it was only necessary to place one end of the arc in contact with a nerve or with the spinal canal, from which the nerves emerge, and with the other end one of the muscles of the leg. Galvani first published these ex- periments in 1791, in is celebrated work, De Viribu* Electricitatis^n Motu Mutculari Commentarius. According to the theory pro- posed in this work, the muscles chiefly contain the animal electricity which itinifested itself in the above experiments, and which he thought was supplied by the nerves and the blood. When the discoveries of Galvani be- came known, the whole civilized world was seized with admiration, and the curiosity to witness his experiments became universal. Du Bois-Reymond says : " Wherever frogs were to be found, and where two different kinds of metal could be procured, everybody was anx- ious to see the mangled limbs of frogs brought to life in this wonderful way. The physiologists believed that at length they should realize their visions of a vital power. The physicians, whom Galvani had somewhat thoughtlessly led on with attempts to explain all kinds of nervous diseases, as sciatica, tetanus, and epi- lepsy, began to believe that no cure was impos- sible." Volta soon opposed the views of Gal- vani, and maintained that the pretended animal electricity was nothing but the electricity de- veloped by the contact of two different metals. Galvani replied that with one metal only the muscular contraction was produced, although very feebly. Volta answered that the metals employed were not pure, and that as they had no homogeneity they acted like two metals. He showed -that even the least physical altera- tion of a part of an arc of one metal was suf- ficient to make it act as if it were composed of two metals. Galvani, however, succeeded in producing contractions without the inter- vention of any metal whatever, by merely applying the nerve of a leg on the muscles or establishing a communication between the muscles and the nerve by a piece of moist animal tissue. Alexander von Humboldt took sides with Galvani against Volta. In employ- ing very irritable frogs he found that there were strong muscular contractions in the fol- lowing circumstances : 1, when the leg of a frog was bent back against the ischiatic nerve, both parts being still originally connected; 2, when the crural nerve and its muscles were connected by a fragment cut from the same nerve ; 8, when a connection was established between two parts of the same nerve by means of some animal tissue. In 1798 Gal- vani died, and the next year Volta discovered the pile; and, as it has been said, he then earned the right of exclaiming, with triumph- ant scorn, "I don't need your frog; give me two metals and a moist rag, and I will pro- duce your animal electricity. Your frog is nothing but a moist conductor, and in this respect it is inferior to my wet rag." For nearly 30 years the supporters of the theory of animal electricity were silenced by the great discovery of Volta. In 1825 Nobili, having rendered extremely sensitive the galvanometer (instrument for the measuring of galvanic cur- rents), thought that the current which produces muscular contractions in the frog's legs might be detected by his instrument. He failed in his first attempt, the contractions taking place while the needle of his instrument stood still ; but after having improved the- instrument he