Chap. II
Of the Animals best fitted for Galvanic Experiments, of the Metals best calculated for making these Experiments, and of Conductors.
Almost every animal can be made to produce these muscular contractions by the Galvanic power, but those called cold blooded are the best. Thus frogs have been found the most convenient, both on account of their size and abundance. They also retain their muscular irritability to the Galvanic influence longer than most other animals, and it is asserted that strong convulsions can be produced in them many hours after the brain and spinal marrow have been destroyed; and also that when pretty far advanced in the process of putrefaction they are capable of Galvanic excitement.
No contractions have been produced in animals killed by corrosive sublimate, nor in those which have been starved to death: but a very slight motion can be made to appear in those killed by opium, the electric shock, or azotic gas.
With regard to the metals used to effect these motions, almost any two will answer the purpose; but the most powerful are the following, viz.
Zinc
Tin
Lead in conjunction with Gold.
Silver.
Molybdena.
Steel.
Copper.
Those which have the most power are placed first; that is zinc and gold, will produce greater muscular contractions than tin and silver, or tin and gold, and so of the rest.
The process by which these wonderful appearances are produced consists in effecting, by means of the Galvanic apparatus, a communication between a nerve and a muscle, in any part of an animal body. The part of the animal upon which the experiment is to be performed is denominated the animal arc: and the Galvanic instruments which form the communication between the muscle and the nerve, are called the excitatory arc. This latter generally consists of three pieces; one fixed to the muscle, another to the nerve, and a third forming a communication between both. This last, called the communicator, may be made of the same metal with either of the others, or be different from both. The best communicators or conductors, are the following.—The list begins with the most perfect.
Malleable platina.
Silver.
Gold.
Quicksilver.
Copper.
Brass.
Tin.
Lead.
Iron.
The human body.
Salt water.
Fresh water.
The metallic ores are not so good conductors as the purified metals, and their conducting power varies, according to the nature of the ores.
The metallic salts are tolerably good conductors.
Dr. Valli observed that human bodies are not all equally good conductors. Out of four persons in a company, he found that when two of them formed the circuit of communication between the nerve and muscles of a frog, the motions took place very readily. When the third person formed the circuit, the motions were very weak; but that, when the fourth person formed the communication, no motion took place. This experiment, he adds, was often repeated with the same success. The effect however may be owing to the different dryness of the skin.
Vitriolic acid, and even alcohol, appear to conduct the Galvanic influence rather better than water.
The veins and arteries are not so good conductors as the nerves; for when a blood-vessel forms part of the circuit of communication, the contractions will take place only when ramifications of the nerves are adhering to it, and if these be carefully separated, the motion will not take place. The same thing may be said of the tendons, the bones, and the membranes; for when either of those parts is separated from the body, and is introduced into the circle of communication between the muscles and nerves of a prepared frog, no motion will ensue; excepting, indeed, when those parts are full of moisture and in immediate contact with the nerve.