How did Luigi Galvani explain the source of electrical force in animal tissue?

Luigi Galvani proposed that animal tissue contained an innate, vital force he termed "animal electricity." He believed this force was distinct from both the "natural" electricity found in lightning or electric fish and the "artificial" electricity produced by friction.1

Galvani theorized that the brain acted as the primary organ for secreting this "electric fluid," which was then conducted through the nerves to the muscles. He likened the interaction between nerves and muscles to the inner and outer surfaces of a Leyden jar, suggesting that the flow of this fluid provided the necessary stimulus for muscular contraction.1

While his contemporary Alessandro Volta argued that the electricity originated from the contact of dissimilar metals, Galvani eventually demonstrated that muscular contraction could occur without metal by touching the nerve of one frog to the muscle of another. This experiment provided the first evidence of bioelectric forces existing within living tissue, though modern science recognizes that his specific concept of a unique "animal electricity" was an incomplete explanation of the underlying physiological processes.1