When the separated leg of a frog was laid upon a plate of iron, and a loadstone was brought in contact both with its nerve and the plate, no contraction was excited. I have often brought frogs, in every state of preparation, as nearly as possible to a very sensible magnetic needle, but no variation in its direction was in any case produced by the contractions of the frogs excited by the metals. ------------------------------------------------------------------------ SECTION III. _What are the relations which subsist between the influence discovered by Galvani, and the muscles, the nervous, and the vascular systems, of animals?_ In proposing to myself a question of this very extensive nature, it will hardly be imputed to me, that I ever entertained, for a moment, the idle expectation of being able completely to solve it. It is prefixed to the following experiments as the most commodious general head under which I could arrange, not only what I had further to say, upon the influence discovered by Galvani, but likewise upon the several physiological subjects, in the examination of which this influence was employed merely as a test. OF THE MUSCLES. As I am acquainted with no criterion by which we can assure ourselves of the complete separation of muscular fibres from nerves, without rendering them objects too minute for accurate experiment; it can never be in our power, so far as I am able to judge, to satisfy ourselves, if this new influence can act immediately upon the muscular fibre. A doubt must always remain, whether nerve has not been present; and from this doubt will arise another still more difficult to solve, whether the influence produced or excited by the metals have passed through the nerve to the muscles? or if it have merely acted as a stimulus to the nerve, serving to rouse that unknown energy, by which nerves are known in certain circumstances to excite muscles to contraction. The following experiments, made upon animals considered by anatomists, in general, as destitute of nerves, may to some appear decisive of this question, but to myself, I confess, they are by no means so. In by far the greater number of animals, we are precluded from the possibility of discovering nerves by their minuteness; yet the actions of these animals, not merely excited by mechanical irritation, but so obviously directed to the attainment of an end, oblige us to infer their existence even where our senses, aided by the best glasses, do not enable us to detect them.