of my friends, Mr Hunter and Mr Thomson, having dissected away the pericardium from a frog’s heart, which had an hour before ceased spontaneously to contract, I removed the muscles, and cellular membrane covering its nerves, and large blood vessels. I then placed one end of a rod of pure silver in contact with one side of these nerves, and blood vessels, and one end of a rod of zinc on the other, both of them at about the distance of the third part of an inch from the auricles of the heart. On bringing the opposite ends of these rods in contact with each other, the auricle first, and then the ventricle of the heart immediately contracted, and repeated their contractions as often as the ends of the metal rods were made to touch each other. When a stick of glass, wax, or wood, was made use of in place of one of the metals, no contraction took place. Contractions, however, were excited by irritating the heart itself with the point of a sharp instrument. The contractions were both more vigorous, and more constant when the metals were placed in contact with the heart itself, than when touching only its blood vessels and nerves. I have several times attempted to trace some of the nerves, which may be seen near the large blood vessels of the heart of a frog, into the heart itself, in order to arm them separated from other parts; but, partly on account of their minuteness, and partly on account of the weak state of my eyes, which does not permit me to look intently at minute objects, I have never been able to succeed. Since making this last experiment, I have repeated it upwards of twenty times. In order to its complete success, it is necessary that the spontaneous contractions of the heart should nearly, if not altogether, have ceased; and, when in this state, the experiment is rendered still more satisfactory by removing the heart from the body of the frog, and laying it upon a plate of zinc. We are then sure that its contractions cannot have been excited, by any mechanical irritation, arising from the contractions of the muscles of the thorax.