Hearts, with their nerves intact, were dissected from two frogs.
The hearts were hooked up to mechanical devices which recorded their heart beats.
Then the vagus nerve of one heart (the donor) was electrically stimulated.
The donor heart immediately slowed down, as expected.
A sample of the fluid passing through the donor heart was taken with a small pipette and this fluid was dripped onto the second heart (the recipient).
The recipient heart slowed down, even though its vagus had not been stimulated. Chemicals released by the donor heart into the fluid were sufficient to produce slowing.