Page:Collected Physical Papers.djvu/283

Rh three elements are in good order, no light-message can be perceived. We must have a sensitive retina, and the conducting nerve, free from injury. Finally, just as the galvanometer will fail to detect a current if it is injured by rough usage, so also after a violent blow, the brain will no longer perceive, though the terminal organ, the retina, and the connecting optic nerve may be intact.

If we take a piece of living muscle whose surface is uninjured, then any two points A and B on such a