Page:Collected Physical Papers.djvu/282

262 When the muscle is continuously excited it gets fatigued. The height of the curve becomes continuously less.

Drugs may act as stimulants, or produce depression, according to their nature. As extreme cases of such depressing agents we may instance poisons, which kill the response of living tissue. All signs of irritability then disappear.

This mechanical method of studying the response of living substances is, however, very limited in its application. For example, when a piece of nerve is stimulated, there is no visible change. When light falls on the retina there is no change of form, but it responds by transmitting to the brain a visual impulse. What, then, is this visual impulse which is sent along the optic nerve, causing the sensation of light?

Thanks to the work of Homgren, Dewar, McKendrick and others, it is possible to answer this question. If we excise an eye, say of a frog, and substitute a galvanometer in the visual circuit in the place of the perceiving brain, it is then found that each time a flash of light falls on the eye there is produced an electric response—that is to say, there is a sudden production of a current, which ceases on the cessation of light-stimulus. Stronger light produces stronger electric response in the galvanometer, just as it produces stronger visual sensation in the brain.

The visual circuit is therefore like an electric circuit. The retina is the sensitive element. The nerve is the conductor. The brain like the responding galvanometer is a detector of the impulse. Unless these