Thank for pointing that out. From your Google Books links:
How great the just perceptible change can be made to become by making the rate of change extremely slow is a matter that still remains for investigation. It is worthy of note that it has been found possible in 5.25 hours to crush a frog's foot, without a sign that the pressure was felt, by screwing down a button at the rate of 0.03 mm per minute. A similar experiment showed that a live frog can actually be boiled without a movement if the water is heated slowly enough; in one experiment the temperature was raised at the rate of 0.002 Celsius per second, and the frog was found dead at the end of 2.5 hours without having moved.(1) If a frog can be crushed or boiled without any evidence that he has noticed it, it is at least an interesting question of what can be accomplished in this direction with human beings.
(1) The literature on these experiments with frogs includes Heinzmann, Euber die Wirkung sehr allmaliger Aenderungen thermischer Reize auf die Empfindungsnerven, "Archiv f. d. ges. Physiol." (Pfluger), 1872, vi. 222. Fratscher, Euber continuirliche und langsame Nervenreizung, "Jenaische Zeitschrift," 1875, N. F. ii. 130. Sedgwick, On the Variation of Reflex Excitability in the Frog induced by changes of Temperature, "Stud. Biol. Lab., Johns Hopkins Univ.," 1882, 385.
