“There can be no question that the practice of vivisection hardens the sensibility of the operator and begets indifference to the infliction of pain, as well as great carelessness in judging of its severity.”
Source: Surgical Anaesthesia (1894), p. 368
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Henry Jacob Bigelow 7
American surgeon 1818–1890Related quotes

"Discovering Darwin", Proceedings of the International Anti-Vivisection and Animal Protection congress, held at Washington, D.C. December 8th to 11th, 1913 (1913), p. 158The only consistent attitude, since Darwin established the unity of life (and the attitude we shall assume, if we ever become really civilised), is the attitude of universal gentleness and humanity.
Source: Something More, A Consideration of the Vast, Undeveloped Resources of Life (1920), p. 86
Context: In our own inner selves, we find operating the same trio—ignorance, indifference, and carelessness. We are ignorant of our own latent capacities, of the degree of our likeness to God, of the possibilities of our lives. We are indifferent to the higher values and are content to the lower level of physical appetites and pleasures. Even when we recognize to some extent our possibilities and when we seek after a fashion to realize them, we grow careless, become swamped by the temporary, and lose sight of the eternal.... A lack of appreciation of the intrinsic worth and latent possibilities of every man we meet, indifference to his welfare, and carelessness as to his rights and privileges, prevent us from living on friendly terms with him.

Sec. 115
Some Thoughts Concerning Education (1693)
Context: Since the great foundation of fear is pain, the way to harden and fortify children against fear and danger is to accustom them to suffer pain. This 'tis possible will be thought, by kind parents, a very unnatural thing towards their children; and by most, unreasonable...
“Societies which inflict pain and discomfort upon their infants tend to neglect them as well.”
"Body Pleasure and the Origins of Violence" (1975)

Sec. 325
The Gay Science (1882)

“The greatness of a nation and its moral progress can be judged by the way its animals are treated.”
Widely attributed to Gandhi, sometimes citing Ramachandra Krishna Prabhu, The Moral Basis of Vegetarianism (1959). (Cf. Ralph Keyes, The Quote Verifier (2006), p. 74.) However, it is not found in that essay http://www.mkgandhi.org/ebks/moralbasis_vegetarianism.pdf nor in any of Gandhi's Complete works. http://animalsmattertogod.com/2013/09/13/mahatma-gandhi-hoax-quote-greatness-of-a-nation-and-its-moral-progress-can-be-judged-by-the-way-that-its-animals-are-treated/
The original quote seems to be by David Strauss, The Old Faith and the New (Der alte und der neue Glaube, 1872, trans. by M. Blind, New York: Henry Holt & Co., 1873), vol. II, ch. 71, p. 59 https://archive.org/stream/oldfaithnewconfe01stra#page/59/mode/2up: The manner in which a nation in the aggregate treats animals, is one chief measure of its real civilization.
Similar quotes, not attributed to Gandhi, are found throughout the twentieth century: e.g. The great actress, Mrs Fiske, once said to me, "The civilization of any country can be told by the way it treats its animals" (Zoe Berkeley, "Zoe Berkeley's Corner", Salinas Index-Journal, 1933-07-01, p. 8).
Attributed to Gandhi since at least 1980: The seal hunt truly is Canada's shame and we would do well to think of the words of Gandhi when he said, "The greatness of a nation and its moral progress can be judged by the way its animals are treated" (Doris Potter, Letter to the editor, The Gazette (Montreal), 1980-03-18, p. 8).
Disputed

The Zero Marginal Cost Society: The Internet of Things, the Collaborative Commons, and the Eclipse of Capitalism (2014)