From Benjamin Rush's "Observations Intended to Favour a Supposition That the Black Color (As It Is Called) of the Negroes Is Derived From the Leprosy":
4. The leprofy induces a morbid infenfibility in the nerves. In countries where the difeafe prevails, it is common to fay that a perfon devoid of fenfibility, has no more feeling than a leper. This insensibility belongs in a peculiar manner to the negroes. Dr. Mofeley fays, "they are void of fenfibility to a furprizing degree. They fleep found in every difeafe, nor does any mental difturbance ever keep them awake. They bear furgical operations much better than white people, and what would be a caufe of infupportable pain to a white man, a negro would almoft difregard. I have amputated the legs of many negroes, who have held the upper part of the limb themfelves." This morbid infenfibility in the negroes difcovers itfelf further in the apathy with which they expofe themfelves to great heat, and the indifference with which they handle coals of fire.
Rush himself cited Treatises Upon Tropical Diseases, p. 475.