The year is conjectured from the fact that 1868 is the year in which CD’s correspondence most frequently mentions the orbicularis muscle; see, for example, Correspondence vol. 16, letter from William Bowman, 2 April [1868]. An early summary mentioned the orbicularis muscle and named the author as Richard Barwell, although the reasons for these choices are no longer clear, possibly as a result of further deterioration of the letter.
In Expression, p. 41, CD mentioned a bright light causing sneezing as the ‘radiation of nerve-force from strongly-excited nerve-cells to other connected cells’.