(Chapter 5) Footnote 12:

The distinction between “hard” motor nerves and “soft” sensory nerves is Galenic. Herophilus had distinguished the two neural functions; Galen added the notion, paraphrased here by Vesalius, that “a soft substance is better suited to receive impressions, whereas a hard one is more suitable for acting.” ( De usu partium 3.633.14, tr. May 1968, p. 397).