(Chapter 10) Footnote 8:

See De ossibus Ch. 6 (2.754.15-18, tr. Singer 1952, p. 771): “The bone of the lower jaw is not single, as one might think, for when boiled it too is separated at the point of the chin, so it is seen to be composite.” Cf. Galen’s commentary on Hippocrates’ De articulis 18a.444.1ff., 18a.459.13ff., where he says the halves are joined by symphysis. But Galen admitted some doubt about this feature, at least in apes ( De anat. adm.; see n. 12 below).