(Chapter 7) Footnote 4:

iugum, -ī n., Gk. zugo/n, to/ = yoke, hence zu/gwma for the arcus zygomaticus. The nomenclature is Hippocratic: Hipp. De articulis 30.6. See Galen In Hippocratis librum de articulis et Galeni in eum commentarii 18a.426.9ff.: “This slender bone, which attaches the upper jaw to the head, is called zygōma by the anatomists because Hippocrates himself said it yoked the two bones, lying close to them like a yoke.”