(Chapter 21) Footnote 9:

Here and throughout the figure legend, the 1555 edition substitutes superior scapulae processus for summus humerus when referring to the acromion. This avoids the ambiguity of the earlier name, which suggests the upper end of the humerus instead of a portion of the scapula. Except where Vesalius is discussing his nomenclature, we shall regularly translate these “acromion.” See also n. 61 below.