(Chapter 23) Footnote 16:

Lat. murus or “wall.” Vesalius is describing a type of light pulley that is open on one side for faster rigging. The side of the wheel facing the pulley’s frame or “wall” does not need to rise as far above the groove because the frame prevents the rope from slipping off on that side. Vesalius will now compare the capitulum of the humerus (P in his figs. 1 and 2) to such a “wall.”