(Chapter 30) Footnote 30:

Instead of this sentence, the 1555 edition inserts the following: “But also for the sake of lightness, Nature did not wish this part of of the femur, which needed to widen on the sides in order to put forth its lower heads, to be smoothly rounded and prominent in its posterior side though it serves for the origin and insertion of no muscles, is not exposed to injuries, and is more or less surrounded by the lower heads of the femur extended posteriorly. Thus is is extremely agreeable to see how Nature besides the function of the parts and their particular use took into consideration strength as well as lightness, and along its length provided the femur with a long and ample cavity filled with marrow which makes this largest bone of the body quite light for its size.”