(Chapter 4) Footnote 47:

This sentence, which is opaque even for Vesalius, is glossed by the middle two illustrations. Punning on the Latin idiom ad unguem, “exactly” (where a seam is too tight to admit a fingernail) and perhaps also on unguis as a kind of bivalve whose halves join along an undulating line curved like a series of fingernails, it compares bone sutures to a fancy sewn seam whose lines resemble a series of fingernails, and to the rabbet joints used to attach the sides to the front and back of a wooden box or the coachwork in a litter.