(Chapter 10) Footnote 4:

Although not articulating with the mental spines, the anterior belly of the digastric muscle originates from the digastric fossa, an area just below and lateral to the mental spines. Because of its intimacy with the geniohyoid muscle, Vesalius probably includes the digastric muscle as one of the two muscles in this portion of the mandible that “draw the jaw downward.”