fontanelle (n.)

also fontanel, 1540s, "hollow between two muscles," from French fontanelle (16c.), from Old French fontenele "small source, fountain, spring; fontanelle," diminutive of fontaine "spring" (see fountain), on analogy of the dent in the earth where a spring arises. In reference to the "hollow" in a baby's skull, it is first recorded 1741.