Fontanel

Occasionally adult fontanel(le)s aren't of natural origin. Someone with a severe head injury (gunshot, equipment accident, etc.) as a small child may require reconstruction of the skull. Obviously as the skull grows, the artificial bits, if any, don't. So you can end up with a gap or lumpy area. These should generally be fully reconstructed at some point, but if the person in question isn't engaging in head-cracking activities, it's not generally urgent unless the gap places the brain in danger of damage from direct contact to that area of the head.

Fon"ta*nel` (?), n. [F. fontanelle, prop., a little fountain, fr. fontaine fountain. See Fountain.]

1. Med.

An issue or artificial ulcer for the discharge of humors from the body.

Wiseman.

2. Anat.

One of the membranous intervals between the incompleted angles of the parietal and neighboring bones of a fetal or young skull; -- so called because it exhibits a rhythmical pulsation.

⇒ In the human fetus there are six fontanels, of which the anterior, or bregmatic, situated at the junction of the coronal and sagittal sutures, is much the largest, and remains open a considerable time after birth.

© Webster 1913.