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BONES. 119
centres of ossification, and act independently until synostosis takes
place.
FoNTAXELLES.—The fontanelles of the head are six in number—two
situated in the median line, anterior and posterior, and four laterally.
They are membranous interspaces formed by the incomplete ossification
at the four angles of the parietal bones (Figs. 61 and 62).
The Anterior Median Fontanelle is situated at the anterior superior
angle of the parietal bones, and is formed by the incomplete ossific con-
P^IG. Gl.
Fig. fi2.

Skull at Birth, showing the Anterior and The Lateral Fontanelles.
Posterior Fontanelles.
dition of these angles as well as the superior angles of the two halves of
the frontal bone. It is quadrilateral in form, its angles extending into
the four sutures belonging to the frontal and parietal bones. It is the
largest of the six fontanelles, and usually remains partially open until
the tenth or fifteenth month after birth, holding in this respect a dose
relation with the rapidity of the developmod of the entire osseous system.
In quickly-closing fontanelles the teeth appear soon and the child walks
early} Sometimes this fontanelle remains open through years of early
life, and it has been known to exist in the adult.
The Posterior Median Fontanelle is situated at the posterior su])erior
angles of the parietal bones and the superior angle of the occipital bone.
It is triangular in outline, the angles extending into tlie sutures formed
by the parietal and occipital bones. This fontanelle is closed at birth
or shortly thereafter, the bones being united by membrane which per-
mits them to move freely upon each other.
The Lateral Fontanelles, four in number, are situated at the inferior
angles of the parietal bones and the bones in immediate juxtaposition
therewith. They are small in size and irregular in form, those situated
posteriorly being the larger. They are closed at birth or soon thereafter.
The fontanelles are gradually closed by the extension of the bones into
the membranes which fill the spaces: It is in this way that the angles
of the bones are completed and the sutures formed.
The posterior lateral and occasionally the anterior fontanelles are filled
^Allen's Human Anatomy.