This bone (Os interparietale) is centrally placed between the squamous
part of the occipital and the parietal bones. It is usually described as
a single bone, although it ossifies from two chief lateral centers, and
is sometimes distinctly paired in skulls of young foals.
The =external surface= (Facies externa) is quadrilateral and is flat and
smooth in the very young foal; later it presents the median external
sagittal crest.
The =internal surface= (Facies cerebralis) presents the =internal
occipital protuberance=, a three-sided process which projects downward
and forward into the cranial cavity between the cerebral hemispheres and
the cerebellum; it has three concave surfaces and three sharp borders
which form part of the tentorium osseum.
The =posterior border= is thick; it joins the squamous part of the
occipital bone. The =lateral= and =anterior borders= are united by
suture with the parietal bones.
=Development.=—The interparietal ossifies in membrane from two chief
lateral centers.[13] It fuses first with the parietals, somewhat later
with the occipital, but the period at which this union takes place is
quite variable.