THE PARIETAL BONES 57 
The external, parietal surface (Facies parietalis) is quadrilateral and is flat and 
smooth in the very young foal; later it presents the parietal crest. 
The internal, cerebral surface (Iacies 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. 
Behind the base of the protuberance there is a transverse groove for the sinus com- 
municans of the dura mater. 
The posterior border is thick; it joins the squamous part of the occipital 
bone. ‘The lateral and anterior borders are 
united with the parietal bones. Squamous part of 
Development.—The interparietal ossi- occipital bone 
fies in membrane from two chief lateral cen- 
ters.! It fuses first with the parietals, some- 
what later with the occipital, but the period 
at which this union takes place is quite 
variable. 
THE PARIETAL BONES 
The two parietal bones (Ossa_parie- 
talia) form the greater part of the roof of 
the cranium; they unite in the median line, 
forming the parietal suture. Each is quadri- 
lateral in outline and has two surfaces and 
four borders. 
The external, parietal surface (Facies 
parietalis) is convex, and is marked by a 
more or less prominent curved line, the ex- 
ternal parietal crest; this is median in its 
posterior part, and is continuous with the 
crest of like name on the interparietal and 
Fic. 34.—INTERPARIETAL AND Squamous Part oF 
occipital; im front it curves outward and is Occrprta, Bone oF NEw-BoRN Foau; VEN- 
continuous with the frontal crest. The sur- TRAL VIEW. 
face lateral to the crest (Planum temporale) Hy der, GH Unvenetsetl eal Gena 
part of occipital bone; 2, interparietal suture; 3, 
enters into the formation of the temporal internal occipital protuberance; 4, transverse groove 
fossa, and is roughened for the attachment for sinus; 5, depression for vermis cerebelli; 6, de- 
of the temporal muscle. Se fae 
The internal, cerebral surface (Facies bone; §, junction of occipital with petro-mastoid; 
cerebralis) is concave. It presentsnumerous 9, 9, junction of interparietal with parietal. 
digital impressions (Impressiones digitatze) 
which correspond to the gyri, and ridges (Juga cerebralia) which correspond to the 
sulci, of the cerebrum. There are also furrows (Sulci vasculosi) for the meningeal 
arteries. Along the medial border there is a sagittal groove (Sulcus sagittalis) for 
the superior longitudinal sinus. 
The anterior border joins the frontal bone at the parieto-frontal suture (Sutura 
parieto-frontalis). 
The posterior border meets the occipital bone at the parieto-occipital suture. 
Below this junction it curves inward and concurs with the temporal bone in the 
formation of the temporal canal (Meatus temporalis). A transverse groove (Sulcus 
transversus) connects this canal with the sagittal sulcus. 
The medial border is thick and serrated. It joins its fellow in great part at the 
parietal suture, but (in the young subject) meets the interparietal at its posterior 
1 According to Martin, there are originally four centers, two anterior and two posterior 
(smaller) ones, which fuse in a variable manner. 
