54 THE SKELETON OF THE HORSE 



smooth in the very young foal; later it presents the median external sagittal 



crest. 



The internal surface (Facies cerebralis) presents the internal occipital pro- 

 tuberance, a three-sided process which projects downward and forward into the 

 cranial cavity between the cerel)ral 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.^ It fuses first with the parietals, somewhat later with the occipital,, but 

 the period at which this union takes place is quite variable. 



The Parietal Bones 



The two parietal bones (Ossa parietalia) form the greater part of the roof of 

 the cranium; they unite in the median line, forming the sagittal suture. Each is 

 quadrilateral in outline and has two surfaces and four borders. 



The external surface (Facies parietalis) is convex, and is marked by a more or 

 less prominent curved line, the external sagittal crest ; this is median in its posterior 

 part, and is continuous with the crest of like name on the occipital bone; in front 

 it curves outward and is continuous with the frontal crest. The surface external to 

 the crest (Planum temporale) enters into the formation of the temporal fossa, and 

 is roughened for the attachment of the temporal muscle. 



The internal or cerebral surface (Facies cerebralis) is concave. It presents 

 numerous digital impressions (Impressiones digitata;) which correspond to the gyri 

 of the cere])rum. There are also furrows (Sulci vasculosi) for the meningeal arte- 

 ries. Along the inner 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 

 coronalis) . 



The posterior border meets the occipital bone at the parieto-occipital suture 

 (Sutura laml)doidea). Below this junction it curves inward and concurs with the 

 temporal bone in the formation of the parie to-temporal canal (Meatus temporalis). 

 A transverse groove (Sulcus transversus) connects this canal with the sagittal 

 sulcus. 



The internal border is thick and serrated. It joins its fellow at the sagittal 

 suture, and (in the young subject) meets the interparietal at the interparietal suture. 

 The line of junction is marked internally by the internal sagittal crest (Crista 

 sagittalis interna). 



The external border is beveled and is overlapped by the squamous temporal 

 bone, forming the parieto-temporal suture (Sutura parieto-squamosa). The angle 

 of junction of the external and posterior borders articulates with the posterior 

 angle of the temi)oral wing of the sphenoid. 



Development. — Each parietal bone ossifies in membrane from a single center. 

 In the young foal the central part of the bone is much mor(^ convex than in the 

 adult and forms a prominence similar to the pronounced tul)er parietale of the 

 young child; the external sagittal crest is not i^resent, and the external surface is 

 smooth. 



The siVfi;itt;il suture is usually closed at four years, the parielo-oceipital at five years, and the 

 parieto-temporal at twelve to lifte(>n years. 



' According to Martin, there are ori>iinally four centers, two anterior and two posterior 

 (smaller) ones, wliich fuse in a variable manner. 



