THE PARIETAL BONES 



57 



occipital bone 



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 (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. 

 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- 

 fies in membrane from two chief lateral cen- 

 ters.^ It fuses first with the parietals, some- 

 what later mth 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 

 parietafis) 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 hke name on the interparietal and 

 occipital; in front it curves outward and is 

 continuous with the frontal crest. The sur- 

 face lateral 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, cerebral surface (Facies 

 cerebralis) is concave. It presents numerous 

 digital impressions (Impressiones digitate) ,. ^ , . , j ^ ^.u 



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 memngeal 

 arteries. Along the medial border there is a sagittal groove (Sulcus sagittahs) for 

 the superior longitudinal sinus. ^a„+.,^„ 



The anterior border joins the frontal bone at the paneto-frontal suture (Sutura 



^^" Thtposteriir border meets the occipital bone at the parieto-occipital suture. 

 Below this junction it curves inward and concurs with the temporal bone m the 

 formation of the temporal canal (Meatus temporahs) A transverse groove (Sulcus 

 transversus) connects this canal with the sagittal sulcus. 



The medial border is thick and serrated. It joms its fellow m 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. 



Inter- 

 parietal 

 bone 



Fig. 34. — Intbrparietal and Squamous Part or 

 Occipital Bone of New-born Foal; Ven- 

 tral ^'IEW. 



1, Junction of interparietal with squamous 

 part of occipital bone; 2, interparietal suture; 3. 

 internal occipital protuberance; 4, transverse groove 

 for sinus; 5, depression for vermis cerebelli; 6, de- 

 pression for hemisphere of cerebellum; 7, junction 

 of squamous part with lateral part of occipital 

 bone; S, junction of occipital with petro-mastoid; 

 9, 9, junction of interparietal with parietal. 



