THE HEAD. 35 



point where the bone is bent posteriorly ; they separate the anterior lateral 

 from the corresponding posterior lateral border, and are occupied by the 

 tuberous portion of the temporal bone. 



Structure. The occipital bone contains much spongy substance. 



Development. It is developed from four centres of ossification : one, the 

 anterior, is single, and forms the occipital protuberance; another, the 

 posterior, also single, forms the basilar process ; the other two are pairs, and 

 comprise each a condyle, with the styloid process and the corresponding 

 condyloid foramen. 



2. The Parietal Bone. 



The parietal is a wide and thin bone, very much arched to form the roof 

 of the cranial cavity. It is bounded above by the occipital bone, below 

 by the frontal, and laterally by the two temporal bones. It offers for study 

 an external and internal face, and a circumference divided into four regions or 

 borders. 



Faces. The external face is convex. It exhibits two curved ridges whose 

 concavity is directed outwards ; these two crests, which are termed the 

 parietal ridges, approach each other and unite superiorly, to be continued 

 with the antero-posterior ridge of the occipital bone ; below they diverge 

 and proceed, one on each side, to join the supraorbital process. They 

 divide the surface of the bone into three portions : two lateral, which are 

 rough and traversed by vascular channels, forming part of the temporal fossse; 

 the third, or middle, is plane, smooth, and of a triangular form, and covered 

 by the skin. The internal face is concave, covered by digital impressions, 

 and grooved by small vascular canals ; it offers, on the median line, and 

 altogether above, the parietal eminence. 1 This trifacial and very salient pro- 

 jection presents at its base, on each side, an excavation elongated trans- 

 versely, into which opens the parieto-temporal canal, and which is destined 

 to lodge a venous sinus. It is continued, in front, by a median crest, which 

 is often replaced by a slight groove, the saggital furrow, bordered by linear 

 imprints. Two other ridges, resulting from the abutment of the lateral 

 border of the bone against the anterior face of the petrous bone, rise from the 

 sides of this eminence and descend to the sphenoid bone ; they separate the 

 cerebral from the cerebellar cavity. 



Borders. The superior border is notched, thick, and slightly dentated ; 

 it articulates with the occipital bone. The inferior border, slightly concave, 

 and deeply dentated, offers an external bevel in its middle portion, and an 

 internal bevel on its sides; it corresponds with the frontal bone. The 

 lateral borders are very thin, and are cut at the expense of the external plate 

 into a wide, sloping edge, which shows a groove destined to form the 

 parieto-temporal canal. A very prominent angle separates each into two 

 portions, an inferior, that articulates by suture with the squamous portion 

 of the temporal bone ; and a superior, curved inwards towards the centre of 

 the cranial cavity ; the latter portion of the lateral border is in contact 

 with the anterior face of the petrous portion of the temporal bone, with 

 which it concurs to form the lateral crest that descends to the parietal 

 eminence. 



Structure. This bone contains much compact tissue, the spongy sub- 

 stance existing only in its middle. 



Development. It is developed from two large centres of ossification, to 



1 The internal occipital eminence of Man. 



