THE HEAD. 



49 



Fig. 27. 



2. The Parietal Bone (Fig. 26). 



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 fare, and a circumference divided into four regions or borders. 



Faces. — The external fare is convex. It exhibits two curved ridges with 

 concavity 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 supra-orbital 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 fossae ; the 

 third, or middle, is plane, smooth, and of a triangu- 

 lar 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 middle line, and altogether above, the 

 parietal protuberance. This trifacial and very 

 salient projection presents at its base, on each side, 

 an excavation elongated transversely, into which 

 opens the parieto-temporal canal, and which 

 lodges 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 protuberance. 



Structure. — This bone contains much compact tissue, the spongy substance 

 existing only in its middle. 



Dei^elopment. — It is developed from two large lateral centres of ossification, to 

 which is added a single centre to form the parietal protuberance. In early life 

 the parietal ridges are absent. 



head of the cat (posterior 

 aspect). 



1, Occipital bone ; 2, zygomatic 

 process of the tempoml bone ; 

 3, t_vmpanic bulb; 4, condyle of 

 the temporal buue ; 5, malar 

 bone ; 6, orbital process of the 

 malar; 7, liitto of frontal bone; 

 8, 8, palatine bones; 9. 9, m;ixil- 

 lary bone; 10, premaxillary bone; 

 11, pterygoid bone; 12, sphenoid 

 bone ; 13, vomer. 



