THE SKELETAL SYSTEM 19 



Parietal Bone (Os Parietale) 



The paired parietal bones are of membranous origin. 

 They constitute the greater part of the roof and a con- 

 siderable part of the sides of the cerebral fossa. Each 

 bone, were it flattened out, would be nearly rectangular. 

 The true shape is obscured externally by a process of 

 the squamosal bone which covers the anteroventral part 

 of the parietal. Note the connections, posteriorly with 

 the interparietal, anteriorly with the frontal, and later- 

 ally with the squamosal bones. The curved inner surface 

 is smooth, but externally the temporal line separates the 

 dorsal from the lateral region of each bone. 



Frontal Bone (Os Frontale) 



The two frontal bones are of membranous origin. They 

 are united in the mid-dorsal line by the frontal suture. 

 Each bone consists of a frontal and an orbital part. The 

 former roofs the skull in front of the parietals, the latter 

 forms the greater part of the side wall of the orbito-tem- 

 poral fossa. The temporal line sharply separates these 

 two portions of the bone externally. The frontal part 

 joins the parietal bone posteriorly at the coronal suture. 

 This part decreases in width anteriorly and articulates 

 with the nasal and premaxillary bones. The orbital part 

 is concave. Its posterior region, partly overlaid by the 

 squamosal bone, forms a portion of the back wall of the 

 orbit. Ventrally it joins the alisphenoid, orbitosphenoid, 

 and maxillary bones — anteriorly the lacrimal and max- 

 illary bones. 



The frontal bones, internally, are the anterior wall of 

 the cerebral fossa, the lateral wall of the olfactory fossa, 

 and they partly inclose the nasal cavity posteriorly. An 



