Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/33

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The paired parietal bones are of membranous origin. They constitute the greater part of the roof and a considerable 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 laterally 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.

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-temporal 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 maxillary 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