

THE PARIETAL BONE 



133 



the bone is developed in cartilage. The number of nuclei for the planum occipitale is usually 

 given as four, two appearing near the middle line about the second month, and two some little 

 distance from the middle line about the third month of fetal life. The planum nuchale of the 

 squama is ossified from two centers, which appear about the seventh week of fetal life and soon 

 unite to form a single piece. Union of the upper and lower portions of the squama takes place 

 in the third month of fetal life. An occasional center (Kerckring) appears in the posterior margin 

 of the foramen magnum during the fifth month; this forms a separate ossicle (sometimes double) 

 which unites with the rest of the squama before birth. Each of the lateral parts begins to 

 ossify from a single center during the eighth week of fetal life. The basilar portion is ossified 

 from two centers, one in front of the other; these appear about the sixth week of fetal life and 

 rapidly coalesce. Mall 1 states that the planum occipitale is ossified from two centers and the 

 basilar portion from one. About the fourth year the squama and the two lateral portions unite, 

 and about the sixth year the bone consists of a single piece. Between the eighteenth and twenty- 

 fifth years the occipital and sphenoid become united, forming a single bone. 



Articulations. The occipital articulates with six bones: the two parietals, the two temporals, 

 the sphenoid, and the atlas. 



The Parietal Bone (Os Parietale). 



The parietal bones form, by their union, the sides and roof of the cranium. Each 

 bone is irregularly quadrilateral in form, and has two surfaces, four borders, 

 and four angles. 



Articulates with opposite parietal bone 

 I 



With mastoid portion of 



With sphenoid temporal bone 



FIG. 132. Left parietal bone. Outer surface. 



Surfaces. The external surface (Fig. 132) is convex, smooth, and marked near 

 ic center by an eminence, the parietal eminence (tuber parietale), which indicates 

 \e point where ossification commenced. Crossing the middle of the bone in an 



1 American Journal of Anatomy, 1906, vol. v. 



