THE PARIETAL BONES. 



59 



a rule, there are four, but there may be only one (Blandin) or as many as eight 

 (Meckel). They appear about the eighth week of foetal life, and soon unite to 

 form a single piece, which is, however, fissured in the direction indicated in 

 the figure. The basilar and two condyloid portions are each developed from 

 a single nucleus, which appears a little later. The upper portion of the 

 tabular surface that is to say, the portion above the transverse fissure is 

 developed from membrane, and may remain separated from the rest of the 

 bone throughout life, when it constitutes the interparietal bone ; the rest of the 

 bone is developed from cartilage. At about the fourth year the tabular and the 

 two condyloid pieces join, and about the sixth year the bone consists of a single 

 piece. At a later period, between the eighteenth and tAventy-.fifth years, the 

 occipital arid sphenoid become united, forming a single bone. 



Articulations. With six bones : two parietal, two temporal, sphenoid, and atlas. 



Attachment of Muscles. To twelve pairs : to the superior curved line are 

 attached the Occipito-frontalis, Trapezius, and Sterno-cleido-mastoid. To the 

 space between the curved lines, the Complexus, 1 Splenius capitis, and Obliquus 

 capitis superior ; to the inferior curved line, and the . space between it and the 

 foramen magnum, the Rectus capitis posticus, major and minor; to the transverse 

 process, the Rectus capitis lateralis ; and to the basilar process, the Rectus capitis 

 anticus, major and minor, and Superior constrictor of the pharynx. 



^opposite parietal l> 



" ' 



Articulates 



Squamous portion of 

 temporal bone. 



FIG. 26. Left parietal bone. External surface. 



The Parietal Bones. 



The Parietal Bones (paries, a wall) form, by their union, the sides and roof of 

 the skull. Each bone is of an irregular quadrilateral form, and presents for 

 examination two surfaces, four borders, and four angles. 4 



1 To these the Biventer cervicis should be added, if it is regarded as a separate muscle. 



