168 



THE SKELETON. 



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 twenty-fifth years, the occipital and sphenoid become united, form- 

 ing 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. 



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. 



Surfaces. The external surf ace (Fig. 133) is convex, smooth, and marked about 

 its centre by an eminence called the parietal eminence, which indicates the point 



FIG. 133. Left parietal bone. External surface. 



where ossification commenced. Crossing the middle of the bone in an arched 

 direction are two well-marked curved lines or ridges, of which the lower is the 

 more distinct and is termed the temporal ridge ; it marks the upper attachment of 

 the temporal muscle and follows a semicircular course across the bone. The upper 

 ridge is less marked, and pursues a similar course across the bone, but about two- 



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



