PARIETAL BONE. 



63 



angle articulates with the mastoid portion of the temporal bone, and generally 

 presents on its inner surface a broad shallow groove for lodging part of the lateral 

 sinus. 



Development. The parietal bone is formed in membrane, being developed by 

 one centre, which corresponds with the parietal eminence, and makes its first 

 appearance about the fifth or sixth week of foetal life. Ossification gradually 



Fig. 26. Left Parietal Bone : Internal Surface. 



Ant . Infer. A.vtjl 



extends from the centre to the circumference of the bone, the angles are conse- 

 quently the parts last formed, and it is in their situation, that the fontanelles exist, 

 previous to the completion of the growth of the bone. 



Articulations. With five bones; the opposite parietal, the occipital, frontal, 

 temporal, and sphenoid. 



Attachment of Muscles. To one only, the Temporal. 



THE FRONTAL BONE. 



This bone, which resembles a cockle-shell in form, consists of two portions . 

 a vertical or frontal portion, situated at the anterior part of the cranium, forming 

 the forehead ; and a horizontal or orbito-nasal portion, which enters into the forma- 

 tion of the roof of the orbits and nose. 



Vertical Portion. External Surface (fig. 27). In the median line, traversing 

 the bone from the upper to its lower part, is occasionally seen a slightly elevated 

 ridge, and in young subjects a suture, which represents the point of union of the 

 two lateral halves of which the bone consists at an early period of life ; in the 

 adult, this suture usually disappears, excepting below. On either side of this 

 ridge, a little below the centre of the bone, is a rounded eminence, the frontal 

 eminence. These eminences vary in size in different individuals, and are occa- 

 sionally unsymmetrical in the same subject. They are especially prominent in 



