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BONES. 69

At birth the sphenoid bone is in three separate parts, exehKling the
sphenoidal tin-binatetl Ijones. The great wings and external })tervgoid
plates have joined the internal pterygoid plates on either side, and the
posterior portion of" the body has joined the anterior portion, including
the lesser wings. The great wings join the body about the end of the
first year. The spheno-turbinated bones unite with the body, and the
posterior surface of the l)ody with the basilar process of the occipital
bone, about the age of puberty.

The Paeietal Bone.
The parietal bones form a large portion of the walls and the greater
part of the roof of the brain-case. They are two in nunilier, lateral in shape, and have two surfaces, an external and an internal ; four
borders anterior, posterior, superior, and inferior ; and four angles, ante-
rior superior, posterior superior, anterior inferior, and posterior inferior.
The External Surface (Fig. 25) is convex in form, the greatest con-
FiG. 25.
W'/?/,

rO^
Left Parietal Bone, external surface.
vexity being in the centre of the bone, forming the parietal eminence
and indicating the point where ossification commences. Below this