=The Cranial Bones.=--The cranium or skull is especially adapted for the protection of the brain and the bones are flat and closely fitted to its surface. They have two layers of bone, the outer and the inner tables, of which the outer is the thicker, and between these is a tissue filled with blood-vessels, the _diploë_. In the infant, whose brain has not yet attained its full size, opportunity must be left for growth and the skull therefore consists of a number of bones with interlocking notched edges, where growth takes place, but in the adult it forms one solid covering of bone. The line where the edges of two cranial bones come together is called a _suture_. The suture between the frontal bone and the forward edges of the two parietal bones is called the _coronal suture_, that between the two parietal bones at the vertex of the skull is known as the _longitudinal_ or _sagittal suture_, and that between the occipital bone and the back edges of the parietal bones as the _lambdoidal suture_. Where the coronal and sagittal sutures meet is a membranous interval known as the _anterior fontanelle_, while the _posterior fontanelle_ is at the juncture of the sagittal with the lambdoidal suture. These fontanelles--so called from the pulsations of the brain that can be seen in them--close after birth either by the extension of the surrounding bones or by the development in them of small bones known as _Wormian bones_, the posterior one closing within a few months, the anterior by the end of the second year. In rickets, however, the anterior fontanelle remains open a long time, sometimes into the fourth year. [Illustration] [Illustration: FIG. 15.--Cranium at birth, showing sutures and fontanelles.] The =frontal bone=, as its name implies, forms the fore part of the head or forehead. It joins the parietal bones above and the temporal bones on either side. At the lower edge are the supra-orbital arches, each with a supra-orbital notch or foramen on its inner margin for the passage of the supra-orbital vessels and nerve, the nerve most affected in neuralgia. Just above the arches on either side are the superciliary ridges, behind which, between the two tables of the skull, lie the frontal sinuses. On the inner surface the frontal sulcus for the longitudinal sinus runs along the median line. The =parietal bones= are the side bones of the skull. They meet each other in the sagittal suture at the median line above and join the frontal and occipital bones at either end, while below they touch upon the temporal bones, the temporal muscles being attached in part along their lower surface. These muscles are inserted into the coronoid process of the lower jaw, which they thus help to raise and to retract. [Illustration: FIG. 16.--Front view of the skull. (After Sobotta.)]