This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
As the child grows, the bones of the face increase more rapidly than do those of the vault. The bony prominences become marked, due to the action of the various muscles of mastication, expression, etc., inserted into them. The face is much larger in size in proportion to the calvarium than was the case in infancy. While in infancy bone is practically homogeneous, in late childhood and early adult life cavities begin to develop in it.
Outer and inner tables of compact tissue are formed, separated by diploic structure. The frontal, ethmoidal, and other air-sinuses are an exaggeration of these diploic spaces. They are lined with mucous membrane and communicate with the nasopharynx. The diploe first begins to appear about the age of ten years, but is not well formed until early adult life. It contains large veins, called the diploic veins, which communicate with the veins both of the inside and outside of the skull. In injuries to the skull bleeding from these veins is rarely troublesome and usually stops spontaneously.
The skull is thinner in the white than in the negro race. It is thickest over the occipital protuberance and mastoid processes. The bone is thinnest in the temporal and lower occipital regions. The two tables are separated widely from one another in the region of the frontal sinuses.
The inner table is thinner and more brittle than the outer one, and in fractures it is almost always more extensively splintered than the outer. In rare cases the outer table may be temporarily depressed by a glancing blow and spring back into place without showing any depression, while the inner table may be fractured.
The two tables are not exactly parallel.
Where the skull is thin, as in the temporal and occipital regions, they are close together; where it is thick, they are farther apart. The outer surface of the skull is comparatively even and smooth. The inner surface is quite uneven, being depressed in places to receive the convolutions of the brain. For this reason it is necessary to use the trephine with great care, as it may cut through on one side of the circle and injure the dura mater before it cuts through the other part.
The sutures of the skull begin to ossify at about the age of forty years and continue to fuse until about the eightieth year. Frontal Sinuses. - The frontal sinuses begin to develop at the age of seven years, but do not increase rapidly in size until puberty. When adult age is reached they are well developed. They may extend well out over the orbits, reaching to within a short distance of the temporal ridge, while in other instances they do not go beyond the supra-orbital notches. In height they may reach the lower portion of the frontal eminences or may cease at the level of the superciliary ridges. The size of the sinus cannot be judged from the size of the bony prominences. Neither is the size nor sex of the individual any criterion. In a small female we have seen them of considerable size. When diseased sufficiently to give rise to symptoms, they will be found to be quite large. They are separated from each other by a septum, and if extensive are divided into several pockets or recesses. They open into the infundibulum, at the anterior extremity of the middle turbinated bone in the middle meatus of the nose. Fracture of the outer wall of the sinus not infrequently occurs without involving the inner table.
Fig. 14. - Transverse section of the skull showing its variations in thickness at different points.
Fig. 15. - Frontal sinus of one side; the anterior wall has been cut away, exposing its interior.
Fig. 16. - Surface chipped away to show the mastoid antrum and cells, the latter unusually well developed.
Fig. 17. - The cerebral blood sinuses.