The bonlers are of various lengths.; the superior is the longest, the iqftrior is the shortest, and the anterior is longer than the posterior. The superior is united to the same border of the opposite bone by the regular interchange of serrations of the outer table ; the anterior and posterior reverse the arrange ment Nvhich obtains in the frontal and occipital bones; that is, they are overlapped in the upper part, while in the lower they overlap those bones ; the inferior is sharp, and merely terminates the articular surfa.ce already al luded to.
1 e angles contained within these borders are the frontal (which is nearly a right angle) formed by the superior and anterior borders; the occipital (more obtuse) by the superior and posterior borders; the mastoidal, truncated and articulated with the rnastoid process of the temporal bone; and the spinous (acute) re ceived on the tip of the great wing of the sphenoid, and intervening between the tem poral and frontal bones. The mastoidal angle is, on its inner surface, traversed by a sulcus (the sulcus latcralis) to lodge the lateral sinus and to transmit it from the occipital to the temporal bone. The spinous angle is deeply grooved on its inner surface by the suicus spinosus for the middle meningeal artery, or the arteria spinalis durm matris; this groove has its place frequently supplied by a canal, then called canalis spinosvs.
Its connexions are with its fellow above ; the temporal and sphenoid beloNv; the frontal before; and the occipital behind.
The parietal, like each half of the frontal bone, is developed from the protuberance; and from this point the ossific matter radiates to wards its several borders. While this process is going on, the part above and the part below the centre form a considerable angle with each other; but this is much effaced when the edges have arrived at their destination, espe cially when the squamous process of the tem poral quits its vertical for its curved position.
_Articulation (2f the cranial bones.—Thesc several bones are locked together so as to form the envelope of the brain, and the mode by which their secure adherence to each other is effected, differs in the summit, on the sides, and in the base of the cranium.
In the calvaria they are united either by the overlapping or by the dove-tailing of their edges, or else by the two modes combined. The inner table does not proceed so far as the external, and the latter being jagged with pro cesses which have no definite form, but which are either tortuous, or narrower at their fixed than at their free extremity, the outer tables are immovably joined by the fixation of the processes of each side into the spaces of the other. By this means the inner tables of the
tWO bones are brought nearly into contact, a thill lamina only of cartilage intervening ; so that on looking into the vault, but little more than a plain line will be noticed. Here, however, there is no overlapping of the outer tables; but the only instance of it is in the junction of the two parietals on the median line, by which, in effect, they form but one bone. On the sides of the skull there is a mere overlapping of the descending by the ascending portions, and to accomplish this, and yet maintain uniformity of surface, those parts of the outer tables .which project beyond the inner are pared off or thinned in opposite directions. Thus the squamous processes of the temporal bones and the great wings of the sphenoid rise upwards from a fixed basis and form a wall which is bevelled off on the inner edge of its outer plate, so as to receive the parietal and frontal bones, the outside of which sustains a corresponding bevelling, by which arra.ngement they are prevented from being thrust outwards. The articulation of the anterior and the posterior with the middle portion of the calvaria, is a modification of the two preceding; that is, the outer table is partly bevelled and partly denticulated. The frontal and occipital bones are symmetrical and single, while there are parietal; and, though these are well united by their mutual interchange of denticulation, they are yet more firmly consolidated by the extension of the frontal and occipital bones on the frontal and occipital angles of the parietals, and on their borders to some distance from those angles; each symmetrical bone thereby forming a spe cies of cramp on the parietals. The edges, however, of the outer tables are not pared to a sharp ridge, but there is left sufficient to be fashioned into processes to maintain the secu rity of the skull in a longitudinal direction. The parietals being thus firmly secured above and below, the intervening portion of their edges is competent to act as girders themselves, and, in fact, we find that the lower part of their anterior and posterior borders overlap the corresponding portions of the frontal and occi pital bones respectively.