It is by a process analogous to the pre ceding that the occipital bone occasionally presents a suture between the upper and under halves of its posterior portion. The elements of those two parts combine among themselves, and the pieces resulting from their union ap proach, and, instead of forming the continuous bone, as we usually see it, they are associated by means of an additional suture.
An anomaly of not very unusual occurrence is the permanence of the suture uniting the two halves of the frontal bone, and which is seldom apparent beyond the second year of extra-uterine life.
There are but few skulls which are perfectly symmetrical, although the variation of one side from the other is generally so slight that the eye does not at once detect it. In numerous cases, however, the want of symmetry forcibly obtrudes itself; sometimes one half is considerably larger than the other ; and in other cases it appears to be thrown out of position, as though, during the time that the parietes were soft, pressure had been applied in front and behind, and, by a sort of rotatory movement, it had been drawn back on one side and pushed forward on the opposite. There does not appear to be an absolute uniformity among the skulls of this description saving that the projections are always situated diagonally with respect to each other ; that is, if it be twisted to the right, the right half of the frontal bone will be in advance of the left; while the posterior part of the left parietal, and the corresponding side of the occipital bone, will project behind the right. This is by far the most prevalent variation, but, occasionally, the left half of the frontal bone is in advance, and, in such instances, the posterior increase will be on the right side.
The change which takes place in advanced age can scarcely be accounted an anomaly. At that period the skull is much more an entire bone than it is in the earlier epochs. The sutures are to a certain extent effaced, and a mere line indicates the former disjunction of the bones. It is on the interior of the skull that these sutures are first effaced, and on the exterior the order of obliteration is from the summit to the base. It has been affirmed that
the volume of the skull diminishes in old age, and that it is susceptible of change, in different directions, after the bones are locked together. It is, however, certain that its external con figuration is somewhat altered, for the promi nences formed by the centres of ossification of the parietal and frontal bones become flattened and undistinguishable from the rest of the parietes ; which, as old age sets in, become thinner than they were previously. This change, however, is but temporary, for, in extreme old age, the skull is thicker and more porous than at any antecedent period of life. This hyper trophy is produced by the recession of the inner from the outer table, and the conversion of some part of the substance of each into a thin spongy tissue ; the dipliie itself sustaining an analogous alteration, by the enlargement of its cells, and the thinning of the plates which form their walls.
Occasional instances occur in which the skull is of inordinate thickness, and this, appa rently, without its being connected with the age of the subject. The late Mr. Joshua Brookes had some sections of a skull, found in a church yard in Lancashire, of nearly three quarters of an inch in thickness; and specimens have been seen of more than an inch. In some of them the diploe is perfect, the augmentation being in the two tables ; in others, and indeed in the majority of specimens, the two tables and the dipliie are confounded together in one thick mass of matter, which is of an ivory hardness. It is not improbable that we might justly refer this condition, as well as some other peculiarities of the cranium, to inflammation of the bone itself, or of its investing membranes. That exostosis is the product of a limited periostitis admits of but little dispute, and it is very likely that those cases of hyperostosis in which there is a uniform deposit of bone, only mark the effect of a more diffused and general inflammation ; the more so, since we meet with these local and general deposits, as well on the inner, as on the outer table of the skull, and for the existence of which it would otherwise be impossible to account.