ABNORMAL CONDITIONS OP TDB CRANIUM.
Most of the abnormal conditions of the cra nium are dependent on circumstances con nected with the evolution of the brain, and are mostly acquired after birth; the only con genital variations being those in which there is a total or a partial privation of its parietes.
There is no vestige of it, or, indeed, of the head itself, in the true acephalous foetus; but, whenever the medulla oblongata is present, the base of the cranium is developed, and often times there are found rudimentary.portions of the other bones (false acephalia and anence phalia).
The parietal or occipital bones, and some times all of them are imperfect in that mal formation termed encephalocele, which, in some cases, is analogous to spina bifida, and, in others, to hernia cerebri. When serous fluid constitutes the tumour, the deficiency of the bones is considerable, owing to the arrestation of the formative process; but when the brain protrudes, their development continues in such a way as to embrace the root of the tumour, and then the calvaria, flattened and in contact with the base, exhibits an opening through which the hernia escaped.
The cranium is said to be, at times, insuffi ciently evolved; the evolution of its parts being accelerated and their coalescence prematurely effected, so that the ossific capsule is formed before the bmin has attained its full growth. It is, however, most probable that in this as in other cases it adapts itself to the brain, and that it is on an imperfect development of that organ that the smallness of the cranium is de pendent; but varieties of this description which are connected with deficiences of mental en dowment will scarcely admit of enumeration.
The parietes of the cranium may be preter naturally thin, without this being dependent on disease; but they are most obviously in that condition in hydrocephalus, in which affection, however, there are two opposite states of the skull.
When the disease occurs in infancy, and persists for any length of time, the bones of the calvaria usually become thin and pellucid ; the spaces between them are of great extent ; and the deposition of the inorganic texture is arrested in such a way that instead of bones we have frequently little more than a membrano -cartilaginous lamina, and. some
times not even that; for instances have been known in which the upper part of the head has been covered by membrane only. This suspension of action, however, is in some instances only temporary. The deposition of ossific matter becomes then more rapid and abundant than under ordinary circumstances; the points of deposit are more numerous than usual; and a skull of' gigantic dimensions and of peculiar and premature hardness is pro duced.
It has been sufficiently explained that the several ossific elements of the cranium unite in definite numbers to produce the bones which we have been occupied in describing. Never theless, it not unusually happens that some of these elements, or, otherwise, adventitious de posits of a similar character, which manifest themselves, do not flow into and combine with the other elements of the bone in which they occur ; but, on the contrary, each in itself forms the centre of an ossific process, and the bone thus formed (be it large or small) articu lates by its circumference to the parts with which it comes into contact. These adven titious pieces are commonly known under the name of ossa Wormiana, because it is supposed that they were first described by Wormius, a physician at Copenhagen in the seventeenth century ;* they are also called ossa triquetra, triangularia, ossa suturarum, ossa supranume raria. They vary in situation, nuniber, and size. In general they are situated in the lambdoidal suture ; they are, however, met with in the sagittal, occasionally in the coronal, and (though rarely) in the squamous suture. One of the most remarkable is that which sometimes replaces the superior angle of the occipital bone, called by Blasius os triangulare or epac tate. Bertin describes one in the situation of the anterior fontanelle.