The skull is subject to very various deviations from its healthy form. It will be sufficient to furnish a general account of them, without entering into a detailed description of any but the most important.

Very considerable malformations, both of the cranial and facial portions of the skull, form an essential part of the conditions already described as a hemicephalus, encephalocele, and hydrencephalocele, of congenital hydrocephalus, fissures of the facial bones, cyclopia, etc. In that rare form of encephalocele, in which the protrusion takes place through the ethmoid bone, the vault of the skull sinks down upon its base in the form of a saddle.

In congenital hypertrophy of the brain, but especially in congenital hydrocephalus, the size of the cranium is strikingly disproportioned to the small dimensions of the facial skull. The cranium may be expanded to a circumference of two feet or more; the frontal and parietal bones, especially, being very large, as well as the membranes filling the interspaces between them: the forehead projects greatly, the orbital plates are forced downwards, and so compress the orbits that they become mere narrow transverse fissures: the squamous portions of the temporal bones, and the broad plates of the occipital, incline towards a horizontal direction, the external meatus of the ears are depressed, and the base of the skull, besides being depressed, is remarkably small in proportion to the cavity of the skull. If, despite the large size of the skull, its ossification should be completed, the margins of the bones reach each other by means of long, ray-like denticulations, or they just meet along a sinuous suture (Harmonia), or else ossa triquetra are developed in the interspaces between them. One of the parietal bones, or one half of the frontal, may be increased in size, whilst the other remains unaltered, and then the sutures follow an unusual direction. This is the ordinary form in chronic congenital hydrocephalus; but if, during some interruption of the disease, certain sutures should have closed, a recurrence of the hydrocephalus will produce material alterations in it; the distension will take an unusual direction, and the head deviate, accordingly, from the ordinary hydrocephalic form.

Deficiency in the development of certain parts of the brain produces important deformities of the skull; such as flattening, or receding of the forehead, flattening of the back of the head, etc. The deformity is very striking, when it occurs only on one side. The most frequent deformities are those in which there is a preponderance of some one diameter, so that the skull is longer, broader, higher, or, in some other direction, greater than natural. Allied to these are the round, the blunt, four-cornered, and similar skulls, and those which are oblique. At one time the obliquity is found to consist in a displacement of the halves of the skull in a logitudinal or a vertical direction: and in this way the law of compensation is carried out, the apparently greater width of the cranial cavity on one side being made up for by its condition on the other. When the obliquity is considerable, the facial skull shares in it. At another time, the obliquity is occasioned by a lateral displacement of the several cranial vertebrae (which Carus names Scoliosis of the skull), so that the mesial line of the base of the skull is curved or serpentine. Other obliquities arise from the atrophy of the bones on one side; they are most marked in the face.