1. Deficiency And Excess Of Development

In cases of Acephalus, the skull is altogether wanting, or is reduced to a merely rudimentary condition. It is liable, also, to various degrees of defect, in Acrania being without any vault, and in Encephalocele and Anencephalus presenting various, but less degrees of the same anomaly. Defects of other parts of it are noticed when the cranial or facial bones are fissured, when certain portions of the brain are wanting, or symmetrical parts of it are fused together, as happens in Cyclopia, etc. Examples are also met with in which the development of the skull is arrested in a less degree, apertures closed by membrane being found in its bones, or large membranous interspaces between those that form the cranial vault: the fontanelles are large, or unusual ones exist, or certain sutures continue permanently unclosed. The cases of this class mostly owe their origin to a preter-naturally large size of the brain - to Hypertrophy or Hydrocephalus.

The number of bones composing the skull is occasionally incomplete; particular bones are wanting altogether, and sometimes their place is supplied by the enlargement of those in the neighborhood; thus the nasal processes of the superior maxillaries may occupy the space which is left by deficiency of the nasal or lachrymal bones.

An excess of development is observed in those cases where more or less of a second head is formed; and premature closure of the sutures and fontanelles constitutes another, but a less degree of the same general condition.

When certain sutures, such as the frontal, do not close, when there are unusual accessory sutures, such, for instance, as a horizontal one through the parietal bones, but especially when Wormian bones exist, the number of the bones of the skull is increased.

The Wormian bones are most common in the lambdoidal suture, and in the squamous; they are less frequent in the coronal and sagittal, and are most rare where the wings of the sphenoid meet the parietal and temporal bones, and the roofs of the orbits. In the lambdoidal and squamous sutures they are not unfrequently very numerous, and even form two or three rows and as many sutures. When they occur in the other sutures they are often only single; and this is true especially of the sutures surrounding the wings of the sphenoid bone. In their situation, as well as in their dimensions and form, they are usually symmetrical, though there are some interesting cases in which those of the one side do not correspond with those on the other. A Wormian bone situated over a fontanelle receives the special denomination of a fonta-nellar bone. Finally, the outer layer of the Wormian bones is usually broader than the inner: sometimes they form part of the outer table of the skull only, and, in rare instances, only of the inner. Their existence is chiefly to be accounted for by the large interspaces which are left between the cranial bones in congenital hypertrophy of the brain and in hydrocephalus.