In addition to the usual centres of ossification of the cranial bones, others may occur in the course of the sutures, giving rise to irregular, isolated, sutural bones (fig. 382). They occur most frequently in the course of the lambdoid suture, but are occasionally seen at the fontanelles, especially the posterior one, the pterion ossicle, sometimes exists between the anterior inferior angle of the parietal bone and the greater wing of the sphenoid hone. These bones vary much in size, but have a tendency to be more or less symmetrical on the two sides of the skull. Their number is generally limited to two or three; but more than a hundred have been found in the skull of a hydrocephalic subject.
Applied Anatomy.-An arrest in the ossifying process may give rise to deficiencies, gaps, or fissures in the cranium, which are of importance from a medico-legal point of view, as they are liable to be mistaken for fractures. The fissures generally extend from the margins towards the centre of a bone, but the gaps may be found in the middle as well as at the edges. In course of time they may become filled with thin laminae of bone. In some instances, however, the gaps are due to absorption of bone already formed. This is especially so when they appear in the centre of a bone such as the parietal, the ossification of which has already been described as occurring in a regular manner radiating from two centres.