Delayed ossification of the parts of a baby’s skull that have not yet grown together. The large fontanel is the space between the forehead bone and the temporal bones; the small fontanel is between the bone at the rear of the skull and the temporal bones. A fontanel consists of rigid periosteum, which protects the blood vessels and brain beneath it. The small fontanel is usually closed (ossified) by the age of six weeks, and the large fontanel by the time the child is 18 months old. There are variations, and closure after the second birthday is not abnormal. The large fontanel provides a useful means of diagnosing certain abnormalities in young children. Normally the skin above the fontanel is on the level of its surroundings. If it sinks, it suggests dehydration; if it is raised, it indicates high pressure within the skull, possibly as the result of hydrocephalus or meningitis, but also caused by vigorous crying. If the fontanel is too small the skull cannot grow properly, and the head is too small (microcephalus); the condition can lead to restricted brain development and mental deficiency. An excessively large fontanel is usually a sign of incipient hydrocephalus; careful checking of the diameter of the skull is important in such cases. Diagnosis is by various tests and radiological examination. Treatment is according to the seriousness of the cause of the abnormality. If the fontanel is late in closing vitamin D can be given; surgery is sometimes necessary.