The skull comprises two main parts: the neurocranium (bones surrounding the brain) and the viscerocranium (bones of the face); each is made of individual bones. The skull contains 22 bones in total; all are firmly interconnected in adults by immobile fibrous joints called sutures. The exception is the single bone of the lower jaw called the mandible, which articulates with the skull at a pair of mobile synovial ‘temporomandibular’ joints. The sutures are not straight lines, but are twisted to interlock the juxtaposed bones, increasing protection for the enclosed brain. In newborns, the bones are not firmly interconnected but are separated by wide fibrous areas at the suture sites to allow mobility and approximation of the bones, in ordeer to decrease the size of skull during birth and to permit the brain to grow, especially during the first year of life. The areas where more than two bones meet are much wider, forming what are called fontanelles. The fontanelles vary in size; the largest is the anterior fontanelle.