Disarticulated bones of the skull
The skull consists of 22 bones; 16 in the neurocranium (brain-case), and 8 in the endocranium (facial bones). At birth, these bones are not secured tightly together - this allows for passage of the infant through the birth canal (newborn heads are often cone-shaped for the first few days, if delivered vaginally), and allows for the rapid expansion of the brain during the first several years of life
By adulthood, however, the skull is one cohesive unit, and one of the strongest and least-deformable structures in nature. For human skulls, upwards of one ton of force (if evenly applied) is required to depress the bone just one centimeter. Of course, fractures and punctures can weaken the structural integrity of the skull significantly.
Anatomical Atlas, Illustrative of the Structure of the Human Body. Henry H. Smith, 1867.