Regions of the Skull

The human skull can be divided into two main regions, the neurocranium and the viscerocranium (see Fig. 1). This basic distinction between regions of the skull becomes important in understanding the normal process of craniofacial development, the mechanisms involved in creating the structures of the skull and face. This regional division can provide a basis for describing the disorders that can occur during development.

The neurocranium is the region of the skull that surrounds and encases the brain and sensory organs (the eyes, middle ear, olfactory bulbs). It is further subdivided into the cranial vault – the portion of the skull that overlies the brain – and the cranial base, the portion which underlies the brain. The cranial base and the cranial vault are each made up of many smaller bones.
     The cranial vault is composed of the following bones : parietal, frontal, squamous temporal, and the squamous part of the occipital. Bone formation in this region occurs primarily by way of intramembranous ossification.
     The cranial base consists of the following bones: basilar and lateral portions of the occipital, sphenoid, ethmoid, and the petrous and mastoid part of the temporal bone (see Fig. 2 and 3). These bones are formed through endochondral ossification, and for this reason the cranial base is sometimes referred to as the chondrocranium.

The viscerocranium is the region of the skull that makes up the bones of the face. The viscerocranium consists of the following bones: nasal, lacrimal, maxilla, palatine, vomer, zygomatic, and mandible (see Fig. 2 and 3). A large portion of the face and neck is derived from structures known as pharyngeal arches. There are five pharyngeal arches numbered from 1 to 6; the viscercranium is primarily formed from Arch 1 and 2. Each contributes not only to the development of a particular portion of the skull, but also to the creation of specific muscles, nerves, and blood vessels.
      The basic pattern and creation of facial bones is concentrated to weeks 4-10 of embryonic development (Sweeney, 1998). The bony and cartilaginous structures of the viscerocranium also develop by both endochondral and intramembranous ossification. For more details on the development of the face, the pharyngeal arches , and arch derivatives, see our Embryonic Development section.

The bones of the skull change dramatically during the first years of life, and growth and development continue through adulthood. (The rotational models above, Fig. 2 and 3, are of a normal young adult, approximately 15 years old.) Any alteration in this normal process of change can result in a craniofacial disorder, potentially producing many different types of head or facial abnormalities. Defects of the head and facial region can affect almost all functions in everyday life, including physical appearance, speech, vision, eating, breathing, and brain function. The cause of craniofacial disorders can be a complex combination of genetics and environment. For information on normal skull development go to the Embryonic, Fetal, or Post-natal pages of Skull Development, or for major craniofacial disorders see the Skull Vault or Oral & Facial pages of Disorders of Development.