The neurocranium is the part of the skull that encloses and protects the brain. It is a composite structure, meaning its different parts have distinct embryological origins. This bony case provides a rigid barrier against injury and offers a stable surface for the attachment of muscles.
The Bones of the Neurocranium
The neurocranium is composed of eight bones that encase the brain. Four of these are single: the frontal bone, the occipital bone, the sphenoid bone, and the ethmoid bone. The frontal bone forms the forehead and the upper part of the eye sockets, while the occipital bone provides the main structure for the back of the skull. The sphenoid is an irregularly shaped bone at the base of the cranium, and the ethmoid is a light bone between the eye sockets that helps form the nasal cavity.
The other four bones are paired. The two parietal bones form the majority of the roof and sides of the cranial cavity. The two temporal bones are located on the sides and base of the skull, housing the structures of the ear.
These eight bones are connected by fibrous joints called sutures. Sutures are made of thick connective tissue and have an irregular, interlocking shape that allows them to tightly join the cranial bones. In adults, these sutures are fused and immovable, creating a solid, continuous protective shell for the brain. This fusion is the final stage of a long developmental process.
Primary Functions
The most apparent function of the neurocranium is the protection of the brain. Its hard, curved structure acts as a shield, absorbing impact and preventing physical trauma to the delicate neural tissues housed within the cranial cavity. The rounded shape of the skullcap helps to deflect and distribute the force from blows to the head, reducing the risk of direct injury.
Beyond its protective role, the neurocranium serves as an attachment site for various muscles. Several muscles of mastication, or chewing, originate from the external surfaces of the neurocranium, allowing for the movements of the jaw. Additionally, it provides a stable anchor for the muscles of the neck, which are responsible for moving and stabilizing the head.
Development from Infancy to Adulthood
The neurocranium of a newborn is markedly different from that of an adult, a design that accommodates birth and early brain growth. An infant’s skull bones are not yet fused and are separated by flexible fibrous seams. These gaps are most noticeable at the fontanelles, often called “soft spots,” which are wider intersections where multiple skull bones meet.
These fontanelles serve two main purposes. During childbirth, the flexibility they provide allows the bony plates of the skull to overlap slightly, changing the shape of the head to facilitate its passage through the birth canal. After birth, the fontanelles and unfused sutures permit the skull to expand rapidly, accommodating the growth the brain undergoes during the first few years of life.
The process of the fontanelles closing and the sutures hardening is known as ossification. The posterior fontanelle at the back of the head closes by about six to eight weeks of age. The larger, diamond-shaped anterior fontanelle on the top of the head remains open much longer, closing between one and two years of age. The sutures themselves will gradually fuse throughout childhood and adolescence, a process that is largely complete by adulthood.
Associated Anatomical Features
The internal surface of the neurocranium is not a smooth, uniform bowl. The floor of the cranial cavity is structured into three distinct, tiered levels known as the cranial fossae. These are the anterior, middle, and posterior fossae, which form step-like depressions that support different parts of the brain. The frontal lobes of the cerebrum rest in the anterior fossa, the temporal lobes sit in the middle fossa, and the cerebellum and brainstem are housed in the posterior fossa.
The bones of the neurocranium are also punctuated by numerous openings called foramina. These holes and canals serve as passageways for the cranial nerves, which transmit sensory and motor information between the brain and the rest of the body. Blood vessels also pass through these foramina to supply the brain with oxygen and nutrients. For example, the foramen magnum is the largest of these openings, located in the occipital bone, and it allows the spinal cord to connect with the brainstem.