The skull serves as the bony framework of the head, providing structural support and protection for the brain and other delicate sensory organs. It shields the central nervous system from external forces. This structure also establishes the foundation for facial features and the entry points for sensory perceptions like sight, smell, and hearing.
Two Main Parts of the Skull
The skull is broadly divided into two primary sections: the neurocranium and the viscerocranium. The neurocranium, often referred to as the braincase, forms the protective vault around the brain. It encloses and safeguards the brain, along with its surrounding membranes and cerebrospinal fluid.
The viscerocranium, also known as the facial skeleton, comprises the bones that form the front and lower parts of the skull. This region contributes to the structure of the face, housing the orbits for the eyes, the nasal cavity, and the oral cavity. The bones of the viscerocranium are involved in functions such as chewing, facial expressions, and providing pathways for air and food.
Bones of the Braincase
The neurocranium consists of eight bones. The frontal bone makes up the forehead and the superior part of the eye sockets, extending posteriorly to form the anterior floor of the cranial cavity. It contributes to the overall facial structure.
Two parietal bones form the majority of the superior and lateral walls of the cranium. These large, flat bones meet at the sagittal suture and articulate with other cranial bones.
The two temporal bones are situated on the sides and base of the skull, inferior to the parietal bones. Each temporal bone contains the external acoustic meatus, which leads to the middle and inner ear structures, facilitating hearing and balance. They also include the mastoid process, a prominent bony projection behind the ear, and the zygomatic process, which contributes to the cheekbone.
The occipital bone forms the posterior and inferior part of the cranium. It features a large opening called the foramen magnum, through which the brainstem connects to the spinal cord. This bone also articulates with the first cervical vertebra, supporting the head’s movement and posture.
The sphenoid bone is a complex, butterfly-shaped bone located at the base of the skull, anterior to the temporal bones. It forms part of the floor of the cranial cavity, the lateral walls of the skull, and the posterior walls of the eye orbits. This bone contains the sella turcica, a saddle-shaped depression that houses the pituitary gland.
The ethmoid bone is a delicate, irregularly shaped bone situated anterior to the sphenoid bone, contributing to the medial wall of the orbit, the roof of the nasal cavity, and the nasal septum. It contains numerous small air cells, forming ethmoidal sinuses, and features the cribriform plate, which allows olfactory nerves to pass from the nasal cavity to the brain.
Bones of the Face
The viscerocranium is composed of 14 bones. Two maxillae, or upper jawbones, fuse to form the central part of the facial skeleton. These bones contain the upper teeth, form the anterior part of the hard palate, and contribute to the floor of the nasal cavity and the lower margin of the eye orbits.
The two zygomatic bones, commonly known as cheekbones, form the prominence of the cheeks and the lateral walls of the eye orbits. Each zygomatic bone articulates with the frontal, temporal, and maxillary bones.
Two small, rectangular nasal bones form the bridge of the nose. These bones articulate with the frontal bone superiorly and the maxillae laterally, providing structural support for the cartilaginous part of the nose.
The two lacrimal bones are located in the medial wall of each eye orbit. Each lacrimal bone contains a groove that forms part of the nasolacrimal canal, which drains tears from the eye surface into the nasal cavity.
The two palatine bones are L-shaped bones located posterior to the maxillae. They form the posterior part of the hard palate, a portion of the floor and lateral walls of the nasal cavity, and a small part of the orbit. These bones are involved in separating the oral and nasal cavities.
The two inferior nasal conchae are scroll-shaped bones that project from the lateral walls of the nasal cavity. They are distinct from the superior and middle conchae, which are parts of the ethmoid bone. These conchae increase the surface area within the nasal cavity, helping to warm, humidify, and filter inhaled air.
The vomer is a thin, flat bone that forms the inferior and posterior part of the nasal septum, which divides the nasal cavity into two halves. It articulates with the sphenoid, ethmoid, maxillae, and palatine bones, providing a central partition within the nose.
The mandible, or lower jawbone, is the only movable bone of the skull. It forms the entire lower jaw and contains the lower teeth. The mandible articulates with the temporal bones at the temporomandibular joints, enabling movements such as chewing and speaking.
Connecting Features and Openings
The bones of the skull are connected by specialized fibrous joints called sutures. The coronal suture joins the frontal bone with the two parietal bones, running transversely across the top of the skull.
The sagittal suture connects the two parietal bones along the midline of the skull, extending from the coronal suture to the lambdoid suture. The lambdoid suture separates the two parietal bones from the occipital bone posteriorly, resembling the Greek letter lambda.
The squamous sutures are located on each side of the skull, joining the parietal bones with the temporal bones. These sutures are unique in that the edges of the bones overlap like scales, providing a strong, beveled articulation.
Foramina are natural openings or passages found in bones. The foramen magnum, located in the occipital bone, is the largest foramen of the skull, allowing the spinal cord to connect with the brainstem. The optic canals, found in the sphenoid bone, transmit the optic nerves from the eyes to the brain. These openings are crucial for the passage of nerves and blood vessels to and from the brain and face.