The cranium is formed of one frontal bone, two parietal bones, one sphenoid, two temporal bones, one occipital bone, and one ethmoid.
The frontal bone forms the anterior part of the cranium, including the superior portion of the orbits (eye sockets), the forehead, and the roof of the nasal cavity. There are two large frontal sinuses in the frontal bone, one located superior to each eye.
The two parietal (pah-ri ‘-e-tal) bones form the sides and roof of the cranium. They are joined at the midline by the sagittal suture and to the frontal bone by the coronal suture.
The occipital (ok-sip’-i-tal) bone forms the posterior portion and floor of the cranium. It contains a large opening, the foramen magnum, through which the brainstem extends to join with the spinal cord. On each side of the foramen magnum are the occipital condyles (kon’-dils), large knucklelike surfaces that articulate with the first vertebra of the vertebral column. The occipital bone is joined to the parietal bones by the lambdoid (lam’doyd) suture.
The temporal bones are located inferior to the parietal bones on each side of the cranium. They are joined to the parietal bones by squamous (skwa’mus) sutures and to the occipital bone by the lambdoid suture. In each temporal bone, an external acoustic meatus leads inward to the eardrum. Just anterior to the external acoustic meatus is the mandibular fossa, a depression that receives the mandibular condyle to form the temporomandibular joint.
Three processes are located on each temporal bone. The zygomatic (zi-go-mat’-ic) process projects anteriorly to join with the zygomatic bone. The mastoid (mas’-toyd) process is a large, rounded projection that is located inferior to the external acoustic meatus. It serves as an attachment site for some neck muscles. The styloid process lies just medial to the mastoid process. It is a long, spikelike process to which muscles and ligaments of the tongue and neck are attached.
The sphenoid (sfe’-noid) forms part of the floor of the cranium, the posterior portions of the orbits, and the lateral portions of the cranium just anterior to the temporal bones. Because it articulates with all other cranial bones, the sphenoid is referred to as the “keystone” of the cranium. On its superior surface at the midline is a saddleshaped structure called the sella turcica (ter’-si-ka), or turkish saddle. It has a depression that contains the pituitary gland. Two sphenoidal sinuses are located just inferior to the sella turcica.
The ethmoid (eth’-moid) forms the anterior portion of the cranium, including part of the medial surface of each orbit and part of the roof of the nasal cavity. The lateral portions contain several air-filled sinuses called ethmoidal cells. The perpendicular plate extends inferiorly to form most of the nasal septum, which separates the right and left portions of the nasal cavity. It joins the sphenoid and vomer posteriorly and the nasal and frontal bones anteriorly.
The superior and middle nasal conchae (kong’-ke, singular, concha) extend from the lateral portions of the ethmoid toward the perpendicular plate. These delicate, scroll-like bones support the mucous membrane and increase the surface area of the nasal wall. The roof of the nasal cavity is formed by the cribriform plate of the ethmoid; the olfactory nerves enter the cranial cavity through foramina in the cribriform plate. On the superior surface where these plates join at the midline is a prominent projection called the crista galli, or cock’s comb. The meninges that envelop the brain are attached to the crista galli.