The word ‘Diploe’ is a general term used to describe the spongy of cancellous region of intervening bone between the hard outer cortical shell and the brittle inner shell (as with the skull). The term is usually used for bones of the skull, but can be used for other short bones. The Parietal foramina are not always present and are found in the posterosuperior region of the parietal bone and form the channel for the emissary vein to connect with the superior sagittal sinus. They sometimes communicate with the occipital vein. The Granular Foveolae can be described as pits found on the inner surface of the skull along the length of the superior sagittal sinus and mark the region where the arachnoid granulations lodge.
Bones of the Calvaria
The frontal bone forms the anterior most part of the superior skull and overlies the frontal lobe. The frontal crest is a ridge in the anterior squamous portion of the frontal bone and is formed by the convergence of the two ridges of the superior sagittal sinus. The fold of dura that divides the two cerebral hemispheres (the falx cerebri) attaches at this point.
The bone forms synarthrosis joints with the adjacent sphenoid, zygomatic and parietal bones. The area where these four bones articulate is termed the anterior pterion which overlies the middle meningeal artery (a branch of the maxillary artery). In infancy the sphenoid fontanel lies here, allowing for the bones of the skull to move over eachother to enable passage of the head through the birth canal. The frontal bone contains the frontal sinus which protects the brain from direct skull trauma.
The occipital bone forms the posterior part of the skull and articulates with the parietal bones superiorly and forms the occipital condyles inferiorly. These condyles articulate with the lateral masses of the atlas. Atlas was the Greek titan who carried the skies, so the C1 vertebra has received an appropriately important name!
The parietal bone articulates with the frontal bones anteriorly and the occipital bones posteriorly. The bones are the largest bones of the skull and articulate in the midline via the sagittal suture. The parietal lobe underneath is involved in sensory perception. Early fusion of the sagittal suture gives rise to scaphocephaly. This bone is imprinted by meningeal arteries that lie within the substance of the dura mater.
Structures and Foramina within the Calvaria
The arterial grooves are indentations of the meningeal arteries on the inner surface of the skull. The middle meningeal artery enters the skull through the foramen spinosum located a few millimetres lateral and posterior to the foramen ovale. The artery supplies blood to the meninges along with the anterior and posterior meningeal arteries. The anterior is a branch of the anterior ethmoidal artery which enters the cranium through the canal of the same name. The posterior artery variably enters the skull through the jugular foramen, foramen magnum or hypoglossal canal.
The superior sagittal sinus runs in the attached margin of the falx cerebri and drains the lateral aspects of the anterior cerebral. It drains into the confluence of the sinuses (located deep to the posterior pterion). The sinus grooves the middle, inner surfaces of the frontal bone, both parietal bones in the sagittal plane and the cruciate eminence of the occipital bone.
The sagittal suture lies in the midline of the brain, extending from the coronal suture to the occipital bone posteriorly. The metopic suture is the anterior continuation of the sagittal suture and fuses at one year of age. Premature fusion causes trigonocephaly. The sutures of the skull are examples of synarthrosis joints, and are part of the group that includes gomphosis (teeth sockets) synchondrosis (a cartilaginous joint joined together by hyaline cartilage) and synostoses. The sagittal suture divides both parietal bones superiorly and lies in the mid sagittal plane. Both parietal bones fuse at around age 30, making it the last suture to fuse. This suture also overlies the superior sagittal sinus.
The lambdoid suture fuses at 25 years of age and lies between the parietal bones superiorly and the occipital bone inferiorly. It extends from the back of the skull to the posterior pterion. The posterior pterion forms at the joining of the occipital, parietal and temporal bone and was formerly the mastoid fontanel in infancy.
The squamous suture lies between the temporal bone inferiorly and the parietal bone superiorly. It joints anterior pterion to posterior pterion.