The parietal bone is one of the largest bones of the human skull, serving as a protective shield for the brain. This paired, roughly quadrilateral bone forms a significant portion of the roof and sides of the cranium. Its robust structure is designed to absorb external forces, maintaining the integrity of the neural tissue inside. Understanding the parietal bone’s anatomy and articulations is central to understanding the normal development and potential vulnerabilities of the human head.
Key Features of the Bone Structure
The parietal bone presents a distinct difference between its outer and inner surfaces. The external surface is convex and generally smooth, prominently featuring the parietal eminence (or boss), which marks the point where ossification began during fetal development. Arcing across this external surface are the superior and inferior temporal lines, which serve as attachment points for the temporalis muscle and its covering fascia.
The internal surface is concave and bears the imprint of the structures it protects. This inner aspect shows shallow depressions corresponding to the convolutions of the cerebral cortex. It features a network of branching furrows known as the meningeal artery grooves, which accommodate the middle meningeal artery and its divisions. Along the upper border, the sagittal sulcus combines with the groove on the opposite parietal bone to form a larger channel for the superior sagittal sinus, a major vein draining blood from the brain.
Sutural Connections to the Skull
The parietal bone forms immovable, fibrous joints (sutures) with five other bones of the cranial vault. These connections interlock like puzzle pieces, providing strength to the skull. The two parietal bones meet at the midline to form the serrated sagittal suture.
Anteriorly, the parietal bone meets the frontal bone, forming the coronal suture. Posteriorly, it connects with the occipital bone, creating the distinctive “V” shape of the lambdoid suture. Laterally, the bone articulates with the temporal bone, forming the squamous suture, a unique overlapping joint. These sutures represent the boundaries of growth centers that allow for the expansion of the skull during brain development.
Trauma and Vascular Vulnerability
The parietal bone’s broad, exposed position makes it frequently involved in cases of blunt force trauma. Impacts can result in various injuries, ranging from a simple linear fracture to a depressed fracture where bone fragments are pushed inward toward the brain. The clinical significance of a fracture in the parietal region is closely tied to its internal anatomical features.
A fracture extending toward the temporal region can lacerate the middle meningeal artery, which travels along the internal surface of the bone. Since this is an arterial vessel, the resulting hemorrhage is high-pressure and rapid, leading to the formation of an Epidural Hematoma (EDH). This accumulation of blood collects between the inner table of the skull and the protective dura mater. As the arterial blood quickly expands, it exerts pressure on the underlying brain tissue, causing rapid neurological decline and requiring surgical intervention.
Developmental Considerations in Infancy
The parietal bone develops through intramembranous ossification, where bone tissue forms directly from a membrane without a preceding cartilage model. This method is characteristic of the flat bones of the cranial vault. At birth, the edges of the parietal bones and adjacent bones are not fully fused, leaving gaps of dense connective tissue called fontanelles.
The parietal bones contribute to the large, diamond-shaped anterior fontanelle (at the junction with the frontal bone) and the smaller, triangular posterior fontanelle (where it meets the occipital bone). These fontanelles permit the slight overlapping of skull plates during birth and allow for rapid brain growth in the first years of life. If the sutures involving the parietal bone fuse prematurely, craniosynostosis occurs. Premature fusion of the sagittal suture, for example, restricts growth perpendicular to the suture line, resulting in an abnormally long and narrow head shape.