The parietal bone is one of the large, curved plates that forms the top and sides of the human skull, and you have two of them, one on each side. Together they make up a significant portion of the cranial vault, the dome-shaped casing that protects the brain. Though it might seem like a simple shield of bone, the parietal bone has a surprisingly rich story involving embryonic development, blood vessel pathways, evolutionary changes that set humans apart from other primates, and practical applications in surgery and forensic science.
Where It Sits and What It Does
Each parietal bone is roughly quadrilateral and gently convex. The two parietal bones meet along the midline at the sagittal suture, which runs from front to back along the top of the head. At the front, they border the frontal bone at the coronal suture, and at the back, they meet the occipital bone at the lambdoid suture. Lower down, each parietal bone adjoins the temporal bone. The point where all four of these bones converge on each side, called the pterion, is one of the thinnest spots on the entire skull.
The inner surface of each parietal bone is not smooth. It is marked by branching grooves carved by the middle meningeal artery, the largest branch of the maxillary artery, which supplies blood to the membranes wrapping the brain and nourishes the frontal and parietal regions of the skull.1Arquivos Brasileiros de Neurocirurgia. Anatomy of the Middle Meningeal Artery These grooves give surgeons and radiologists landmarks for identifying the artery’s path on imaging scans.
How the Parietal Bone Forms Before Birth
Unlike the bones in your arms or legs, the parietal bone does not develop from a cartilage template. It forms directly from connective tissue through a process called intramembranous ossification. Each parietal bone begins from two separate centers of bone-forming activity that appear near the parietal eminence, the gentle bump you can feel on the side of your head, around the eighth week after conception. These two centers merge by about the fourth month, and bone then spreads outward from that fused center toward the edges.2Journal of Korean Neurosurgical Society. Development and Growth of the Normal Cranial Vault: An Embryologic Review
By 14 weeks of fetal development, both parietal bones show extensive ossification, but the process is far from complete at birth. The sutures between the parietal bones and their neighbors remain broad and flexible, especially in the region where the parietal bone meets the temporal bone. This flexibility is what allows an infant’s head to compress slightly during birth and what gives the soft spots, or fontanelles, their characteristic give. The sutures also serve as growth zones, allowing the skull to expand as the brain grows rapidly during the first years of life.3Journal of Korean Neurosurgical Society. Development and Growth of the Normal Cranial Vault: An Embryologic Review
The Parietal Foramen and Its Veins
Near the back of each parietal bone, close to the sagittal suture, there is often a small hole called the parietal foramen. Not everyone has one, and when present it varies in size. A scalp vein threads through this opening, connecting veins on the outside of the skull with the superior sagittal sinus, a major drainage channel inside the skull.4PubMed. The parietal foramen anatomy: studies using dry skulls, cadaver and in vivo MRI These connecting vessels are called emissary veins, and they have no valves, meaning blood can flow through them in either direction.5PubMed. Clinical and surgical implications of emissary venous anatomy
That two-way flow has real consequences. On the helpful side, emissary veins may play a role in regulating brain temperature by allowing cooled blood from the scalp to reach the intracranial sinuses. They also serve as a safety valve for intracranial pressure. On the harmful side, the same valve-free pathway can carry infections from the scalp or outer tissues directly into the dural venous sinuses, which is a serious complication.6PubMed Central. Morphology and topography of the parietal emissary foramina in South Indians: an anatomical study Knowledge of where these foramina sit and how large they are matters during neurosurgery, because cutting through one unexpectedly can cause significant bleeding.7PubMed Central. Morphology and topography of the parietal emissary foramina in South Indians: an anatomical study
Fractures and the Vulnerable Temporal-Parietal Region
The parietal bone is reasonably thick across most of its surface, but the area where it meets the temporal bone is a weak spot. A blow to the side of the head here can fracture the bone and, because the middle meningeal artery runs along grooves on the inner surface, the same impact can tear the artery. A ruptured middle meningeal artery causes an epidural hematoma, a rapidly expanding pocket of blood between the skull and the brain’s outer membrane. This is a neurosurgical emergency. Research into the biomechanics of this region has examined how much force the temporal-parietal area can withstand under different kinds of loading, from falls to direct impacts, and the results inform helmet design and vehicle safety standards.8PubMed Central. Biomechanics of temporo-parietal skull fracture
Fracture patterns on parietal bone can also tell a forensic story. In cases of repeated blunt-force trauma, the parietal bone can develop a distinctive stair-step fracture pattern, where multiple contiguous fractures form from loading the head at different times with the force hitting slightly different spots each time.9PubMed Central. An unusual blunt force trauma pattern and mechanism to the cranial vault: Investigation of an atypical infant homicide Recognizing that pattern can help forensic pathologists distinguish a single accidental impact from repeated inflicted injury.
Sagittal Craniosynostosis
The sagittal suture between the two parietal bones is supposed to stay open throughout childhood, gradually fusing in adulthood. When it closes prematurely, a condition called sagittal craniosynostosis, the skull cannot widen normally. Because the brain keeps growing anyway, it compensates by elongating front-to-back, producing a narrow, boat-shaped head known as scaphocephaly. This is the most common type of premature suture fusion, affecting roughly one in 5,000 children.10PubMed. Correction of sagittal craniosynostosis using a novel parietal bone fixation technique: results over a 10-year period
Beyond the visible head shape, sagittal craniosynostosis can raise intracranial pressure and affect neurological development.11PubMed Central. Isolated Sagittal Craniosynostosis: A Comprehensive Review When the sagittal suture is locked, growth accelerates at the sutures that remain open, particularly the metopic and coronal sutures, as the skull tries to accommodate brain expansion through whatever pathways are still available.12PubMed Central. Cranial growth in isolated sagittal craniosynostosis compared with normal growth in the first 6 months of age Treatment typically involves surgery during the first year of life, either an open procedure that reshapes the parietal bones or a less invasive approach that removes a strip of bone along the fused suture and relies on helmet therapy to guide the skull into a rounder shape as the brain grows.
Other Conditions That Affect the Parietal Bone
A rare genetic condition called foramina parietalia permagna produces abnormally large holes in the parietal bones, sometimes several centimeters across. These openings result from incomplete ossification during embryonic development and are caused by mutations in either the MSX2 or ALX4 genes.13PubMed. Bilateral Foramina Parietalia Permagna – A Calvarial Defect Caused by Haploinsufficiency of the Msh Homeobox 2 Gene: A Case Report and Current Literature Review Most people with this condition are asymptomatic and discover the defect incidentally on imaging. In severe cases, however, the brain may be partially exposed beneath only skin and membrane, making the area vulnerable to injury.
The parietal bone is also one of the skull bones most commonly affected by eosinophilic granuloma, a benign but locally destructive lesion that eats through bone.14International Journal of Surgery Case Reports. Eosinophilic granuloma affecting the parietal bone of the skull: A case report and literature review On CT scans, it appears as a sharply defined hole punched in the bone. These lesions can start within the spongy middle layer of the skull and expand outward and inward, sometimes pressing on the brain’s outer covering.15Brain and Development. Eosinophilic granuloma of bone: Two case reports Treatment ranges from observation to surgical removal, depending on size and symptoms.
The Parietal Bone as a Source for Bone Grafts
Surgeons rebuilding the face after trauma or tumor removal often need pieces of bone, and the parietal region of the skull turns out to be an excellent donor site. The bone here has a sandwich structure: two hard outer layers with a spongy middle layer called the diploë. A surgeon can split the parietal bone along this spongy layer and harvest the outer table for grafting while leaving the inner table intact, keeping the brain protected throughout.
A study that tracked patients who received these split calvarial grafts for facial reconstruction found an overall graft survival rate above 95% across 95 transplanted pieces. Resorption was negligible in most cases, and the grafts held up well for rebuilding cheekbones, orbital floors, and nasal structures. The only complete graft losses occurred in patients who had undergone tumor surgery, not in trauma cases.16JAMA Otolaryngology–Head & Neck Surgery. Midfacial Reconstruction Using Calvarial Split Bone Grafts One reason these grafts perform so well is that calvarial bone, being formed from membrane rather than cartilage, tends to resist resorption better than bone taken from the hip or rib.
What Parietal Bones Reveal About Human Evolution
The shape of the parietal bone turns out to be one of the features that most clearly distinguishes the modern human skull from those of our closest evolutionary relatives. Modern humans have noticeably bulging parietal bones that give the skull its characteristic rounded, globular profile. Neanderthals, despite having brain volumes similar to or even larger than ours, had flatter parietal regions and more elongated braincases.17PubMed. A morphometric comparison of the parietal lobe in modern humans and Neanderthals The difference is not just about overall brain size, since Neanderthals matched us in volume. Instead, it reflects the expansion of specific brain regions beneath the parietal bone, particularly the precuneus, a part of the brain involved in visuospatial processing, self-awareness, and episodic memory.18PubMed Central. Evidence for expansion of the precuneus in human evolution
The internal structure of parietal bone has also drawn attention from paleoanthropologists. Homo erectus was long thought to have uniquely thick skull bones, which some researchers interpreted as an adaptation for protection. More recent analysis complicates that picture. After controlling for brain size, Homo erectus skulls are not unusually thick compared with some living primates. However, there is a meaningful difference in composition: in Homo erectus and other members of our genus, the parietal bone is dominated by the spongy diploë layer, whereas in non-human primates with similarly thick skulls, the thickness comes mainly from the hard cortical outer layers.19PubMed. Cranial vault thickness in primates: Homo erectus does not have uniquely thick vault bones That thick-diploë architecture may be a defining trait of the genus Homo rather than something specific to Homo erectus.
The Lost Third Eye
In many reptiles and some amphibians alive today, there is a functioning photoreceptive organ on top of the head, often called the “third eye” or parietal eye, that sits in an opening in the parietal bone called the parietal foramen. This organ helps regulate circadian rhythms and sun exposure. Modern mammals lack this opening entirely, but the evolutionary record shows how it was gradually lost.
Studies of therapsids, the group of ancient animals that eventually gave rise to mammals, reveal that the parietal foramen shrank and disappeared multiple times across different lineages in a convergent fashion. Among the group most closely related to mammals, the Probainognathia, the parietal foramen was completely lost before true mammals appeared.20Acta Palaeontologica Polonica. The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids This gradual disappearance tracks with what researchers interpret as the loss of the pineal organ’s light-sensing function as these animals transitioned toward warm-blooded physiology and no longer needed a light-sensitive patch on the skull for thermoregulation.21PubMed. Physiological implications of the abnormal absence of the parietal foramen in a late Permian cynodont (Therapsida) Humans retain a pineal gland buried deep inside the brain that produces melatonin, but the bony window that once exposed it to direct sunlight closed hundreds of millions of years ago.
Sagittal Crests and Jaw Muscles in Apes
In some primates, the parietal bones carry a prominent bony ridge running along the sagittal suture called a sagittal crest. This is not a sign of pathology; it forms because the temporalis muscles, which power the jaw during chewing, grow so large that they need more surface area for attachment. When the two muscle sheets from either side meet at the top of the skull, the bone responds by building up a ridge where they overlap.
Gorillas show the most dramatic sagittal cresting among the great apes. About 93% of male western lowland gorillas and 58% of females develop a sagittal crest. In orangutans, roughly 39% of males develop one, but no females do. Chimpanzees and gibbons show much lower frequencies.22PubMed Central. Sagittal crest formation in great apes and gibbons In gorillas, the crest can rise as high as 5 cm above the skull surface, and the nuchal crest at the back can form a shelf of bone up to 4 cm wide.23Proceedings of the Zoological Society of London. Cranial Crests in the Anthropoidea Humans never develop sagittal crests because our jaw muscles are comparatively small, a shift linked to changes in diet and cooking that reduced the mechanical demands on our chewing apparatus.
Forensic and Archaeological Clues from the Parietal Bone
Forensic anthropologists have long tried to estimate age at death from suture closure, since the sagittal suture between the parietal bones gradually fuses over the course of adulthood. In practice, this method has serious limitations. A machine-learning analysis of suture closure patterns found that the degree of fusion along the sagittal suture is useful only as a supporting method for age estimation, not a reliable primary one.24Forensic Imaging. Reliability of sagittal suture maturation for age-at-death prediction assessed by means of machine learning techniques The variability between individuals is simply too large: some people’s sagittal sutures are almost entirely fused by their 40s, while others retain open segments well into old age.
The parietal bone also shows up prominently in archaeological studies of intentional cranial modification, a practice documented across cultures on every inhabited continent. Various devices, boards, cloths, or cradle bindings, applied to an infant’s still-malleable skull would redirect growth, often flattening or raising the parietal region.25PubMed Central. Morphological consequences of artificial cranial deformation: Modularity and integration Some forms of deformation specifically lowered the cranial vault starting from bregma, the point where the two parietal bones meet the frontal bone, producing a distinctly flattened profile.26Camera Praehistorica. Artificial cranial deformation: typology and geometric morphometrics These modified skulls are instantly recognizable in the archaeological record and have been a source of both scientific study and popular fascination, occasionally being misidentified as evidence of non-human origins by enthusiasts unfamiliar with the practice.