Parietal bone
Encyclopedia
The parietal bones are bones in the human
skull
which, when joined together, form the sides and roof of the cranium
. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall.
(tuber parietale), which indicates the point where ossification
commenced.
Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the temporalis.
Above these lines the bone is covered by the galea aponeurotica
(epicranial aponeurosis); below them it forms part of the temporal fossa, and affords attachment to the temporalis muscle.
At the back part and close to the upper or sagittal border is the parietal foramen, which transmits a vein to the superior sagittal sinus
, and sometimes a small branch of the occipital artery
; it is not constantly present, and its size varies considerably.
; the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border.
Along the upper margin is a shallow groove, which, together with that on the opposite parietal, forms a channel, the sagittal sulcus
, for the superior sagittal sinus
; the edges of the sulcus afford attachment to the falx cerebri
.
Near the groove are several depressions, best marked in the skulls of old persons, for the arachnoid granulations (Pacchionian bodies).
In the groove is the internal opening of the parietal foramen when that aperture exists.
Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist.
Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture.
, lying behind the frontal bones. In many non-mammalian tetrapod
s, they are bordered to the rear by a pair of postparietal bones that may be solely in the roof of the skull, or slope downwards to contribute to the back of the skull, depending on the species. In the living tuatara
, and many fossil species, a small opening, the parietal foramen, lies between the two parietal bones. This opening is the location of a third eye
in the midline of the skull, which is much smaller than the two main eyes.
Human
Humans are the only living species in the Homo genus...
skull
Human skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
which, when joined together, form the sides and roof of the cranium
Human skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall.
External
The external surface [Fig. 1] is convex, smooth, and marked near the center by an eminence, the parietal eminenceParietal eminence
The external surface of the parietal bone is convex, smooth, and marked near the center by an eminence, the parietal eminence , which indicates the point where ossification commenced....
(tuber parietale), which indicates the point where ossification
Ossification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...
commenced.
Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the temporalis.
Above these lines the bone is covered by the galea aponeurotica
Galea aponeurotica
The galea aponeurotica is a tough layer of dense fibrous tissue which covers the upper part of the cranium; behind, it is attached, in the interval between its union with the Occipitales, to the external occipital protuberance and highest nuchal lines of the occipital bone; in front, it forms a...
(epicranial aponeurosis); below them it forms part of the temporal fossa, and affords attachment to the temporalis muscle.
At the back part and close to the upper or sagittal border is the parietal foramen, which transmits a vein to the superior sagittal sinus
Superior sagittal sinus
The superior sagittal sinus , within the human head, is an unpaired area along the attached margin of falx cerebri. It allows blood to drain from the lateral aspects of anterior cerebral hemispheres to the confluence of sinuses...
, and sometimes a small branch of the occipital artery
Occipital artery
The occipital artery arises from the external carotid artery opposite the facial artery, its path is below the posterior belly of digastric to the occipital region. This artery supplies blood to the back of the scalp and sterno-mastoid muscles...
; it is not constantly present, and its size varies considerably.
Internal
The internal surface [Fig. 2] is concave; it presents depressions corresponding to the cerebral convolutions, and numerous furrows (grooves) for the ramifications of the middle meningeal arteryMiddle meningeal artery
The middle meningeal artery is typically the third branch of the first part of the maxillary artery, one of the two terminal branches of the external carotid artery...
; the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border.
Along the upper margin is a shallow groove, which, together with that on the opposite parietal, forms a channel, the sagittal sulcus
Sagittal sulcus
The internal surface of the squama frontalis of the frontal bone is concave and presents in the upper part of the middle line a vertical groove, the sagittal sulcus, the edges of which unite below to form a ridge, the frontal crest; the sulcus lodges the superior sagittal sinus, while its margins...
, for the superior sagittal sinus
Superior sagittal sinus
The superior sagittal sinus , within the human head, is an unpaired area along the attached margin of falx cerebri. It allows blood to drain from the lateral aspects of anterior cerebral hemispheres to the confluence of sinuses...
; the edges of the sulcus afford attachment to the falx cerebri
Falx cerebri
The falx cerebri, also known as the cerebral falx, so named from its sickle-like form, is a strong, arched fold of dura mater which descends vertically in the longitudinal fissure between the cerebral hemispheres....
.
Near the groove are several depressions, best marked in the skulls of old persons, for the arachnoid granulations (Pacchionian bodies).
In the groove is the internal opening of the parietal foramen when that aperture exists.
Borders
- The sagittal border, the longest and thickest, is dentated (has toothlike projections) and articulates with its fellow of the opposite side, forming the sagittal suture.
- The squamous border is divided into three parts: of these:
- the anterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid;
- the middle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squama of the temporal;
- the posterior part is thick and serrated for articulation with the mastoid portion of the temporalTemporal boneThe temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...
.
- The frontal border is deeply serrated, and bevelled at the expense of the outer surface above and of the inner below; it articulates with the frontal boneFrontal boneThe frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
, forming half of the coronal sutureCoronal sutureThe coronal suture is a dense, fibrous connective tissue joint that separates the frontal and parietal bones of the skull. At birth, the bones of the skull do not meet.-Pathology:...
. The point where the coronal suture intersects with the sagittal suture forms a T-shape and is called the bregma.
- The occipital border, deeply denticulated (finely toothed), articulates with the occipital boneOccipital boneThe occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...
, forming half of the lambdoid sutureLambdoid sutureThe lambdoid suture is a dense, fibrous connective tissue joint on the posterior aspect of the skull that connects the parietal and temporal bones with the occipital bone.Its name comes from its lambda-like shape....
. That point where the sagittal suture intersects the lambdoid suture is called the lambdaLambdaLambda is the 11th letter of the Greek alphabet. In the system of Greek numerals lambda has a value of 30. Lambda is related to the Phoenician letter Lamed . Letters in other alphabets that stemmed from lambda include the Roman L and the Cyrillic letter El...
, because of its resemblance to the Greek letter.
Angles
- The frontal angle is practically a right angle, and corresponds with the point of meeting of the sagittal and coronal sutures; this point is named the bregmaBregmaThe bregma is the anatomical point on the skull at which the coronal suture is intersected perpendicularly by the sagittal suture.-Location:The bregma is located at the intersection of the coronal suture and the sagittal suture on the superior middle portion of the calvarium...
; in the fetal skull and for about a year and a half after birth this region is membranous, and is called the anterior fontanelleAnterior fontanelleThe anterior fontanelle is the largest fontanelle, and is placed at the junction of the sagittal suture, coronal suture, and frontal suture; it is lozenge-shaped, and measures about 4 cm in its antero-posterior and 2.5 cm in its transverse diameter...
.
- The sphenoidal angle, thin and acute, is received into the interval between the frontal bone and the great wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes a canal, for the anterior divisions of the middle meningeal arteryMiddle meningeal arteryThe middle meningeal artery is typically the third branch of the first part of the maxillary artery, one of the two terminal branches of the external carotid artery...
.
- The occipital angle is rounded and corresponds with the point of meeting of the sagittal and lambdoidal sutures—a point which is termed the lambda; in the fetus this part of the skull is membranous, and is called the posterior fontanellePosterior fontanelleThe posterior fontanelle is a gap between bones in the human skull, triangular in form and situated at the junction of the sagittal suture and lambdoidal suture. It generally closes in 6-8 weeks from birth. A delay in closure is associated with congential hypothyroidism....
.
- The mastoid angle is truncated; it articulates with the occipital bone and with the mastoid portion of the temporal, and presents on its inner surface a broad, shallow groove which lodges part of the transverse sinus. The point of meeting of this angle with the occipital and the mastoid part of the temporal is named the asterionAsterion (anatomy)The asterion is the point on the skull corresponding to the posterior end of the parietomastoid suture.-Location:In human anatomy, the asterion is a visible, so-called craniometric, point on the exposed skull, just behind the ear, where three cranial sutures meet:* the lambdoid,* parieto-mastoid,...
.
Ossification
The parietal bone is ossified in membrane from a single center, which appears at the parietal eminence about the eighth week of fetal life.Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist.
Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture.
In other animals
In non-human vertebrates, the parietal bones typically form the rear or central part of the skull roofSkull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...
, lying behind the frontal bones. In many non-mammalian tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, they are bordered to the rear by a pair of postparietal bones that may be solely in the roof of the skull, or slope downwards to contribute to the back of the skull, depending on the species. In the living tuatara
Tuatara
The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...
, and many fossil species, a small opening, the parietal foramen, lies between the two parietal bones. This opening is the location of a third eye
Parietal eye
A parietal eye, also known as a parietal organ or third-eye or pineal eye, is a part of the epithalamus present in some animal species...
in the midline of the skull, which is much smaller than the two main eyes.