Q.Describe in detail the structure of the human skull.

The sphenoid is called the keystone bone because, like the keystone of an arch, it sits centrally at the base of the skull and locks together (articulates with) nearly all the other cranial bones.

The human cranium is made up of eight cranial bones: 1 frontal, 2 parietal, 1 occipital, 2 temporal, 1 ethmoid, and 1 sphenoid.

The sphenoid bone is an irregular, bat-shaped (or butterfly-shaped) bone situated at the base of the skull, roughly in the middle of the cranial floor. It is positioned between the frontal bone anteriorly and the occipital bone posteriorly, and it also articulates laterally with the temporal bones, and with the ethmoid bone in front.

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The sphenoid, situated at the base of the skull, is the cranial keystone bone.

In an arch, the keystone is the central wedge-shaped stone that locks all the other stones in place.

Among the eight cranial bones, the sphenoid bone lies centrally at the base of the cranium and articulates with almost every other cranial bone (frontal, parietal, temporal, occipital, ethmoid).

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The sphenoid, a bat/butterfly-shaped bone at the base of the skull, is called the keystone bone because it locks all the other cranial bones together.

The cranium is made of eight bones: 1 frontal, 2 parietal, 2 temporal, 1 occipital, 1 sphenoid and 1 ethmoid.

Why the sphenoid is the "keystone": A keystone is the central wedge-shaped stone at the top of an arch that locks all the other stones in place — remove it and the arch collapses. Similarly, the sphenoid bone sits centrally at the base of the skull and articulates with nearly all the other cranial bones (frontal, parietal, temporal, occipital and ethmoid), interlocking and stabilising the entire cranial floor.

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"Dicondylic" means the skull articulates with the vertebral column through two occipital condyles, as seen in humans and other mammals.

The human skull is attached to the vertebral column at the atlanto-occipital joint. On the occipital bone, at the margin of the foramen magnum, there are two rounded, smooth, bony projections called occipital condyles.

These two occipital condyles fit into corresponding facets on the atlas (the first cervical vertebra), forming the atlanto-occipital joint. Because there are two points/condyles of articulation, the human skull is described as dicondylic.

Reason/significance:

• The paired condyles provide a broader, more stable base of support for the relatively heavy human skull, which is important given the upright (bipedal) posture of humans. …

The sphenoid bone is the keystone bone of the cranium; it lies at the base (middle of the floor) of the skull and articulates with all the other cranial bones.

The human cranium (brain box) is made of several bones joined by immovable sutures. Among them, one bone is called the keystone bone.

• The sphenoid bone is a single, butterfly-shaped bone situated at the base (floor) of the cranium, in the middle region. …

A suture is an immovable fibrous joint between skull bones; the coronal and sagittal sutures are two well-known examples.

A suture is an immovable (fibrous) joint found between the flat bones of the skull, where the bone margins interlock and are held together by dense connective tissue, allowing no movement and providing protection to the brain.

Two cranial sutures:

1. Coronal suture - runs transversely across the top of the skull, joining the frontal bone (forehead) to the two parietal bones.
2. Sagittal suture - runs along the midline of the skull roof (front to back), joining the two parietal bones to each other. …

The sphenoid bone is called the keystone of the cranium because it is centrally placed at the base of the skull and locks together all the other cranial bones, like the keystone of an arch.

The human cranium is formed of eight bones - frontal, parietal (2), temporal (2), occipital, sphenoid and ethmoid. Among these, the sphenoid bone is unique in its position and shape: it is a bat- or butterfly-shaped bone that lies at the base of the skull, roughly in the middle, anterior to the temporal bones and basi-occipital region.

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Cranial sutures are immovable fibrous joints between the flat bones of the skull; the coronal, sagittal, lambdoid and squamosal sutures are the main ones.

The cranium (8 bones) is joined together by immovable fibrous joints called sutures, which lock the skull bones firmly to protect the brain. Two examples:

1. Coronal suture: located between the frontal bone (forehead) and the two parietal bones, running transversely across the top of the skull.
2. Sagittal suture: located between the two parietal bones, running along the midline from front to back on top of the skull. …

The sphenoid is the keystone bone of the cranium; it lies at the base/floor of the skull articulating with every other cranial bone.

The cranium (brain-box) is formed of several bones fitted together like an arch. The sphenoid bone is termed the keystone because, like the keystone of an arch, it lies centrally and locks with all the surrounding cranial bones — the frontal, parietals, temporals, occipital and ethmoid — so that it holds the whole cranial vault together. This is part of the human skeletal-system content in Class 11 Biology.

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