Q.Describe the structure of a synovial joint.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — The Human Skull, Joints and Skeletal Bones
The human skeleton provides the framework and the joints that muscles pull against, and the skull is one of its most distinctive parts. The cranium (braincase) is built of several separate flat bones — the frontal, the two parietals, the two temporals, the occipital, the sphenoid and the ethmoid — that interlock at immovable, jagged joints called sutures; for example, the coronal suture joins the frontal bone to the two parietal bones, the sagittal suture runs between the two parietal bones along the midline, and the lambdoid suture joins the parietal bones to the occipital bone at the back of the skull. Sitting centrally at the base of the cranium, the sphenoid bone touches and helps lock together nearly every other cranial bone, much as a keystone locks the stones of an arch in place — which is why it is called the “keystone bone” of the cranium.
The skull rests on the vertebral column through two rounded knobs on the occipital bone, called occipital condyles, which fit into matching depressions on the first neck vertebra (the atlas). Because there are two condyles rather than one, the human skull is described as dicondylic; this two-point articulation gives a stable, largely fixed connection (unlike the single-condyle, more mobile joint of reptiles and birds) while still allowing controlled nodding movements at the atlanto-occipital joint.
Inside the middle ear lie three tiny bones — the malleus, incus and stapes, together called the ear ossicles — which form a lever chain that carries and amplifies sound vibrations from the eardrum to the fluid-filled inner ear. Their evolutionary origin lies in the jaw apparatus of reptilian ancestors: the malleus and incus are derived from bones that once formed part of the lower and upper jaw joint (the articular and quadrate bones respectively), while the stapes has an even older origin, tracing back to the hyomandibula of early jawed vertebrates. Their repurposing from jaw-joint bones to sound-conducting ossicles is one of the most often cited examples of how existing structures are modified for a new function over evolutionary time.
Away from the skull, different joints of the body are shaped for the particular movement each location needs. The joint between the atlas and the axis (the first two neck vertebrae) is a pivot joint, allowing the head to rotate from side to side; the joint at the base of the thumb, between its carpal and metacarpal bone, is a saddle joint, which is what lets the thumb swing across the palm to oppose the fingers. The atlanto-axial joint (again, atlas on axis) is this same pivot joint, while the joint between the femur and the acetabulum of the hip bone is a ball-and-socket joint, permitting movement in almost every direction — flexion, extension, abduction, adduction and rotation. The sutures joining the bones of the cranium are fibrous, immovable joints, in clear contrast to the joints between the small tarsal bones of the foot, which are gliding (plane) joints allowing only a limited sliding movement against one another. …
A synovial joint has articular cartilage, a joint capsule (fibrous layer + synovial membrane), a fluid-filled synovial cavity, and reinforcing ligaments. …
A synovial joint is the freely movable type of joint, and it has a characteristic structure. The ends of the two bones meeting at the joint are capped with smooth articular cartilage, which reduces friction between them during movement. Surrounding the joint is a joint capsule, whose outer layer is tough fibrous connective tissue that holds the joint together, while its inner layer is a synovial membrane that secretes synovial fluid. This fluid fills the enclosed synovial cavity between the bone ends, lubricating the joint and supplying nutrients to the avascular cartilage. Ligaments, bands of tough connective tissue crossing or reinforcing t …
- Calling the synovial fluid part of the cartilage rather than a secretion of the s …
- CBSE 2026Set ANNUAL2 marksQ.Name the keystone bone of the cranium. Where is it located?
›Reveal solutionSolution
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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- CBSE 2026Set ANNUAL2 marksQ.Name the keystone bone of the cranium. Where is it located?
›Reveal solutionSolution
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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- CBSE 2025Set ANNUAL2 marksQ.Name the keystone bone of the cranium. Where is it located?
›Reveal solutionSolution
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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- CBSE 2025Set ANNUAL2 marksQ.Human skull is described as dicondylic skull. Give reason.
›Reveal solutionSolution
"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. …
- CBSE 2023Set ANNUAL2 marksQ.Name the keystone bone of the cranium. Where is it located?
›Reveal solutionSolution
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. …
- CBSE 2020Set ANNUAL2 marksQ.Name two cranial sutures and their locations.
›Reveal solutionSolution
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:
- Coronal suture - runs transversely across the top of the skull, joining the frontal bone (forehead) to the two parietal bones.
- Sagittal suture - runs along the midline of the skull roof (front to back), joining the two parietal bones to each other. …
- CBSE 2019Set ANNUAL2 marksQ.Name the keystone bone of the cranium. Where is it located ?
›Reveal solutionSolution
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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- CBSE 2018Set ANNUAL2 marksQ.Name two cranial sutures and their locations.
›Reveal solutionSolution
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:
- Coronal suture: located between the frontal bone (forehead) and the two parietal bones, running transversely across the top of the skull.
- Sagittal suture: located between the two parietal bones, running along the midline from front to back on top of the skull. …
- CBSE 2018Set ANNUAL2 marksQ.Name the keystone bone of the cranium. Where is it located ?
›Reveal solutionSolution
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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