Axial skeleton — A/L Biology

What must I remember about Axial skeleton?

• The human axial skeleton consists of 80 bones organized along the central vertical longitudinal axis: the Skull (22 bones: 8 cranial + 14 facial), associated bones (6 auditory ossicles + 1 hyoid bone), the Vertebral Column (26 bones: 24 vertebrae + sacrum + coccyx), and the Thoracic Cage (24 ribs + 1 sternum).
• The Skull protects the brain within the neurocranium (frontal, parietals, temporals, occipital, sphenoid, ethmoid joined by immovable sutures) and supports facial musculature/sense organs; the base of occipital bone contains the Foramen Magnum where spinal cord exits, flanked by occipital condyles articulating with the Atlas vertebra (C1).
• Vertebral Column has 5 regions: Cervical (7 vertebrae; C1 Atlas lacks body and articulates for 'yes' nodding motion, C2 Axis has odontoid process/dens for 'no' rotation motion; all cervicals feature transverse foramina for vertebral arteries), Thoracic (12 vertebrae articulating with ribs via costal facets), Lumbar (5 large vertebrae with massive bodies for weight-bearing), Sacrum (5 fused vertebrae), and Coccyx (4 fused rudimentary vertebrae).
• Vertebral column curves: Four adult curvatures provide resilience, shock absorption, and balance for bipedal erect posture: two primary (thoracic and sacral kyphoses, present at birth) and two secondary (cervical lordosis developing when infant holds head up, lumbar lordosis developing upon walking).
• Thoracic Cage: Sternum (manubrium, body, xiphoid process) and 12 pairs of ribs protecting heart and lungs: True ribs (pairs 1-7, attach directly to sternum via individual costal cartilages), False ribs (pairs 8-10, attach indirectly to cartilage of rib 7), and Floating ribs (pairs 11-12, lack anterior sternal attachment).

Test yourself (1 minute)

1. Intervertebral discs structure

   Outer fibrous annulus fibrosus surrounding inner gel-like nucleus pulposus; act as shock absorbers; herniation ('slipped disc') compresses adjacent spinal nerves causing sciatica.

2. Atlas (C1) and Axis (C2) functional anatomy

   Atlanto-occipital joint allows sagittal nodding ('Yes' motion); Atlanto-axial joint (dens/odontoid process pivoting in atlas ring) allows rotational head movement ('No' motion).

3. Primary vs Secondary spinal curvatures

   Primary kyphotic curves (thoracic, sacral) retain fetal anterior concavity; secondary lordotic curves (cervical, lumbar) develop anterior convexity postnatally during motor milestones.

Practise Axial skeleton questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 5

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Next in the syllabus

Where does Axial skeleton fit in the syllabus?

Year 2 · Term 1