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AN1.1-2 | Anatomical terminology — Part 2

Body Movements: What Joints Do

Now that you know the planes, you can describe movements — because each movement occurs in a specific plane.

Figure: Body Movements: What Joints Do

Flexion and Extension
• Flexion = decreasing the angle between body parts (bending)
• Extension = increasing the angle (straightening)
• Both occur in the sagittal plane
• Example: Bending the elbow is flexion; straightening it is extension

Abduction and Adduction
• Abduction = moving away from the midline
• Adduction = moving toward the midline
• Both occur in the coronal (frontal) plane
• Mnemonic: ABduction = ABsent from midline; ADduction = ADding back to midline

Rotation
• Movement around a longitudinal axis
• Medial rotation = rotating toward the midline; Lateral rotation = rotating away
• Occurs in the transverse plane

Special Forearm Movements:
• Pronation = rotating the forearm so the palm faces downward/backward
• Supination = rotating so the palm faces upward/forward
• Mnemonic: Supination = holding a bowl of Soup

Special Ankle Movements:
• Dorsiflexion = pulling the foot upward (toward the dorsum/shin)
• Plantar flexion = pointing the foot downward (like a ballet dancer pointing toes)
• Inversion = sole of foot turning inward (medially)
• Eversion = sole turning outward (laterally)

Circumduction = combination of flexion, extension, abduction, adduction in sequence — creates a cone-shaped range of motion (e.g., shoulder joint).

Bone Composition: Why Bone Is Neither Chalk Nor Steel

AN1.2 asks you to describe the composition of bone and bone marrow. Bone is remarkable because it is simultaneously hard AND flexible.

Figure: Bone Composition: Why Bone Is Neither Chalk Nor Steel

Organic Component (~35% of dry weight):
• Primarily collagen — a protein that provides flexibility and tensile strength
• Made by bone-forming cells called osteoblasts
• Without collagen, bone would shatter on impact like glass

Inorganic Component (~65% of dry weight):
• Primarily hydroxyapatite — a crystalline calcium-phosphate mineral
• Provides hardness and compressive strength
• Without it, bone would bend like rubber

Think of bone as reinforced concrete: collagen = steel rebar (flexibility), hydroxyapatite = concrete (hardness). Neither component alone would be structural; together they are outstanding.

Cross-subject link: You are studying Basic Biochemistry this week — the same calcium ions that form hydroxyapatite in bone are regulated by parathyroid hormone (PTH) and calcitonin. When you encounter calcium homeostasis in Biochemistry, connect it here.

Bone Marrow: The Body Blood Factory

Inside the hollow spaces of bone lives a specialised tissue called bone marrow. There are two types:

Figure: Bone Marrow: The Body Blood Factory

Red bone marrow:
• Found in the spaces of spongy (cancellous) bone — mainly in flat bones (sternum, ileum) and epiphyses of long bones in adults
• The site of haematopoiesis — the production of all blood cells (red cells, white cells, platelets)
• Clinical relevance: A bone marrow biopsy (taken from the posterior iliac crest) diagnoses leukaemia, anaemia, and other blood disorders

Yellow bone marrow:
• Found in the medullary cavity — the hollow shaft of long bones
• Consists mainly of adipocytes (fat cells) — essentially stored fat
• In severe blood loss or anaemia, yellow marrow can convert back to red marrow — a remarkable adaptive response

Think of the medullary cavity as a storage tank for fuel (fat), while the spongy regions are the production floor (blood cells).

Mnemonic: Red marrow = Red blood cells produced; Yellow marrow = Yellow fat stored