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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.

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.

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:

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