The Skeletal System: Functions, Structure & Bones

Functions of the Skeletal System

• The skeletal system has five main functions.
• It provides support by holding up the body and soft organs.
• It protects vital organs like the brain (skull) and heart/lungs (ribs).
• It enables movement with muscles.
• It stores minerals like calcium and phosphorus.
• Bone marrow produces red and white blood cells.

Structure of the Skeletal System

• Bones are rigid structures.
• Joints are connections between bones that allow movement.
• Cartilage cushions joints and provides structure.
• Ligaments connect bones to other bones.

Types of Bones (Classified by Shape)

• Long Bones are found in arms & legs (e.g., femur, humerus).
• Short Bones are cube-shaped (e.g., carpals, tarsals).
• Flat Bones are thin and often curved (e.g., skull, ribs).
• Irregular Bones have complex shapes (e.g., vertebrae, pelvis).

Types of Bone Tissue

• Compact Bone has a hard, dense outer layer.
• Compact bone contains osteons.
• It's found in the shafts of long bones.
• Spongy Bone is lighter and porous.
• Spongy bone contains trabecular plates and bone marrow.
• It's found in ends of long bones & inside flat bones.

Structure of a Long Bone

• Diaphysis: The shaft (middle part) of the bone.
• Epiphysis: The ends of the bone.
• Epiphyseal Disc (Growth Plate): Area where bone growth occurs.
• Medullary Cavity: Hollow area inside bone that stores bone marrow.
• Periosteum: Tough outer covering of the bone.
• Articular Cartilage: Covers joints for smooth movement.

Bone Formation and Growth

• Intramembranous Ossification: Flat bones develop from connective tissue.
• Endochondral Ossification: Most bones develop from cartilage and slowly harden.
• Bones grow taller at the Epiphyseal Disc (Growth Plate).
• Osteoblasts invade cartilage and form new bone.
• Hormones influence growth.
• Osteoclasts hollow out the bone, allowing bone to grow wider.
• Osteoblasts add new bone to the outer surface.
• Resorption is the process of bone breakdown and reformation.

Bone Markings (Bumps & Grooves on Bones)

• Condyle: Large, round knob
• Epicondyle: Small bump near a condyle
• Head: Rounded top of a bone
• Crest: Ridge-like structure
• Spine: Sharp projection.
• Tuberosity: Large, rough bump.
• Trochanter: Large projection on the femur.
• Foramen: Hole for nerves or blood vessels.
• Fossa: Depression or groove.
• Meatus: Tunnel-like passageway.
• Sinus: Air-filled cavity in bone.

Types of Bone Fractures (Breaks)

• Simple (Closed) Fracture: Bone breaks but does not pierce the skin.
• Compound (Open) Fracture: Bone breaks through the skin.
• Greenstick Fracture: Incomplete break, common in children.

Axial Skeleton (Central Bones)

• Skull: cranium (8 bones), facial bones (14 bones), and middle ear bones (3 per ear).
• Hyoid Bone supports the tongue, and does not connect to other bones.
• Vertebral Column: Spine (cervical, thoracic, lumbar, sacrum, coccyx).
• Thoracic Cage: Ribs, sternum, and thoracic vertebrae.

Cranial Bones (8 bones)

• Frontal, Parietal, Temporal, Occipital, Sphenoid, Ethmoid

Facial Bones (14 bones)

• Mandible, Maxilla, Zygomatic, etc.
• Sinuses are air-filled spaces that lighten the skull and enhance voice resonance.

Appendicular Skeleton (Limbs & Girdles)

• Shoulder Girdle: Clavicle & Scapula.
• Arm Bones: Humerus, Radius, Ulna.
• Hand Bones: Carpals, Metacarpals, Phalanges.
• Pelvic Girdle (Coxal Bones): Ilium, Ischium, Pubis.
• Leg Bones: Femur, Patella, Tibia, Fibula.
• Foot Bones: Tarsals, Metatarsals, Phalanges.

Joints (Articulations) and Movements

• Immmovable Joints are found in the skull (sutures).
• Slightly Movable Joints: Intervertebral discs.
• Freely Movable Joints: Most joints in the body.
• Articular Cartilage covers ends of bones.
• Synovial Fluid lubricates the joint.
• Ligaments connect bones to bones.
• Bursae are fluid-filled sacs that reduce friction.

Types of Synovial Joints

• Hinge: Elbows, Knees.
• Ball-and-Socket: Shoulders, Hips.
• Pivot: Neck (Atlas & Axis).
• Saddle: Thumb joint.
• Gliding: Wrist.
• Condyloid: Knuckles.

Types of Joint Movements

• Flexion-Extension: Bending and straightening.
• Abduction-Adduction: Moving limb away or toward the body.
• Rotation: Turning movement.
• Circumduction: Circular movement.
• Supination-Pronation: Rotating palm up/down.
• Inversion-Eversion: Turning foot inward/outward.

Final Notes

• The knee is the largest and most complex joint.
• The hip is very stable due to its deep socket.