The skeletal system is the body's scaffolding, providing structural support, protection for vital organs, and a framework for muscle movement. This entire system is divided into two major complementary groups of bones: the Axial Skeleton and the Appendicular Skeleton.
I. The Axial Skeleton: The Central Core
The axial skeleton forms the central vertical axis of the body, offering robust protection and support. It includes the skull, vertebral column (spine), ribs, and sternum.
A. The Skull
The skull is composed of the cranium (protecting the brain) and the face. Most bones in the mature skull are joined by immovable, fibrous joints called sutures.
| Bone Category | Key Bones | Description/Function |
| Cranium (8 Bones) | Frontal (1), Parietal (2), Temporal (2), Occipital (1), Sphenoid (1), Ethmoid (1) | The cranium surrounds and protects the brain. The Sphenoid bone is centrally located, articulating with four other cranial bones. The Occipital bone is at the back and base. |
| Face (14 Bones) | Zygomatic (2, cheekbones), Maxilla (1, upper jaw), Nasal (2), Vomer (1, nasal septum), Mandible (1) | Provides structural support for the face. The Mandible (lower jaw) is the only movable bone in the skull. |
B. The Vertebral Column (Spine)
The spine consists of 26 individual bones called vertebrae. It provides flexible support and protects the spinal cord.
- Cervical (C1–C7): 7 vertebrae in the neck. $\text{C}1$ (Atlas) and $\text{C}2$ (Axis) form a pivot joint that allows head turning.
- Thoracic (T1–T12): 12 vertebrae that articulate with the ribs.
- Lumbar (L1–L5): 5 large, strong vertebrae in the lower back, bearing the most weight.
- Sacrum (1): Fusion of 5 sacral vertebrae; articulates with the two hip bones ($\text{sacroiliac}$ joints).
- Coccyx (1): Fusion of 4–5 small coccygeal vertebrae (the tailbone remnant).
Note: Intervertebral discs of fibrous cartilage lie between adjacent vertebrae, acting as symphysis joints that cushion and absorb shock.
C. Ribs and Sternum (Thoracic Cage)
The ribs and sternum enclose and protect the vital organs of the chest (heart and lungs).
- Ribs (12 Pairs): All attach posteriorly to the thoracic vertebrae.
- The first 7 pairs are True Ribs (articulate directly with the sternum).
- Pairs 8–10 are False Ribs (cartilage joins the cartilage of the 7th rib).
- The last 2 pairs (11 and 12) are Floating Ribs (no anterior sternal attachment).
Sternum (Breastbone): A flat, muscular bone in the middle of the chest, composed of the manubrium (top), body (middle), and xiphoid process (tip).
II. The Appendicular Skeleton: Movement and Appendages
The appendicular skeleton facilitates movement and consists of the bones of the shoulder girdle, upper limbs, pelvic girdle, and lower limbs.
A. Shoulder Girdle and Upper Limb
The shoulder (pectoral) girdle attaches the upper limbs to the axial skeleton.
| Structure | Bones | Key Features |
| Shoulder Girdle | Clavicle (2, collar bone), Scapula (2, shoulder bone) | The Scapula features the glenoid cavity (for the humerus head) and the acromion process (articulates with the clavicle). |
| Upper Limb | Humerus (upper arm), Ulna and Radius (forearm) | The Humerus head forms the shoulder joint. The Ulna and Radius form the elbow joint. |
| Hand & Wrist | Carpals (8 wrist bones in 2 rows), Metacarpals (5 palm bones), Phalanges (14 finger bones) | Each finger has three Phalanges; the thumb has two. |
B. Pelvic Girdle and Lower Limb
The pelvic girdle (hip bones) provides a strong connection between the lower limbs and the trunk, forming a basin with the sacrum and coccyx.
| Structure | Bones | Key Features |
| Pelvic Girdle | Innominate/Hip Bones (2) | Each hip bone is formed by the fusion of the Ilium (superior part), Ischium (inferior/posterior part), and Pubis. The Acetabulum is the socket for the femur head. |
| Pelvis | Hip Bones, Sacrum, Coccyx | Divided into the greater pelvis (above the pelvic brim) and the lesser pelvis (below the brim). |
| Lower Limb | Femur (thigh bone), Tibia (shin bone), Fibula, Patella (kneecap) | The Femur is the longest and heaviest bone. The Patella is a sesamoid (joint-embedded) bone. |
| Foot & Ankle | Tarsals (7 ankle bones), Metatarsals (5 foot bones), Phalanges (14 toe bones) | Tarsals include the talus and calcaneus. The foot structure is supported by arches formed by ligaments and muscles. |
III. Classification of Bones by Shape
Bones are categorized into four types based on their overall shape, which often correlates with their function:
- Long Bones: Longer than they are wide. Characterized by a central shaft (diaphysis) and ends (epiphyses).
- Examples: Humerus, Femur, Phalanges (most limb bones, excluding wrists/ankles).
Short Bones: Cube-shaped.
- Examples: Carpals (wrist bones) and Tarsals (ankle bones).
Flat Bones: Thin, flattened, and often curved.
- Examples: Ribs, Scapulae (shoulder blades), Cranium bones.
Irregular Bones: Do not fit into the other categories due to complex shapes.
- Examples: Vertebrae, Facial bones.
Bone Structure Note: Short, flat, and irregular bones are primarily composed of spongy bone covered by a thin layer of compact bone. The spongy bone contains red bone marrow. Long bones contain a central medullary cavity filled with yellow bone marrow (mostly adipose tissue) in adults. The rest of the bone surface is covered by the periosteum, a fibrous membrane that contains blood vessels and osteoblasts (bone-forming cells).
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