ANATOMY - Skeletal System

 

1. Bones: Number, Type, and Classification

Total Count

• Adult Skeleton: 206 bones.
• Infant Skeleton: ~270 to 300 bones (many fuse together during growth).

Classification by Shape & Type

Bones are classified into five main types based on their shape:

• Long Bones: Longer than they are wide. Act as levers.
• Examples: Femur (longest/strongest), Humerus, Tibia, Fibula, Radius, Ulna.
• Short Bones: Cube-shaped, provide stability and some movement.
• Examples: Carpals (wrist), Tarsals (ankle).
• Flat Bones: Thin, flattened, usually curved. Protect vital organs and provide large surfaces for muscle attachment.
• Examples: Cranial bones (skull), Sternum, Ribs, Scapula.
• Irregular Bones: Complex shapes that don't fit other categories.
• Examples: Vertebrae, Sacrum, Mandible, Hyoid.
• Sesamoid Bones: Small, round bones embedded in tendons. They protect tendons from stress and wear.
• Examples: Patella (kneecap) — largest sesamoid bone in the body. Note: Pisiform (a carpal bone) is also considered a sesamoid bone.

Bone Marrow Location

·         Red Marrow (Hemopoiesis - Blood cell production): Found in the spongy (cancellous) tissue of flat bones (sternum, ilium) and the epiphyses of long bones.

·         Yellow Marrow (Fat storage): Found in the medullary cavity of the diaphysis (shaft) of long bones.

2. Axial vs. Appendicular Skeleton: Identification & Features

The skeleton is divided into two functional parts:

Feature	Axial Skeleton (80 Bones)	Appendicular Skeleton (126 Bones)
Core Function	Protects central organs (brain, spinal cord, heart, lungs).	Facilitates movement and locomotion.
Components	Skull, Vertebral Column, Thoracic Cage, Auditory Ossicles, Hyoid.	Pectoral Girdle, Upper Limbs, Pelvic Girdle, Lower Limbs.

Axial Skeleton Breakdown (80 Bones)

• Skull (22 bones): * Cranium (8): Frontal, Parietal (2), Temporal (2), Occipital, Sphenoid, Ethmoid.
• Facial (14): Maxilla (2), Zygomatic (2), Mandible (1—only movable bone of skull), Nasal (2), Lacrimal (2), Palatine (2), Inferior Nasal Conchae (2), Vomer (1).
• Auditory Ossicles (6 bones): Malleus, Incus, Stapes (smallest bone in the body). Located in the middle ear.
• Hyoid Bone (1 bone): U-shaped bone in the neck. Does not articulate with any other bone.
• Vertebral Column (26 bones in adults): * Cervical (7, including C1 Atlas and C2 Axis), Thoracic (12), Lumbar (5), Sacrum (1 - fused), Coccyx (1 - fused).
• Thoracic Cage (25 bones): Sternum (1) + Ribs (12 pairs = 24).
• True Ribs: 1–7 (attach directly to sternum).
• False Ribs: 8–10 (attach to cartilage of rib 7).
• Floating Ribs: 11–12 (no anterior attachment).

Appendicular Skeleton Breakdown (126 Bones)

• Pectoral Girdle (4): Clavicle (2 - collar bone), Scapula (2 - shoulder blade).
• Upper Limbs (60): Humerus (2), Radius (2 - lateral/thumb side), Ulna (2 - medial/pinky side), Carpals (16 - wrist), Metacarpals (10 - palm), Phalanges (28 - fingers).
• Pelvic Girdle (2): Hip bones (Coxal bones). Formed by the fusion of Ilium, Ischium, and Pubis.
• Lower Limbs (60): Femur (2), Patella (2), Tibia (2 - medial/shin bone), Fibula (2 - lateral), Tarsals (14 - ankle), Metatarsals (10 - sole), Phalanges (28 - toes).

Side Determination Quick Rules

• Clavicle: S-shaped; smooth surface is superior, rough surface is inferior; medial end is blocky, lateral end is flat.
• Humerus: Head faces medially and posteriorly; Olecranon fossa (deep depression) is on the posterior side.
• Femur: Smooth spherical head faces medially; Patellar surface is anterior; Linea aspera (rough ridge) is posterior.

3. Joints: Classification and Types

Joints (articulations) are classified either structurally (what holds them together) or functionally (how much they move).

A. Structural Classification

1. Fibrous Joints: Connected by dense fibrous connective tissue. No joint cavity. Generally immovable.
• Sutures: Found only between skull bones.
• Syndesmoses: Bones connected by a ligament (e.g., distal tibiofibular joint).
• Gomphoses: "Peg-in-socket" joint (e.g., teeth in alveolar sockets).
2. Cartilaginous Joints: Connected by cartilage. No joint cavity. Slightly movable.
• Synchondroses: Connected by hyaline cartilage (e.g., epiphyseal plates in growing long bones, rib 1 to sternum).
• Symphyses: Connected by fibrocartilage (e.g., intervertebral discs, pubic symphysis).
3. Synovial Joints: Fluid-filled joint cavity is present. Freely movable. (See breakdown below).

B. Functional Classification

• Synarthrosis: Immovable joints (e.g., skull sutures).
• Amphiarthrosis: Slightly movable joints (e.g., pubic symphysis, intervertebral discs).
• Diarthrosis: Freely movable joints (all synovial joints).

C. Types of Synovial Joints (Crucial for MCQs)

Synovial Joint Type	Type of Movement	Classic Examples
Ball-and-Socket	Multiaxial (highest range of motion)	Shoulder joint, Hip joint
Hinge	Uniaxial (Flexion/Extension only)	Elbow joint, Knee joint, Ankle joint, Interphalangeal joints
Pivot	Uniaxial (Rotation only)	Atlanto-axial joint (C1-C2 "No" movement), Proximal radioulnar joint (pronation/supination)
Condyloid / Ellipsoid	Biaxial (Flexion/Extension, Abduction/Adduction)	Wrist joint (radiocarpal), Metacarpophalangeal joints (knuckles)
Saddle	Biaxial (Same as condyloid but greater freedom)	First carpometacarpal joint (Thumb)
Plane / Gliding	Nonaxial (Gliding/sliding movements)	Intercarpal joints (wrist bones), Intertarsal joints (ankle bones)

• Where is the deltoid tuberosity located? Humerus
• What bone features the "Acetabulum" (hip socket)? Coxal (Hip) bone
• What bone features the "Glenoid cavity"? Scapula
• Fluid found in diarthrodial joints? Synovial fluid (secreted by the synovial membrane; acts as a lubricant).