The Skeletal System

Overview and Subdivisions

The skeletal system provides the structural framework for the human body and is essential for movement, protection, and various physiological functions. It is divided into two main subdivisions:

• Axial Skeleton: Includes the skull, vertebral column, and rib cage (thorax). It forms the central axis of the body.

• Appendicular Skeleton: Comprises the bones of the limbs (arms and legs), as well as the pectoral (shoulder) and pelvic (hip) girdles.

Components of the Skeletal System

• Bones: Rigid organs that form the skeleton.

• Joints: Articulations where two or more bones meet.

• Cartilages: Flexible connective tissue found in joints and other areas.

• Ligaments: Bands of connective tissue that connect bones to other bones.

Functions of Bones

• Support: Provide structural support for the body.

• Protection: Shield soft organs (e.g., skull protects the brain).

• Movement: Serve as levers for muscles to act upon.

• Mineral Storage: Store minerals such as calcium and phosphorus.

• Fat Storage: Store fats in yellow bone marrow.

• Blood Cell Formation (Hematopoiesis): Occurs in red bone marrow.

Bone Count and Types of Bone Tissue

• Number of Bones in Adult Skeleton: 206

• Compact Bone: Dense, smooth, and homogeneous tissue.

• Spongy Bone: Less dense, contains open spaces, and is found at the ends of long bones and inside flat bones.

Classification of Bones by Shape

• Long Bones: Longer than they are wide (e.g., femur).

• Short Bones: Cube-shaped (e.g., talus).

• Flat Bones: Thin and flattened (e.g., sternum).

• Irregular Bones: Complex shapes (e.g., vertebra).

Structure of a Long Bone

• Diaphysis: Shaft of the bone.

• Periosteum: Outer covering of the diaphysis.

• Epiphysis: Ends of the bone (proximal and distal).

• Articular Cartilage: Hyaline cartilage covering the epiphyses.

• Epiphyseal Plate: Growth plate of hyaline cartilage in children.

• Epiphyseal Line: Remnant of the epiphyseal plate in adults.

• Marrow (Medullary) Cavity: Central cavity containing bone marrow.

Bone Marrow Types and Locations

• Yellow Marrow: Stores fat; found in the medullary cavity of adults.

• Red Marrow: Site of hematopoiesis; found in the medullary cavity of children and in spongy bone and epiphyses of adults.

Microscopic Anatomy of Compact Bone

• Osteocytes: Mature bone cells.

• Lacunae: Small cavities containing osteocytes.

• Lamellae: Concentric rings of bone matrix.

• Haversian (Central) Canal: Central channel containing blood vessels and nerves.

• Haversian System (Osteon): Structural unit of compact bone.

• Canaliculi: Tiny canals connecting lacunae.

• Volkmann’s Canal: Perforating canals connecting Haversian canals.

Axial and Appendicular Skeleton

Axial Skeleton

• Components: Skull, vertebral column, bony thorax (rib cage).

• Bone Count: Approximately 80 bones.

Appendicular Skeleton

• Components: Limbs, pectoral girdle, pelvic girdle.

• Bone Count: Approximately 126 bones.

Skull Anatomy

• Cranial Bones (8): Frontal, occipital, ethmoid, sphenoid, parietal (2), temporal (2).

• Facial Bones (14): Maxillae (2), mandible, nasal (2), zygomatic (2), inferior nasal conchae (2), vomer, lacrimal (2), palatine (2).

• Joints: Bones joined by sutures, except the mandible.

Key Skull Features

• Sella Turcica: Depression in the sphenoid bone housing the pituitary gland.

• Foramen: Openings for nerves and blood vessels.

• Crista Galli: Part of the ethmoid bone separating olfactory bulbs.

Skull Sutures

• Coronal: Between frontal and parietal bones.

• Sagittal: Between the two parietal bones.

• Lambdoidal: Between parietal and occipital bones.

• Squamous: Between parietal and temporal bones.

Special Bones and Features

• Hyoid Bone: Does not articulate with other bones; supports the tongue and aids in swallowing and speech.

• Fontanels: Soft spots in infant skulls allowing flexibility during birth; close as the child grows.

• Sinuses: Air-filled spaces in bones around the nasal cavity; lighten the skull and amplify voice.

Vertebral Column and Thorax

Vertebral Column

• Cervical Vertebrae: 7 (neck region)

• Thoracic Vertebrae: 12 (upper back)

• Lumbar Vertebrae: 5 (lower back)

Spinal Curvatures

• Primary Curvatures: Thoracic and sacral; present at birth.

• Secondary Curvatures: Cervical (develops when baby lifts head) and lumbar (develops when baby walks).

Special Cervical Vertebrae

• Atlas (C1): Lacks a body; supports the skull.

• Axis (C2): Has a dens (odontoid process) for head rotation.

• Transverse Foramen: Unique to cervical vertebrae; passage for arteries.

Bony Thorax

• Sternum: Manubrium, body, xiphoid process.

• Ribs:

  • True Ribs: 7 pairs, directly attached to sternum.

  • False Ribs: 5 pairs, indirectly attached or not attached to sternum (last 2 pairs are floating ribs).

Appendicular Skeleton: Limbs and Girdles

Pectoral (Shoulder) Girdle

• Scapula (shoulder blade)

• Clavicle (collarbone)

Upper Limb Bones

• Arm: Humerus

• Forearm: Radius and ulna

• Hand: Carpals (8 per hand), metacarpals, phalanges

Pelvic Girdle and Pelvis

• Coxal Bone: Formed by fusion of ilium, ischium, and pubis

• Pelvic Girdle: Two coxal bones and the sacrum

• Pelvis Function: Protects reproductive organs, urinary bladder, and part of the large intestine

• Male vs. Female Pelvis: Female pelvis is wider, more circular, shallower, and lighter; male pelvis is narrower and taller

Lower Limb Bones

• Leg: Tibia (weight-bearing), fibula

• Foot: Tarsals (7 per foot; calcaneus is largest, talus is second largest), metatarsals, phalanges

Joints (Articulations)

Definition and Function

• Joint (Articulation): A site where two or more bones meet

• Functions: Hold bones together and allow for mobility

Movements of Joints

• Supination: Rotating the forearm so the palm faces up

• Pronation: Rotating the forearm so the palm faces down

• Adduction: Moving a limb toward the midline

• Abduction: Moving a limb away from the midline

• Other Movements: Flexion, extension, rotation, circumduction, etc.

Classification of Joints

• Functional Classification (by movement):

  • Synarthroses: Immovable joints (e.g., skull sutures)

  • Amphiarthroses: Slightly movable joints (e.g., intervertebral discs)

  • Diarthroses: Freely movable joints (e.g., shoulder, hip)

• Structural Classification (by composition):

  • Fibrous Joints: Bones joined by fibrous tissue (e.g., sutures in the skull)

  • Cartilaginous Joints: Bones joined by cartilage (e.g., first rib to sternum, intervertebral discs)

  • Synovial Joints: Bones separated by a joint cavity filled with synovial fluid (e.g., shoulder, hip, knee)

Features of Synovial Joints

• Articular Cartilage: Covers bone ends

• Articular Capsule: Encloses joint cavity

• Joint Cavity: Contains synovial fluid

• Reinforcing Ligaments: Strengthen the joint

Types of Synovial Joints

Summary Table: Bone Types and Examples

Key Terms and Definitions

• Hematopoiesis: The process of blood cell formation in red bone marrow.

• Epiphyseal Plate: Growth plate of cartilage in children; site of bone lengthening.

• Foramen: Opening in bone for passage of nerves and blood vessels.

• Suture: Immovable joint between skull bones.

• Fontanel: Soft spot in infant skull where bone formation is incomplete.

Additional info:

• For detailed diagrams and identification, refer to textbook figures or lecture slides as indicated in the original notes.

• Movements such as flexion, extension, rotation, and circumduction are best understood with visual aids or by practicing the motions.