Skeletal System: Structure, Function, and Classification

Skeletal System Overview

Subdivisions of the Skeleton

The human skeleton is divided into two main subdivisions, each with distinct anatomical components and functions.

• Axial Skeleton: Includes the skull, vertebral column, and rib cage (thorax).

• Appendicular Skeleton: Comprises the bones of the limbs (arms and legs).

Parts of the Skeletal System

The skeletal system consists of several types of connective tissues that provide structure and facilitate movement.

• Bones

• Joints

• Cartilages

• Ligaments

Functions of Bones

Bones serve multiple essential functions in the body:

• Support the body

• Storage of minerals (especially calcium) and fats

• Protection of soft organs

• Attachment for skeletal muscles

• Blood cell formation (hematopoiesis)

Number of Bones in the Adult Skeleton

• There are 206 bones in the adult human skeleton.

Bone Tissue Types and Structure

Types of Bone Tissue

Bone tissue is classified into two main types based on density and structure:

• Compact bone: Dense, smooth, and homogeneous.

• Spongy bone: Less dense, with more open spaces.

Bone Shapes and Examples

Bones are classified by shape, which relates to their function and location.

• Long bones: e.g., femur

• Short bones: e.g., talus (in the foot)

• Flat bones: e.g., sternum (breast bone)

• Irregular bones: e.g., vertebra

Structure of a Long Bone

Long bones have several distinct anatomical regions:

• Diaphysis: Shaft

• Periosteum: Outside covering of the diaphysis

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

• Articular cartilage: Hyaline cartilage covering the external surface of each epiphysis

• Epiphyseal plate: Flat plate of hyaline cartilage seen in growing bone

• Epiphyseal line: Remnant of epiphyseal plate in adult bones

• Medullary cavity: Cavity inside the shaft that houses bone marrow

Bone Marrow: Yellow vs. Red

Bone marrow is found within the medullary cavity and exists in two forms:

• Yellow marrow: Consists mostly of fats and nutrients; located in the medullary cavity in adults.

• Red marrow: Responsible for blood cell formation (hematopoiesis); located in the medullary cavity in children, but only in spongy bone and epiphyses in adults.

Microscopic Anatomy of Compact Bone

Compact bone contains several specialized structures:

• Osteocytes: Mature bone cells

• Lacunae: Small cavities containing osteocytes

• Lamellae: Concentric rings of bone matrix

• Haversian (Central) canal: Central channel for blood vessels and nerves

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

• Canaliculi: Tiny canals connecting lacunae

• Volkmann's Canal: Perforating canals for blood vessels and nerves

Axial and Appendicular Skeleton

Main Divisions of the Appendicular Skeleton

The appendicular skeleton includes the limbs and their supporting structures.

• Limbs: Appendages

• Pectoral girdle: Shoulder joints

• Pelvic girdle: Hip joints

• Total bones: 126

Cranial and Facial Bones

The skull is composed of cranial and facial bones, joined by sutures (except the mandible).

• 8 cranial bones: Frontal, occipital, ethmoid, sphenoid, parietal (2), temporal (2)

• 14 facial bones: Maxilla (2), mandible, nasal (2), zygomatic (2), inferior nasal conchae (2), vomer, lacrimal (2), palatine (2)

Special Skull Features

• Sella turcica: Depression in the sphenoid bone that protects the pituitary gland

• Cribriform plate: Part of the ethmoid bone; allows olfactory nerves to pass through

• Sutures: Coronal, sagittal, lambdoidal, squamous

Hyoid Bone

The hyoid bone does not directly articulate with other bones. It serves as a moveable base for the tongue and aids in swallowing and speech.

Fontanels

Fontanels are soft spots in the fetal skull where the sutures have not fully formed. They allow for flexibility during birth.

Sinuses

Sinuses are mucosa-lined air pockets surrounding the nasal cavity. They lighten the skull and amplify sound.

Vertebral Column

The vertebral column is divided into regions, each with a specific number of vertebrae:

• Cervical vertebrae: 7

• Thoracic vertebrae: 12

• Lumbar vertebrae: 5

Curvatures of the Spine

The secondary curvatures are cervical and lumbar. Cervical curvature develops when a baby begins to lift its head; lumbar curvature develops when the child begins to walk upright.

Parts of the Vertebra

• Body

• Vertebral arch

• Processes

Special Features: Cervical Vertebrae, Atlas, and Axis

• All cervical vertebrae have transverse foramen.

• Atlas (C1): Unique because it does not have a body.

• Axis (C2): Has a process called the dens, which serves as the rotation point for the head.

Thorax and Appendicular Skeleton

Bony Thorax

• Sternum: Manubrium, body, xiphoid process

• Ribs: 12 pairs

• True ribs: Connected by hyaline cartilage (7 pairs)

• False ribs: Not attached to sternum (5 pairs, including 2 pairs of floating ribs)

Pectoral (Shoulder) Girdle

• Scapula

• Clavicle

Bones of the Arm

• Arm: Humerus

• Forearm: Radius & ulna

Bones of the Hand

• Carpals (wrist): 8

• Metacarpals (palm)

• Phalanges (fingers/digits)

Pelvic Girdle

• Three fused bones: Ilium, ischium, pubis

• Two coxal bones & the sacrum

Pelvis Function

The pelvis protects organs in the lower abdomen, including parts of the digestive and reproductive systems.

Bones of the Lower Limb

Bones of the Leg

• Tibia: Weight-bearing bone

• Fibula

Bones of the Foot

• Tarsals: 7 (calcaneus is the largest, talus is second largest)

• Metatarsals

• Phalanges

Joints and Articulations

Definition of a Joint

A joint is an articulation where two or more bones meet.

Definition of an Articulation

An articulation is another term for a joint.

Joint Movements

Joints allow for various movements, including supination, pronation, adduction, abduction, etc.

Joint Functions

• Bind bones together

• Allow for mobility

Classification of Joints

• Functional classification (by movement):

  • Synarthroses: Immovable joints

  • Amphiarthroses: Slightly movable joints

  • Diarthroses: Freely movable joints

• Structural classification (by structure):

  • Fibrous joints: Sutures in the skull

  • Cartilaginous joints: First rib connects to the sternum, intervertebral discs

  • Synovial joints: Freely movable joints (shoulder joint, hip joint, knee joint)

Features of Synovial Joints

• Articular cartilage

• Articular capsule

• Joint cavity

• Reinforcing ligaments

Synovial Joint Shapes

There are six synovial joint shapes, each with specific movement capabilities:

• Nonaxial

• Uniaxial

• Biaxial

• Multiaxial

Examples: Shoulder joint (multiaxial), elbow joint (uniaxial), wrist joint (biaxial)

Summary Table: Bone Types and Examples

Bone Shape	Example
Long bone	Femur
Short bone	Talus
Flat bone	Sternum
Irregular bone	Vertebra

Summary Table: Vertebral Column Regions

Region	Number of Vertebrae
Cervical	7
Thoracic	12
Lumbar	5

Key Equations and Terms

• Hematopoiesis: Formation of blood cells in the bone marrow.

• Articular cartilage: Hyaline cartilage covering bone surfaces in synovial joints.

• Epiphyseal plate: $\text{Epiphyseal Plate} = \text{Hyaline Cartilage}$ (in growing bones)

Additional info: Some context and definitions have been expanded for clarity and completeness.