Skeletal System: Structure, Function, and Classification of Bones

The Skeletal System

Parts of the Skeletal System

The skeletal system is a complex framework that provides support, protection, and movement for the human body. It consists of several key components:

• Bones (skeleton): The rigid structures that form the skeleton.

• Joints: Articulations where two or more bones meet, allowing movement.

• Cartilages: Flexible connective tissue that cushions joints and forms parts of the skeleton.

• Ligaments: Strong bands of connective tissue that connect bones and stabilize joints.

The skeleton is subdivided into two main regions:

• Axial skeleton: Includes the skull, vertebral column, and rib cage.

• Appendicular skeleton: Includes the limbs and girdles (shoulder and pelvic).

Functions of the Bones

Major Roles of Bones in the Body

Bones perform several essential functions that are critical for survival and health:

• Support: Bones provide a structural framework for the body.

• Protection: Bones shield vital organs from injury.

  • Skull and vertebrae protect the brain and spinal cord.

  • Rib cage protects organs in the thoracic cavity.

• Movement: Bones act as levers for muscles, enabling movement.

• Storage: Bones store minerals (especially calcium and phosphorus) and fats (in the marrow cavity).

• Blood cell formation (hematopoiesis): Occurs in the red bone marrow.

Classification of Bones

Types of Osseous (Bone) Tissue

The adult skeleton contains 206 bones, which are composed of two main types of bone tissue:

• Compact bone: Dense, smooth, and homogeneous tissue that provides strength.

• Spongy bone: Consists of small needlelike pieces (trabeculae) and many open spaces, contributing to lightweight structure and housing marrow.

Bone Classification by Shape

Bones are classified into four groups based on their shapes:

• Long bones: Longer than they are wide, with a shaft and enlarged ends. Mostly compact bone with spongy bone at the ends. Examples: femur, humerus.

• Flat bones: Thin, flattened, and usually curved. Composed of two layers of compact bone with spongy bone in between. Examples: most skull bones, ribs, sternum.

• Short bones: Generally cube-shaped, mostly spongy bone with an outer layer of compact bone. Sesamoid bones (e.g., patella) form within tendons. Examples: carpals (wrist), tarsals (ankle).

• Irregular bones: Complex shapes that do not fit other categories. Examples: vertebrae, hip bones.

Structure of Bone

Long Bone Anatomy

Long bones have a specialized structure that supports their function:

• Diaphysis (shaft): Makes up most of the bone's length and is composed of compact bone, providing strength and support.

• Periosteum: The outer covering of the diaphysis, made of dense fibrous connective tissue. Perforating (Sharpey's) fibers anchor the periosteum to the underlying bone.

Example: The femur, a long bone, has a diaphysis surrounded by periosteum, which is essential for bone growth and repair.

Summary Table: Bone Classification by Shape

Bone Type	Shape/Features	Main Examples
Long	Longer than wide; shaft with enlarged ends; mostly compact bone	Femur, Humerus
Flat	Thin, flattened, curved; two layers of compact bone with spongy bone between	Skull bones, Ribs, Sternum
Short	Cube-shaped; mostly spongy bone with outer compact bone; includes sesamoid bones	Carpals, Tarsals, Patella (sesamoid)
Irregular	Complex shapes; do not fit other categories	Vertebrae, Hip bones

Additional info: The periosteum is vital for bone health, serving as an attachment point for tendons and ligaments and containing cells important for bone growth and repair.