Divisions and Classifications of the Human Skeletal System

Overview of the Human Skeleton

• The human skeleton is comprised of a large network of bones interconnected by joints.

• The skeletal system is organized into two primary divisions:

  • The Axial Skeleton.

  • The Appendicular Skeleton.

The Axial Skeleton

• The term axial indicates that the bones lie along the midline of the body.

• The midline refers to the vertical center of the body, running "right down the middle."

• Identifying components of the Axial Skeleton:

  • The skull.

  • The vertebral column.

  • The sternum.

  • The laryngeal skeleton: Located in the neck region.

  • The thoracic cage.

The Appendicular Skeleton

• The appendicular skeleton consists of all bones that are attached to the axial skeleton.

• Identifying components of the Appendicular Skeleton:

  • The pectoral girdle: Described as a "fancy name for your shoulder."

  • The upper limbs: Located on both sides of the torso.

  • The pelvic girdle: Commonly known as the hips.

  • The lower limbs.

• A simple rule for classification: if a bone is not part of the axial skeleton, it is attached to it and thus part of the appendicular skeleton.

Classification of Bones by Structure and Shape

• Bones in the body are classified based on two main criteria:

  1. Their structure.

  2. Their shape.

• There are six primary categories of bones.

Long Bones

• Characteristics: Long bones are defined as being more "long and thin."

• Anatomical Features: They consist of a central shaft (diaphysis) and two distinct ends (epiphyses).

• Function: Their structural design is specifically suited to support the weight of the body.

• Named Examples:

  • Femur: A classic example of a long bone located in the thigh.

  • Radius and Ulna: The bones of the forearm.

  • Humerus: The bone of the upper arm.

Flat Bones

• Characteristics: These bones are thin and flat.

• Anatomical Features: They possess two parallel surfaces.

• Named Examples:

  • Scapula: The shoulder blade; when viewed sideways, its thinness and parallel surfaces are evident.

  • Cranium: The bones that make up the skull.

  • Sternum.

  • Ribs.

Sutural Bones

• Characteristics: These are small, flat bones.

• Location: They are found lying in between the flat bones of the skull.

• Variability: The specific number of sutural bones varies from person to person.

• Representation in Models: Because the number varies individually, sutural bones are typically not included in general anatomical skeletal models.

Short Bones

• Characteristics: Short bones are small and more "cube shaped."

• Named Examples:

  • Carpals: The small bones located in the wrist region.

  • Tarsals: The short bones found in the ankle that provide stability and support during movement.

    

Irregular Bones

• Characteristics: This is a "catch all category" for bone shapes that do not cleanly fit into other classifications.

• Anatomical Features: These bones often have complex shapes with various ridges.

• Named Examples:

  • Vertebra: A single unit of the spine is a prime example of an irregular bone due to its complex structure.

Sesamoid Bones

• Characteristics: These bones are small and round in shape.

• Development: Sesamoid bones uniquely develop inside of tendons.

• Function: They serve to reinforce the tendon and provide it with greater strength and stability.

• Named Examples:

  • Patella: The kneecap, located in the anterior (front) portion of the knee joint.

  • Hand Tendons: Sesamoid bones can also be found within some tendons in the hand.

• Variability: While every person possess a patella, the presence of sesamoid bones in other tendons, such as those in the hands, varies from person to person.