The Skeletal System: Axial and Appendicular Skeleton Study Notes

The Skeletal System

Overview

The skeletal system is a complex framework of bones and connective tissues that provides support, protection, and movement for the human body. It is divided into two major subdivisions: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Forms the central axis of the body and includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Comprises the limbs and girdles (pectoral and pelvic) that attach them to the axial skeleton.

Axial Skeleton

Main Components

The axial skeleton consists of bones that form the longitudinal axis of the body, providing protection for vital organs and structural support.

• Skull: Protects the brain and forms the structure of the face.

• Vertebral Column: Supports the head and trunk, protects the spinal cord.

• Thoracic Cage: Includes the ribs and sternum, protects the heart and lungs.

Appendicular Skeleton

Main Components

The appendicular skeleton is responsible for movement and manipulation of objects. It includes the bones of the limbs and the girdles that attach them to the axial skeleton.

• Pectoral Girdle: Clavicle and scapula, which attach the upper limbs to the trunk.

• Upper Limbs: Humerus, radius, ulna, carpals, metacarpals, and phalanges.

• Pelvic Girdle: Hip bones (ilium, ischium, pubis), which attach the lower limbs to the trunk.

• Lower Limbs: Femur, tibia, fibula, tarsals, metatarsals, and phalanges.

Surface Features of Bones

Classification and Functions

Bones have various surface features that serve as sites for muscle attachment, articulation with other bones, and passage of nerves and blood vessels. These features are classified into articulations, extensions/projections, depressions, and passages.

Category	Feature	Description
Articulations	Condyle	A rounded knob that articulates with another bone (e.g., occipital condyles of the skull).
	Facet	A smooth, flat, slightly concave or convex articular surface (e.g., facets of the vertebrae).
	Head	The prominent expanded end of a bone, sometimes rounded (e.g., head of the femur).
Extensions and Projections	Crest	A narrow ridge (e.g., iliac crest of the pelvis).
	Epicondyle	A projection superior to a condyle (e.g., medial epicondyle of the femur).
	Line	A slightly raised, elongated ridge (e.g., nuchal lines of the skull).
	Process	Any bony prominence (e.g., mastoid process of the skull).
	Protuberance	A bony outgrowth or protruding part (e.g., mental protuberance of the chin).
	Spine	A sharp, slender, or narrow process (e.g., spine of the scapula).
	Tubercle	A small, rounded process (e.g., greater tubercle of the humerus).
	Tuberosity	A rough, elevated surface (e.g., tibial tuberosity).
Depressions	Alveolus	A pit or socket (e.g., tooth socket).
	Fossa	A shallow, broad, or elongated basin (e.g., mandibular fossa).
	Sulcus	A groove for a tendon, nerve, or blood vessel (e.g., intertubercular sulcus of the humerus).
Passages	Canal	A tubular passage or tunnel in a bone (e.g., condylar canal of the skull).
	Fissure	A slit through a bone (e.g., orbital fissures behind the eye).
	Foramen	A hole through a bone, usually round (e.g., foramen magnum of the skull).
	Meatus	An opening into a canal (e.g., acoustic meatus of the ear).

Example: Application of Surface Features

• The foramen magnum in the occipital bone allows passage of the spinal cord.

• The mastoid process of the temporal bone serves as an attachment site for neck muscles.

• The glenoid cavity of the scapula articulates with the head of the humerus to form the shoulder joint.

Summary Table: Axial vs. Appendicular Skeleton

Skeleton Division	Main Components	Primary Functions
Axial Skeleton	Skull, vertebral column, thoracic cage	Protection of brain, spinal cord, and thoracic organs; support
Appendicular Skeleton	Pectoral girdle, upper limbs, pelvic girdle, lower limbs	Movement, manipulation of environment

Key Terms and Definitions

• Bone: A rigid organ that constitutes part of the vertebrate skeleton, providing structure and protection.

• Articulation: The site where two bones meet.

• Process: A projection or outgrowth of bone.

• Foramen: An opening or hole in a bone for passage of nerves and blood vessels.

Additional info:

• The axial skeleton contains 80 bones, while the appendicular skeleton contains 126 bones.

• Surface features of bones are essential for understanding muscle attachment, joint formation, and the passage of neurovascular structures.