The Skeletal System: The Appendicular Skeleton

The Appendicular Skeleton

Introduction

The appendicular skeleton is one of the two major divisions of the human skeletal system, consisting of the bones that support the appendages (limbs). It is essential for movement and interaction with the environment.

• Pectoral girdle: Composed of the clavicle and scapulae, it connects the upper limbs to the axial skeleton.

• Upper limbs: Includes the humerus, ulna, radius, carpals, metacarpals, and phalanges.

• Pelvic girdle: Formed by the coxal (hip) bones, it attaches the lower limbs to the axial skeleton.

• Lower limbs: Consists of the femur, tibia, fibula, patella, tarsals, metatarsals, and phalanges.

Key Point: The appendicular skeleton contains 126 bones, facilitating movement and supporting the body's weight during locomotion.

Overview of the Skeletal System

Classification of Bones

The human skeleton is divided into two main parts:

• Axial skeleton: 80 bones (skull, vertebral column, rib cage)

• Appendicular skeleton: 126 bones (limbs and girdles)

Example: The appendicular skeleton includes the bones of the arms, legs, shoulders, and hips.

Appendicular Skeleton Components

Region	Bones	Number
Pectoral girdle	Clavicle, Scapula	4 (2 clavicles, 2 scapulae)
Upper limbs	Humerus, Ulna, Radius, Carpals, Metacarpals, Phalanges	60
Pelvic girdle	Hip bones (coxal bones)	2
Lower limbs	Femur, Patella, Tibia, Fibula, Tarsals, Metatarsals, Phalanges	60

The Pectoral Girdle and Upper Limb

The Pectoral Girdle

The pectoral girdle anchors the upper limbs to the trunk and provides attachment points for muscles that move the shoulder and arm.

• Clavicle: A long bone with two ends—sternal end (medial, articulates with the sternum) and acromial end (lateral, articulates with the scapula).

• Scapula: Also known as the shoulder blade, it features the glenoid cavity (socket for the humerus), acromion (articulates with clavicle), and coracoid process (muscle attachment).

Key Point: The pectoral girdle allows for a wide range of motion at the shoulder joint.

Clavicle Structure

• Sternal end: Rounded, articulates with the manubrium of the sternum.

• Acromial end: Flattened, articulates with the acromion of the scapula.

• Function: Acts as a strut to keep the upper limb away from the thorax, allowing for maximum mobility.

Scapula Structure

• Glenoid cavity: Shallow socket for the head of the humerus.

• Acromion: Lateral extension, articulates with the clavicle.

• Coracoid process: Anterior projection for muscle attachment.

• Spine: Prominent ridge on the posterior surface.

Upper Limb Bones

Major Bones of the Upper Limb

The upper limb consists of several bones that allow for complex movements and dexterity.

• Humerus: The arm bone, articulates proximally with the scapula and distally with the radius and ulna.

• Ulna: Medial bone of the forearm, forms the elbow joint with the humerus.

• Radius: Lateral bone of the forearm, allows for rotation of the wrist.

• Carpals: Eight small bones forming the wrist.

• Metacarpals: Five bones forming the palm.

• Phalanges: Fourteen bones forming the fingers (proximal, middle, distal).

Example: The humerus is the largest bone in the upper limb and is essential for arm movement.

Pelvic Girdle and Lower Limb

The Pelvic Girdle

The pelvic girdle supports the weight of the upper body and protects pelvic organs. It consists of two coxal (hip) bones.

• Coxal bones: Each formed by the fusion of three bones—ilium, ischium, and pubis.

• Acetabulum: Deep socket for articulation with the femur.

• Obturator foramen: Large opening for nerves and blood vessels.

Key Point: The pelvic girdle is adapted for weight-bearing and stability.

Lower Limb Bones

The lower limb bones are specialized for supporting body weight and enabling locomotion.

• Femur: The thigh bone, longest and strongest bone in the body.

• Patella: Kneecap, a sesamoid bone that protects the knee joint.

• Tibia: Medial bone of the lower leg, bears most of the weight.

• Fibula: Lateral bone of the lower leg, provides muscle attachment.

• Tarsals: Seven bones forming the ankle.

• Metatarsals: Five bones forming the foot.

• Phalanges: Fourteen bones forming the toes.

Example: The femur articulates with the pelvis at the acetabulum, forming the hip joint.

Summary Table: Major Bones of the Appendicular Skeleton

Region	Bone	Number (per skeleton)
Pectoral girdle	Clavicle	2
Pectoral girdle	Scapula	2
Upper limb	Humerus	2
Upper limb	Ulna	2
Upper limb	Radius	2
Upper limb	Carpals	16
Upper limb	Metacarpals	10
Upper limb	Phalanges	28
Pelvic girdle	Hip bones	2
Lower limb	Femur	2
Lower limb	Patella	2
Lower limb	Tibia	2
Lower limb	Fibula	2
Lower limb	Tarsals	14
Lower limb	Metatarsals	10
Lower limb	Phalanges	28

Additional info: The appendicular skeleton is crucial for movement, manipulation of objects, and interaction with the environment. It is distinct from the axial skeleton, which provides central support and protection for vital organs.