The Appendicular Skeleton: Structure and Function

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The Appendicular Skeleton

Overview of the Appendicular Skeleton

The appendicular skeleton consists of 126 bones that form the limbs and the pectoral and pelvic girdles. These structures enable movement and support the attachment of muscles necessary for locomotion and manipulation of the environment.

Main Divisions: Pectoral girdle, upper limbs, pelvic girdle, and lower limbs
Function: Facilitates movement and interaction with the environment
Key Bones: Clavicle, scapula, humerus, radius, ulna, carpals, metacarpals, phalanges, coxal bones, femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges

Labeled diagram of the human skeleton highlighting the appendicular skeleton

The Pectoral Girdle

Structure and Articulations

The pectoral (shoulder) girdle connects the upper limbs to the trunk and consists of the clavicles and scapulae. It provides attachment points for muscles and allows a wide range of shoulder movements.

Clavicle (Collarbone): S-shaped bone with a medial (sternal) end articulating with the manubrium of the sternum (sternoclavicular joint) and a lateral (acromial) end articulating with the acromion of the scapula (acromioclavicular joint).
Scapula (Shoulder Blade): Broad, flat, triangular bone with the glenoid cavity that articulates with the humerus to form the glenohumeral (shoulder) joint.

Anatomy of the pectoral girdle, showing clavicle and scapula Labeled diagram of the scapula, anterior and posterior aspects

Bones of the Upper Limb

Regions and Major Bones

The upper limb is divided into the arm (brachium), forearm (antebrachium), and hand. Each upper limb contains 30 bones, specialized for mobility and dexterity.

Arm: Contains the humerus, which articulates proximally with the scapula and distally with the radius and ulna.
Forearm: Composed of the radius (lateral) and ulna (medial in anatomical position).
Hand: Includes the carpal bones (wrist), metacarpals (palm), and phalanges (fingers).

Labeled diagram of the humerus, anterior and posterior views Labeled diagram of the radius and ulna, anterior and posterior views Labeled diagram of the bones of the hand, anterior and posterior views

Key Articulations and Features

Shoulder Joint: Formed by the head of the humerus and the glenoid cavity of the scapula.
Elbow Joint: Formed by the distal humerus (trochlea and capitulum) articulating with the ulna and radius.
Wrist Joint: Formed by the distal radius articulating with the scaphoid and lunate carpal bones.
Hand: Carpus (8 carpal bones in two rows), 5 metacarpals, and 14 phalanges (proximal, middle, distal; thumb has only proximal and distal).

The Pelvic Girdle and Pelvis

Structure and Function

The pelvic girdle consists of two coxal (hip) bones, each formed by the fusion of the ilium, ischium, and pubis. The pelvis includes the coxal bones, sacrum, and coccyx, providing support for the lower limbs and protecting pelvic organs.

Ilium: Superior and largest part of the coxal bone; features the iliac crest and anterior superior iliac spine for muscle attachment.
Ischium: Posteroinferior part; the ischial tuberosity supports body weight when sitting.
Pubis: Anterior part; forms the pubic symphysis joint with the opposite pubis via fibrocartilage.
Acetabulum: Deep socket for articulation with the femoral head (hip joint).
Obturator Foramen: Large opening covered by connective tissue for muscle attachment.
Sacroiliac Joint: Articulation between the auricular surfaces of the ilium and sacrum, attaching the pelvic girdle to the axial skeleton.

Bones of the Lower Limb

Regions and Major Bones

The lower limb is specialized for weight-bearing and locomotion, consisting of 30 bones per limb. It is divided into the thigh, leg, and foot.

Thigh: Contains the femur, the longest and heaviest bone, articulating proximally with the acetabulum and distally with the tibia and patella.
Leg: Composed of the tibia (medial, weight-bearing) and fibula (lateral, non-weight-bearing, muscle attachment).
Foot: Includes the tarsal bones (ankle), metatarsals, and phalanges (toes).

Key Articulations and Features

Hip Joint: Formed by the head of the femur and the acetabulum of the coxal bone.
Knee Joint: Formed by the femoral condyles and the tibial condyles; patella is embedded in the quadriceps tendon.
Ankle Joint: Formed by the distal tibia (medial malleolus) and fibula (lateral malleolus) articulating with the talus.
Foot: 7 tarsal bones (including talus and calcaneus), 5 metatarsals, and 14 phalanges (hallux has 2, other toes have 3 each).

Summary Table: Major Bones of the Appendicular Skeleton

Region	Bones	Key Joints
Pectoral Girdle	Clavicle, Scapula	Sternoclavicular, Acromioclavicular, Glenohumeral
Upper Limb	Humerus, Radius, Ulna, Carpals, Metacarpals, Phalanges	Shoulder, Elbow, Wrist, Finger joints
Pelvic Girdle	Ilium, Ischium, Pubis (Coxal bone)	Sacroiliac, Pubic Symphysis, Hip
Lower Limb	Femur, Patella, Tibia, Fibula, Tarsals, Metatarsals, Phalanges	Hip, Knee, Ankle, Toe joints

Additional info: The appendicular skeleton is essential for movement, muscle attachment, and protection of internal organs. Its joints allow for a wide range of motion, especially in the upper limbs, while the lower limbs are adapted for stability and support during standing and walking.