The Appendicular Skeleton: Structure and Function

The Appendicular Skeleton

The appendicular skeleton consists of 126 bones that facilitate movement and support the limbs. It includes the bones of the pectoral and pelvic girdles, as well as those of the upper and lower limbs.

Pectoral Girdle

The pectoral (shoulder) girdle connects the upper limbs to the trunk and provides attachment points for muscles that move the shoulders and arms.

• Clavicle: An S-shaped bone that acts as a strut between the sternum and scapula.

  • The sternal end articulates with the manubrium of the sternum, forming the sternoclavicular joint (the only bony connection between the pectoral girdle and axial skeleton).

  • The acromial end articulates with the acromion of the scapula, forming the acromioclavicular joint.

• Scapula: A large, flat, triangular bone located on the posterior thorax.

  • The glenoid cavity is a shallow depression that articulates with the head of the humerus, forming the glenohumeral (shoulder) joint.

  • The acromion is a prominent posterior projection that articulates with the clavicle.

Bones of the Upper Limb

Each upper limb contains 30 bones, divided into the arm, forearm, and hand.

Arm (Brachium)

• Humerus: The only bone of the arm, extending from the shoulder to the elbow.

  • The head articulates with the glenoid cavity of the scapula.

  • The trochlea (medial) articulates with the ulna; the capitulum (lateral) articulates with the radius.

  • Coronoid fossa: Anterior depression for the coronoid process of the ulna (elbow flexed).

  • Radial fossa: Anterior depression for the head of the radius (elbow flexed).

  • Olecranon fossa: Posterior depression for the olecranon process of the ulna (elbow extended).

Forearm (Antebrachium)

• Ulna: Medial bone of the forearm (in anatomical position).

  • Olecranon: Forms the point of the elbow; fits into the olecranon fossa of the humerus.

  • Trochlear notch: Articulates with the trochlea of the humerus.

  • Coronoid process: Fits into the coronoid fossa of the humerus when flexed.

  • Radial notch: Lateral depression for articulation with the head of the radius (proximal radioulnar joint).

  • Styloid process: Distal projection for ligament attachment at the wrist.

• Radius: Lateral bone of the forearm.

  • Head: Articulates with the capitulum of the humerus and the radial notch of the ulna.

  • Radial tuberosity: Attachment for the biceps brachii muscle.

  • Styloid process: Stabilizes the wrist joint.

  • Ulnar notch: Articulates with the head of the ulna (distal radioulnar joint).

• Interosseous membrane: Connective tissue sheet uniting the radius and ulna.

Wrist (Carpus) and Hand

• Carpus: Composed of 8 carpal bones arranged in two rows (proximal and distal).

  • Proximal row (lateral to medial): Scaphoid, Lunate, Triquetrum, Pisiform

  • Distal row (lateral to medial): Trapezium, Trapezoid, Capitate, Hamate

• Metacarpals: Five bones forming the palm; numbered I-V from lateral (thumb) to medial (little finger).

• Phalanges: 14 bones in each hand; thumb (pollex) has 2 (proximal and distal), fingers have 3 each (proximal, middle, distal).

Pelvic Girdle and Pelvis

The pelvic girdle supports the weight of the upper body and protects pelvic organs. It consists of two hip (coxal) bones, each formed by the fusion of three bones.

• Ilium: Superior, largest part; features the iliac crest and anterior superior iliac spine (muscle attachment).

• Ischium: Posteroinferior part; includes the ischial tuberosity (bears weight when sitting).

• Pubis: Anterior part; forms the pubic symphysis (joint between left and right hip bones, united by fibrocartilage).

• Acetabulum: Deep socket for articulation with the head of the femur (hip joint).

• Obturator foramen: Large opening covered by connective tissue; muscle attachment site.

• Pelvis: Formed by two coxal bones, sacrum, and coccyx.

• 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

The lower limb contains 30 bones, supporting body weight and enabling locomotion. It is divided into the thigh, leg, and foot.

Thigh

• Femur: The longest and heaviest bone in the body; single bone of the thigh.

  • Head: Articulates with the acetabulum of the coxal bone.

  • Greater trochanter: Large, lateral projection for muscle attachment.

  • Lesser trochanter: Small, medial prominence for muscle attachment.

  • Linea aspera: Prominent posterior ridge for muscle attachment.

  • Distal condyles: Articulate with the tibia to form the knee joint.

• Patella: Large sesamoid bone (kneecap) embedded in the quadriceps tendon; patellar ligament attaches to the tibia.

Leg

• Tibia: Medial, larger bone of the leg; bears weight.

  • Tibial tuberosity: Anterior surface, attachment for the patellar ligament.

  • Medial malleolus: Distal projection that articulates with the talus (ankle joint).

• Fibula: Lateral, slender bone; does not bear weight, serves as muscle attachment.

  • Head: Articulates with the lateral condyle of the tibia (proximal tibiofibular joint).

  • Lateral malleolus: Distal projection that articulates with the talus (ankle joint).

Foot

• Tarsus (Ankle): Composed of 7 tarsal bones:

  • Talus: Articulates with the tibia and fibula to form the ankle joint.

  • Calcaneus: Heel bone; largest tarsal bone, attachment for the Achilles tendon.

  • Cuboid, Navicular, Medial/Intermediate/Lateral Cuneiforms: Form the rest of the tarsus.

• Metatarsals: Five bones numbered I-V from medial (great toe) to lateral; articulate proximally with tarsal bones and distally with phalanges.

• Phalanges: 14 bones in each foot; great toe (hallux) has 2 (proximal and distal), other toes have 3 each (proximal, middle, distal).

Summary Table: Major Bones of the Appendicular Skeleton

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

Additional info:

• The appendicular skeleton is essential for movement, muscle attachment, and supporting the weight of the body during locomotion.

• Joints such as the shoulder, elbow, hip, and knee are examples of synovial joints, allowing a wide range of motion.