Appendicular Skeleton: Structure and Function

Appendicular Skeleton

Overview

The appendicular skeleton consists of 126 bones that include the pectoral and pelvic girdles, as well as the bones of the upper and lower limbs. It is primarily responsible for facilitating movement and supporting the attachment of muscles.

• Pectoral Girdle: Connects the upper limbs to the trunk.

• Pelvic Girdle: Connects the lower limbs to the trunk.

• Upper Limbs: Arms, forearms, and hands.

• Lower Limbs: Thighs, legs, and feet.

Pectoral Girdle

Structure and Articulations

The pectoral girdle (shoulder girdle) is composed of the clavicles and scapulae, providing attachment points for the upper limbs and facilitating a wide range of motion.

• Clavicle: S-shaped bone; medial (sternal) end articulates with the manubrium of the sternum (forms the sternoclavicular joint), the only bony connection to the axial skeleton. Lateral (acromial) end articulates with the acromion of the scapula (forms the acromioclavicular joint).

• Scapula: Large, flat, triangular bone. The glenoid cavity is a shallow depression that articulates with the head of the humerus, forming the glenohumeral (shoulder) joint.

Bones of the Upper Limb

Regions and Major Bones

Each upper limb contains 30 bones, divided into three regions: arm (brachium), forearm (antebrachium), and hand.

• Arm (Brachium): Contains the humerus.

• Forearm (Antebrachium): Contains the radius and ulna.

• Hand: Contains carpal bones (wrist), metacarpals (palm), and phalanges (fingers).

Humerus

• Proximal End: Head articulates with the glenoid cavity of the scapula (shoulder joint).

• Distal End: Articulates with the radius and ulna at the elbow. The trochlea (medial) articulates with the ulna; the capitulum (lateral) articulates with the radius.

• Fossae: Coronoid fossa (anterior, receives coronoid process of ulna when flexed), radial fossa (anterior, receives head of radius when flexed), olecranon fossa (posterior, receives olecranon process of ulna when extended).

Ulna and Radius

• Ulna: Medial bone of the forearm. The olecranon forms the point of the elbow. The trochlear notch articulates with the trochlea of the humerus. The coronoid process fits into the coronoid fossa of the humerus when flexed. The radial notch articulates with the head of the radius (proximal radioulnar joint). The styloid process is a connective tissue attachment point at the wrist.

• Radius: Lateral bone of the forearm. The head articulates with the capitulum of the humerus and the radial notch of the ulna. The radial tuberosity is an attachment for the biceps brachii muscle. The styloid process helps stabilize the wrist. The ulnar notch articulates with the head of the ulna (distal radioulnar joint).

• Interosseous Membrane: Sheet of connective tissue uniting the ulna and radius.

Wrist (Carpus), Hand, and Fingers

• Carpus: 8 carpal bones arranged in two rows (proximal and distal). Proximal row (lateral to medial): scaphoid, lunate, triquetrum, pisiform. Distal row: trapezium, trapezoid, capitate, hamate.

• Metacarpals: 5 bones, numbered I-V from lateral (thumb) to medial (little finger).

• Phalanges: 14 bones per hand. The thumb (pollex) has 2 phalanges (proximal and distal); each finger has 3 (proximal, middle, distal).

Pelvic Girdle and Pelvis

Structure and Articulations

The pelvic girdle consists of two hip (coxal) bones, each formed by the fusion of three bones: ilium, ischium, and pubis. The pelvis includes the two coxal bones, sacrum, and coccyx.

• Ilium: Superior, largest part. Iliac crest is the superior margin; anterior superior iliac spine is a muscle attachment point.

• Ischium: Posteroinferior region. Ischial tuberosity bears body weight when sitting.

• Pubis: Anterior region. Pubic symphysis is the joint uniting left and right hip bones via fibrocartilage.

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

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

• Sacroiliac Joint: Auricular surface of ilium articulates with sacrum, attaching pelvic girdle to axial skeleton.

Bones of the Lower Limb

Regions and Major Bones

Each lower limb contains 30 bones, divided into the thigh, leg, and foot.

• Thigh: Contains the femur.

• Leg: Contains the tibia and fibula.

• Foot: Contains tarsal bones (ankle), metatarsals, and phalanges (toes).

Femur

• Head: Proximal, rounded end 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 End: Medial and lateral condyles articulate with the tibia to form the knee joint.

Patella (Kneecap)

• Large sesamoid bone embedded in the quadriceps tendon.

• Quadriceps tendon attaches to superior and anterior surfaces; patellar ligament attaches to inferior and anterior surfaces.

Tibia and Fibula

• Tibia: Larger, medial bone of the leg. Tibial tuberosity is the attachment for the patellar ligament. Medial malleolus forms part of the ankle joint with the talus.

• Fibula: Slender, lateral bone. Does not bear weight; serves as a muscle attachment site. Head articulates with the lateral tibial condyle (proximal tibiofibular joint). Lateral malleolus forms part of the ankle joint with the talus.

Tarsus (Ankle), Foot, and Toes

• Tarsus: 7 tarsal bones: talus, calcaneus, cuboid, navicular, medial cuneiform, intermediate cuneiform, lateral cuneiform.

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

• Calcaneus: Largest tarsal bone; attachment for the Achilles tendon.

• Metatarsals: 5 bones, numbered I-V from medial (great toe) to lateral (little toe).

• Phalanges: 14 bones per foot. The hallux (great toe) has 2 phalanges (proximal and distal); other toes have 3 (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 locomotion and manipulation of the environment.

• Joints formed by these bones allow for a wide range of movements, including flexion, extension, abduction, adduction, and rotation.