Bones Part 2: The Appendicular Skeleton – Structure, Function, and Variation

Overview of the Appendicular Skeleton

Introduction to the Appendicular Skeleton

The appendicular skeleton consists of 126 bones that enable movement and manipulation of objects. It includes all bones outside the axial skeleton, specifically the limbs and their supportive girdles.

• Limbs: The bones of the arms and legs.

• Girdles: Structures that attach the limbs to the axial skeleton.

Key Functions:

• Facilitates locomotion and manipulation.

• Provides attachment points for muscles.

Girdles of the Appendicular Skeleton

Pectoral Girdle

The pectoral girdle (shoulder girdle) connects the upper limbs to the trunk and provides a base for arm movement. It consists of two clavicles and two scapulae.

• Clavicle: Articulates medially with the manubrium (sternoclavicular joint) and laterally with the scapula (acromioclavicular joint).

• Scapula: Does not join the axial skeleton directly, allowing for greater mobility.

Features:

• Light structure, high mobility.

• Shallow glenoid cavity for flexibility, but less stability.

Example: The primate shoulder is adapted for brachiation (arm swinging), as seen in gibbons and siamangs.

Pelvic Girdle

The pelvic girdle attaches the lower limbs to the trunk and supports visceral organs. It is composed of two coxal (hip) bones, which connect at the pubic symphysis and to the sacrum.

• Acetabulum: Deep socket that holds the head of the femur.

• Provides stability for weight-bearing and movement.

Bones of the Upper Limbs

Humerus

The humerus is the only bone of the arm (brachium), articulating with the scapula at the shoulder and with the radius and ulna at the elbow.

• Head: Rounded, fits into the glenoid cavity.

• Anatomical neck: Margin of joint capsule.

• Surgical neck: Common fracture site.

• Greater and lesser tubercles: Muscle attachment points.

• Distal features: Medial/lateral epicondyles, condyle (trochlea and capitulum), coronoid and olecranon fossae.

Ulna and Radius

The forearm (antebrachium) consists of two long bones: the ulna (medial) and the radius (lateral).

• Ulna: Forms the elbow (olecranon process), articulates with the humerus and radius.

• Radius: Disk-shaped head, articulates with the ulna and wrist, radial tuberosity for biceps attachment.

• Interosseous membrane: Connects the shafts of the ulna and radius.

Bones of the Wrist and Hand

The wrist and hand are composed of carpal bones, metacarpals, and phalanges.

• Carpus (wrist): 8 carpal bones (4 proximal, 4 distal) allow wrist flexibility.

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

• Phalanges (fingers): 14 bones; thumb (pollex) has 2, other fingers have 3 each.

Mnemonic for carpal bones: "Some Lovers Try Positions That They Can't Handle" (Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate).

Bones of the Lower Limbs

Pelvic Girdle and Hip Bones

The pelvic girdle consists of two coxal bones, each formed by the fusion of the ilium, ischium, and pubis.

• Ilium: Articulates with the sacrum, features the iliac crest and fossa.

• Ischium: Features the ischial spine and tuberosity ("sit bones").

• Pubis: Forms the pubic symphysis, obturator foramen, and pectineal line.

Femur

The femur is the thigh bone, the longest and strongest bone in the body.

• Head: Articulates with the acetabulum.

• Neck: Connects head to shaft.

• Greater and lesser trochanters: Muscle attachment sites.

• Distal features: Medial/lateral epicondyles and condyles, patellar surface.

Patella

The patella (kneecap) is a sesamoid bone formed within the tendon of the quadriceps femoris.

• Base attaches quadriceps femoris.

• Apex attaches patellar ligament.

Tibia and Fibula

The leg is composed of the tibia (medial, weight-bearing) and fibula (lateral, muscle attachment).

• Tibia: Articulates with femur (knee) and talus (ankle), features medial/lateral condyles, tibial tuberosity, medial malleolus.

• Fibula: Does not contribute to knee joint, features head, lateral malleolus, interosseous membrane binds to tibia.

Bones of the Foot

The foot is composed of tarsal bones, metatarsals, and phalanges, supporting body weight and enabling movement.

• Tarsus: 7 tarsal bones, including the calcaneus (heel bone) and talus (ankle bone).

• Metatarsals: 5 long bones, numbered I-V from medial to lateral.

• Phalanges: 14 bones; big toe (hallux) has 2, other toes have 3 each.

Arches of the foot: Longitudinal (medial/lateral) and transverse arches provide support and flexibility.

Comparing Male and Female Pelvises

Sexual Dimorphism in the Pelvis

The pelvis shows distinct differences between males and females, reflecting adaptations for childbirth in females.

Feature	Male Pelvis	Female Pelvis
General Structure	Heavier, rougher	Smoother, lighter
Pelvic Inlet	Narrow, heart-shaped	Wide, circular
Pubic Arch	Narrow (<90°)	Broad (>100°)
Sacrum & Coccyx	Curved, projects forward	Less curved, projects downward
Obturator Foramen	Oval	Triangular
Pelvic Outlet	Smaller	Larger

Additional info: The female pelvis is adapted for childbirth, with a wider pelvic inlet and outlet, and less curvature of the sacrum and coccyx.

Age-Related Changes and Variation in the Skeleton

Variation and Growth

Studying the skeleton reveals key characteristics such as muscle strength, medical history, sex, age, and body size. The appendicular skeleton changes throughout life, affecting height and body proportions.

• At birth, the head and trunk are proportionally larger than the limbs.

• Lower limbs grow faster than the trunk, reaching an upper/lower body ratio of 1:1 by age 10.

• Bone ridges and mass reflect muscle strength and activity.

• Fusion of bones and measurements help determine age and sex.

Additional info: Age-related changes include fusion of growth plates, changes in bone density, and remodeling in response to activity and hormonal changes.