Anatomy & Physiology I: The Appendicular Skeleton – Upper and Lower Extremities

The Appendicular Skeleton

Overview

The appendicular skeleton consists of the bones of the limbs and the girdles that attach them to the axial skeleton. It is essential for movement and manipulation of the environment. The major regions include the pectoral (shoulder) girdle, upper extremity, pelvic (hip) girdle, and lower extremity.

Pectoral (Shoulder) Girdle

Structure and Function

• Pectoral girdle consists of two bones on each side: the anterior clavicle and the posterior scapula.

• These girdles and their associated muscles form the shoulders and attach the upper limbs to the axial skeleton.

• They provide insertion points for many muscles that move the upper limbs.

• High flexibility and mobility, but less stability compared to other joints.

Key Features

• The only attachment point of the pectoral girdles to the axial skeleton is at the sternoclavicular joints.

• The glenoid cavity of the scapula is small, shallow, and poorly reinforced by ligaments, making the shoulder joint flexible but prone to dislocation.

Clavicle

Anatomy and Function

• Clavicles (collarbones) are slender, doubly curved long bones palpable along their length.

• Each clavicle is rounded medially (sternal end) and flattened laterally (acromial end).

• The superior surface is smooth; the inferior surface is ridged and grooved for muscle and ligament attachment.

• Clavicles act as braces, holding the scapulae and arms laterally away from the thorax.

• Transmit forces from the upper limb to the axial skeleton, but resist compression poorly and are prone to fracture.

• Clavicles enlarge and strengthen in response to muscle pull, especially in manual laborers.

Scapula

Structure and Borders

• Scapulae (shoulder blades) are triangular flat bones located on the dorsal thorax between ribs 2 and 7.

• Each scapula has three borders:

  • Superior border: shortest and sharpest.

  • Vertebral (medial) border: parallels the vertebral column.

  • Axillary (lateral) border: meets the armpit and contains the glenoid cavity.

• The glenoid cavity articulates with the humerus, forming the shoulder joint.

Key Landmarks

• Acromion: lateral projection that articulates with the clavicle at the acromioclavicular (AC) joint.

• Coracoid process: anterior projection anchoring the biceps muscle; bounded by the suprascapular notch.

• Spine: prominent ridge on the posterior surface.

• Fossae:

  • Infraspinous fossa: below the spine.

  • Supraspinous fossa: above the spine.

  • Subscapular fossa: anterior surface.

Upper Extremity

Major Bones

• Humerus: largest bone of the upper limb; articulates proximally with the scapula and distally with the radius and ulna.

• Radius and Ulna: forearm bones; radius is lateral (thumb side), ulna is medial (little finger side).

• Carpals: eight wrist bones arranged in two rows.

• Metacarpals: five bones forming the palm.

• Phalanges: fourteen bones forming the fingers.

Humerus – Proximal End

• Head: fits into the glenoid cavity.

• Greater and lesser tubercles: muscle attachment sites.

• Intertubercular sulcus: guides tendon of biceps muscle.

• Surgical neck: common fracture site.

• Deltoid tuberosity: attachment for deltoid muscle.

Forearm: Radius and Ulna

• Olecranon process (ulna): forms the elbow.

• Radial notch (ulna): articulates with the head of the radius.

• Styloid processes: distal projections for ligament attachment.

Carpals, Metacarpals, and Phalanges

• Carpal bones (wrist): arranged in two rows; mnemonic "Some Lovers Try Positions That They Can't Handle" helps remember their order:

  • Scaphoid

  • Lunate

  • Triquetrum

  • Pisiform

  • Trapezium

  • Trapezoid

  • Capitate

  • Hamate

• Metacarpals: numbered I–V from thumb to little finger.

• Phalanges: each finger has three (proximal, middle, distal), except the thumb (two).

Pelvic (Hip) Girdle

Structure and Function

• Formed by two coxal bones (hip bones), each composed of three fused bones: ilium, ischium, and pubis.

• Attaches lower limbs to the axial skeleton and supports pelvic organs.

• Acetabulum: deep socket for articulation with the femur.

Ischium and Pubis

• Ischium: forms the posteroinferior part of the hip bone; features include the ischial spine, tuberosity, and lesser sciatic notch.

• Pubis: forms the anterior portion; features include the pubic crest, pubic symphysis, and obturator foramen.

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

Pelvic Inlet and Outlet

• Pelvic inlet: widest dimension; important for childbirth.

• Pelvic outlet: bounded by ischial tuberosities and pubic arch.

Sexual Dimorphism: Male vs. Female Pelvis

The male and female pelvis differ in structure to accommodate childbirth in females.

Feature	Male Pelvis	Female Pelvis
General Structure	Tilted backward	Tilted forward, modified for childbearing
Bone Thickness	Greater, heavier	Less, lighter
Acetabula	Larger, closer together	Smaller, farther apart
Pubic Angle	More acute (<90°)	Broader (>90°), more rounded
Sacrum	Narrow, longer, more curved	Wider, shorter, less curved
Coccyx	Less movable, curves ventrally	More movable, straighter
Pelvic Inlet	Narrow, heart-shaped	Wider, oval from side to side
Pelvic Outlet	Narrower, ischial tuberosities longer, sharper, point more medially	Wider, ischial tuberosities shorter, farther apart and everted

Sacrum and Coccyx

Sacrum

• Formed by fusion of five sacral vertebrae (S1–S5).

• Features include the sacral canal, sacral promontory, anterior and posterior sacral foramina, and sacral hiatus.

• Articulates with the ilium at the sacroiliac joint.

Coccyx

• Formed by fusion of 3–5 small vertebrae.

• Serves as an attachment site for ligaments and muscles of the pelvic floor.

Lower Extremity

Major Bones

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

• Patella: kneecap, protects the knee joint.

• Tibia: medial, weight-bearing bone of the leg.

• Fibula: lateral, non-weight-bearing bone of the leg.

• Tarsals: seven ankle bones.

• Metatarsals: five bones forming the foot arch.

• Phalanges: fourteen bones forming the toes.

Tibia and Fibula

• Tibia: features include the medial and lateral condyles, tibial tuberosity, anterior border (crest), and medial malleolus.

• Fibula: features include the head and lateral malleolus.

Tarsals, Metatarsals, and Phalanges

• Tarsal bones: mnemonic "Canada's territory nurses make competent independent choices linking concepts constantly" helps remember their order:

  • Calcaneus (heel bone)

  • Talus (ankle bone)

  • Navicular

  • Medial cuneiform

  • Intermediate cuneiform

  • Lateral cuneiform

  • Cuboid

• Metatarsals: numbered I–V from big toe to little toe.

• Phalanges: each toe has three (proximal, middle, distal), except the big toe (two).

Arches of the Foot

• Medial longitudinal arch: highest arch, important for shock absorption.

• Lateral longitudinal arch: lower, provides balance.

• Transverse arch: runs across the foot, helps distribute weight.

Additional info:

• The appendicular skeleton is crucial for locomotion and manipulation, and its structure reflects the functional demands placed on the limbs.

• Sexual dimorphism in the pelvis is a key consideration in forensic anthropology and obstetrics.