Bones of the Skeletal System

Why This Matters

The skeletal system isn't just a collection of 206 bones to memorize—it's a framework that reveals how form follows function in the human body. You're being tested on your ability to explain why bones have specific shapes, how they articulate with each other, and what functional roles they play in protection, support, and movement. Understanding the skeletal system means recognizing patterns: long bones for leverage, flat bones for protection, irregular bones for specialized functions.

When you encounter exam questions about bones, you'll need to connect structure to function and location to purpose. Think about the difference between weight-bearing bones and stabilizing bones, or why certain bones are fused while others remain articulated. Don't just memorize names—know what concept each bone illustrates, whether that's leverage mechanics, organ protection, joint mobility, or load distribution. This conceptual approach will serve you well on both multiple-choice and free-response questions.

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Axial Skeleton: Protection of Vital Structures

The axial skeleton forms the central axis of the body and prioritizes protection over mobility. These bones create bony enclosures around the most critical organs—brain, spinal cord, heart, and lungs.

Skull (Cranium)

• 22 bones total—8 cranial bones form the protective vault around the brain, while 14 facial bones create the structure for sensory organs and jaw function
• Sutures connect cranial bones with fibrous joints that allow slight movement during birth but fuse in adulthood for maximum protection
• Paranasal sinuses reduce skull weight and provide resonance chambers for voice production

Vertebrae

• 33 vertebrae divided into five regions—7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal
• Vertebral foramen creates the vertebral canal that houses and protects the spinal cord throughout its length
• Regional specialization reflects function: cervical vertebrae allow head rotation, thoracic articulate with ribs, lumbar bear the most weight

Sternum

• Three fused parts—manubrium (superior), body (middle), and xiphoid process (inferior, cartilaginous until middle age)
• Sternal angle marks the junction between manubrium and body, serving as a clinical landmark at the level of the second rib
• Central attachment point for ribs via costal cartilage, completing the anterior thoracic cage

Ribs

• 12 pairs classified by attachment—true ribs (1-7) attach directly to sternum, false ribs (8-10) attach via shared cartilage, floating ribs (11-12) have no anterior attachment
• Costal cartilage provides flexibility to the thoracic cage during breathing while maintaining structural integrity
• Intercostal spaces between ribs house muscles essential for respiration mechanics

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Appendicular Skeleton: The Pectoral Girdle and Upper Limb

The pectoral girdle connects the upper limb to the axial skeleton with an emphasis on mobility over stability. The shoulder sacrifices bony reinforcement for an exceptional range of motion, relying instead on muscles and ligaments.

Clavicle (Collarbone)

• Strut function—holds the shoulder joint away from the thorax to maximize arm mobility and range of motion
• Most frequently fractured bone due to its subcutaneous position and role in transmitting forces from falls on outstretched hands
• Only bony attachment between the upper limb and axial skeleton, articulating with the sternum medially and scapula laterally

Scapula (Shoulder Blade)

• Flat, triangular bone that "floats" on the posterior thorax, held in place by muscles rather than direct skeletal articulation
• Glenoid cavity forms the shallow socket of the shoulder joint—its small size allows mobility but reduces stability
• Multiple processes (acromion, coracoid, spine) serve as attachment sites for rotator cuff and other shoulder muscles

Humerus

• Long bone of the upper arm with a spherical head that articulates with the scapula's glenoid cavity
• Distal condyles (trochlea and capitulum) articulate with the ulna and radius to form the elbow joint
• Deltoid tuberosity provides attachment for the deltoid muscle, essential for arm abduction

Radius

• Lateral forearm bone (thumb side) that rotates around the ulna during pronation and supination
• Radial head articulates with the capitulum of the humerus and the radial notch of the ulna
• Distal end is wider and forms the major articulation with carpal bones at the wrist

Ulna

• Medial forearm bone (pinky side) that forms the primary hinge joint at the elbow
• Olecranon process forms the bony point of the elbow and fits into the olecranon fossa of the humerus during extension
• Trochlear notch wraps around the trochlea of the humerus, creating a stable hinge for flexion and extension

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The Hand: Precision and Dexterity

The bones of the hand are arranged to maximize both grip strength and fine motor control. The progressive decrease in bone size from wrist to fingertips allows for increasingly precise movements.

Carpals (Wrist Bones)

• 8 small bones arranged in two rows—proximal row (scaphoid, lunate, triquetrum, pisiform) and distal row (trapezium, trapezoid, capitate, hamate)
• Scaphoid is the most commonly fractured carpal bone due to its position spanning both rows
• Gliding joints between carpals allow the wrist's complex movements including flexion, extension, and circumduction

Metacarpals

• 5 long bones numbered I-V from thumb to pinky, forming the palm of the hand
• Metacarpal heads form the knuckles and articulate with the proximal phalanges
• First metacarpal (thumb) has a unique saddle joint with the trapezium, enabling opposition

Phalanges (Fingers)

• 14 bones per hand—2 in the thumb (proximal and distal) and 3 in each finger (proximal, middle, distal)
• Hinge joints between phalanges allow flexion and extension essential for grip
• Distal phalanges support the fingernails and contain dense sensory receptors for fine touch discrimination

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Appendicular Skeleton: The Pelvic Girdle and Lower Limb

The pelvic girdle and lower limb prioritize stability and weight-bearing over mobility. These bones are larger, denser, and more firmly attached to the axial skeleton than their upper limb counterparts.

Pelvis (Os Coxae)

• Three fused bones—ilium (superior), ischium (posterior-inferior), and pubis (anterior-inferior)—that fuse at the acetabulum
• Acetabulum forms a deep socket for the femoral head, providing much greater stability than the shoulder's glenoid cavity
• Sexual dimorphism—female pelvis is wider and shallower with a larger pelvic inlet to accommodate childbirth

Sacrum

• 5 fused vertebrae forming a triangular bone that wedges between the two hip bones
• Sacroiliac joints connect the sacrum to the ilium, transferring weight from the spine to the lower limbs
• Sacral foramina allow passage of sacral spinal nerves to the lower body

Femur

• Longest and strongest bone in the body, comprising approximately one-quarter of total body height
• Angle of inclination (~125°) between the neck and shaft optimizes weight transfer while allowing leg adduction
• Condyles at the distal end articulate with the tibia to form the knee joint; the linea aspera provides posterior muscle attachment

Patella (Kneecap)

• Largest sesamoid bone in the body, embedded within the quadriceps tendon
• Increases mechanical advantage of the quadriceps by increasing the angle of pull during knee extension
• Articulates with the femur at the patellofemoral joint on the anterior surface of the femoral condyles

Tibia

• Weight-bearing bone of the lower leg, receiving forces transmitted from the femur at the knee
• Medial and lateral condyles articulate with the femoral condyles; the tibial tuberosity anchors the patellar ligament
• Medial malleolus forms the bony prominence on the inner ankle

Fibula

• Non-weight-bearing bone that runs parallel to the tibia on the lateral side
• Lateral malleolus forms the outer ankle prominence and provides lateral ankle stability
• Muscle attachment site for muscles of the lateral leg compartment, including the fibularis muscles

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The Foot: Weight-Bearing and Locomotion

The foot bones form arches that distribute weight and provide spring during walking. The arrangement balances stability for standing with flexibility for propulsion.

Tarsals (Ankle Bones)

• 7 bones including the talus (articulates with tibia/fibula) and calcaneus (heel bone, largest tarsal)
• Talus transmits body weight from the tibia to the foot; it has no muscle attachments
• Calcaneus serves as the attachment point for the Achilles tendon and forms the posterior pillar of the longitudinal arch

Metatarsals

• 5 long bones numbered I-V from big toe to little toe, forming the metatarsal arch
• First metatarsal is the shortest and thickest, bearing significant weight during push-off in gait
• Metatarsal heads form the "ball of the foot" and bear weight during the toe-off phase of walking

Phalanges (Toes)

• 14 bones per foot—2 in the great toe (hallux) and 3 in each of the lateral four toes
• Great toe lacks the middle phalanx, similar to the thumb, but functions primarily for balance and propulsion rather than grasping
• Shorter than finger phalanges because toes prioritize stability and push-off over dexterity

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Quick Reference Table

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Self-Check Questions

1. Which two bones form the pectoral girdle, and how does their attachment to the axial skeleton differ from the pelvic girdle's attachment?

2. Compare the radius and ulna: which bone dominates at the elbow joint, and which dominates at the wrist? What functional advantage does this arrangement provide?

3. Both the acetabulum and glenoid cavity are joint sockets—what structural difference between them explains why the hip is more stable but less mobile than the shoulder?

4. If an FRQ asks you to explain how bone structure reflects function, which three bones would you choose to illustrate the relationship between shape and weight-bearing capacity?

5. The hand has 8 carpals while the foot has 7 tarsals. How does this difference relate to the functional priorities of each region (manipulation vs. weight-bearing)?