Chapter 7: The Skeletal System – Human Anatomy & Physiology

The Skeletal System

Structure of the Skeleton and Skeletal Cartilages

The human skeleton consists of approximately 206 bones and associated cartilages, forming the framework for the body and providing protection, support, and movement. It is divided into axial and appendicular components.

• Skull: The most complex structure, with 22 bones (8 cranial, 14 facial).

• Vertebral Column: 33 vertebrae; top 24 encase the spinal cord, sacrum and coccyx are fused.

• Thoracic Cage: 12 pairs of ribs, sternum, part of vertebral column; protects thoracic organs.

• Pectoral Girdle: Clavicle and scapula; supports upper limb.

• Upper Limb: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Two pelvic bones (ilium, ischium, pubis) and sacrum; supports lower limb.

• Lower Limb: Femur, tibia, fibula, tarsals, metatarsals, phalanges.

Structural Divisions:

• Axial Skeleton: Skull, vertebral column, thoracic cage; protection.

• Appendicular Skeleton: Girdles, upper and lower limbs; motion.

Bone Markings

Bones display surface features called bone markings, which serve as sites for muscle attachment, passageways for nerves and blood vessels, and articulation points.

• Depressions: Allow passage of vessels/nerves or articulation (e.g., facet, fossa, groove).

• Openings: Enclose delicate structures and allow passage (e.g., canal, foramen).

• Projections: Sites for ligament/tendon attachment or articulation (e.g., condyle, epicondyle, process, line).

Bone Marking	Description	Example
Facet	Shallow concave surface	Articular facet for vertebrae
Fossa	Indentation in bone	Humerus: distal portion with olecranon fossa
Groove	Long indentation	Rib: costal groove

Skull Structure

Overview of Skull Structure

The skull is composed of cranial and facial bones, united by immovable joints called sutures (except the mandible). It houses the brain and sensory organs.

• Cranial Bones: Frontal, occipital, ethmoid, sphenoid (single); temporal, parietal (paired).

• Facial Bones: Mandible, vomer (single); maxilla, zygomatic, nasal, lacrimal, palatine, inferior nasal concha (paired).

Cranial Cavity and Other Cavities

The cranial cavity surrounds the brain, with the cranial vault (calvaria) and cranial base. Other cavities include orbits (eyes), nasal cavity (smell), oral cavity (teeth/tongue), and paranasal sinuses (voice resonance).

• Anterior, Middle, Posterior Cranial Fossae: Support the brain.

• Paranasal Sinuses: Air-filled spaces in frontal, sphenoid, ethmoid, maxillary bones.

Skull Bone Tables

Tables classify cranial and facial bones, describing their features and functions.

Skull Views

Multiple views of the skull (anterior, lateral, posterior, superior, inferior, medial) help identify bone locations and relationships.

Disarticulated Skull

Viewing a disarticulated skull helps understand how individual bones fit together, like a 3D puzzle.

Cavities of the Skull

The orbit, nasal cavity, paranasal sinuses, and oral cavity are formed by multiple bones and house sensory organs.

• Orbit: Formed by 7 bones; houses the eyeball and lacrimal gland.

• Nasal Cavity: First part of the respiratory tract; divided by the nasal septum.

• Paranasal Sinuses: Lighten the skull, enhance voice resonance; susceptible to sinusitis.

• Oral Cavity: Houses teeth, tongue, salivary glands; first part of digestive tract.

Fetal Skull and Forensic Anatomy

Fontanels (soft spots) in the fetal skull allow flexibility during birth and ossify by 18–24 months. Skull features can help determine sex, age, and ethnic heritage in forensic analysis.

• Fontanels: Anterior, posterior, sphenoid, mastoid.

• Sex Differences: Forehead slope, supraorbital ridge, mandibular angle, mastoid process size.

Hyoid Bone

The hyoid bone is a small, C-shaped bone in the neck, suspended by muscles and ligaments. It serves as an attachment point for muscles involved in swallowing and speech.

Vertebral Column

Overview of the Vertebral Column

The vertebral column consists of 33 vertebrae, grouped into cervical, thoracic, lumbar, sacral, and coccygeal regions. It supports the body and protects the spinal cord.

• Cervical: 7 vertebrae (neck)

• Thoracic: 12 vertebrae (articulate with ribs)

• Lumbar: 5 vertebrae (lower back)

• Sacral: 5 fused (sacrum)

• Coccygeal: 3–5 fused (coccyx)

Spinal Curvatures: Primary (thoracic, sacral) and secondary (cervical, lumbar).

Abnormal Spinal Curvatures

Abnormal curvatures include scoliosis (lateral), lordosis (exaggerated cervical/lumbar), and kyphosis (exaggerated thoracic). These may require medical intervention depending on severity.

Structure of Vertebrae

Vertebrae share common features: body (centrum), vertebral foramen, pedicles, laminae, vertebral arch, articular processes, transverse and spinous processes.

• Body: Weight-bearing, separated by intervertebral discs.

• Vertebral Foramen: Passage for spinal cord.

• Processes: Sites for muscle attachment and articulation.

Cervical Vertebrae

Cervical vertebrae are the smallest, with transverse foramina for arteries/veins. The atlas (C1) and axis (C2) are specialized for head movement.

• Atlas (C1): No body/spinous process; articulates with occipital condyles.

• Axis (C2): Dens (odontoid process) projects into atlas.

Thoracic Vertebrae

Thoracic vertebrae are larger, with heart-shaped bodies and costal facets for rib articulation.

Lumbar Vertebrae

Lumbar vertebrae are the largest and heaviest, with kidney-shaped bodies and thick processes.

Comparison Table: Cervical, Thoracic, Lumbar Vertebrae

Comparison tables highlight differences in size, shape, and features among vertebrae types.

Vertebral Compression Fractures

Compression fractures reduce vertebral height, often due to trauma or osteoporosis. Types include wedge and burst fractures, with treatments ranging from rest to surgery.

Sacrum and Coccyx

The sacrum forms the posterior pelvic wall, fusing by age 20–25. The coccyx, composed of 4 fused vertebrae, forms the tailbone.

Intervertebral Discs

Intervertebral discs are pads between vertebrae, composed of a nucleus pulposus (shock absorber) and anulus fibrosus (fibrocartilage ring). They allow flexibility and absorb impact.

Herniated Disc

A herniated (slipped) disc occurs when the anulus fibrosus tears, allowing the nucleus pulposus to protrude and compress nerves, causing pain and weakness.

Thoracic Cage

Structure of the Thoracic Cage

The thoracic cage includes the sternum, 12 pairs of ribs, and thoracic vertebrae, protecting vital organs and supporting respiration.

• Sternum: Manubrium, body, xiphoid process.

• Ribs: True (1–7), false (8–12), floating (11–12).

Structure of a Typical Rib

Ribs have a head, neck, tubercle, angle, shaft, and costal groove. They articulate with vertebrae and costal cartilage.

Pectoral Girdle and Upper Limb

Pectoral Girdle

The pectoral girdle consists of the clavicle and scapula, supporting the upper limb and providing attachment points for muscles.

Humerus

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

Forearm: Radius and Ulna

The radius and ulna are held together by an interosseous membrane, forming the forearm and articulating at the elbow and wrist.

Carpals, Metacarpals, Phalanges

The wrist (carpals) consists of eight bones, the hand (metacarpals) of five, and the fingers (phalanges) of fourteen.

Pelvic Girdle and Lower Limb

Pelvis and Pelvic Bones

The pelvic girdle consists of the sacrum and two pelvic bones (ilium, ischium, pubis), forming the pelvis and supporting the lower limb.

Femur and Patella

The femur is the largest bone, articulating with the pelvis at the hip and the tibia at the knee. The patella is a sesamoid bone within the quadriceps tendon.

Tibia and Fibula

The tibia and fibula form the leg, held together by an interosseous membrane and forming the ankle joint.

Tarsals, Metatarsals, Phalanges

The ankle (tarsals) consists of seven bones, the foot (metatarsals) of five, and the toes (phalanges) of fourteen. The foot has three arches for support.

Study Boost: Mnemonics and Visual Analogies

Memory Devices

• PEST of 6: Parietal, Ethmoid, Sphenoid, Temporal, Occipital, Frontal (cranial bones)

• Breakfast at 7, Lunch at 12, Dinner at 5: 7 cervical, 12 thoracic, 5 lumbar vertebrae

• Some Lunchers Try Peppers That They Can’t Handle: Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate (carpal bones)

• This College Needs Me In Lab Classes: Talus, Calcaneus, Navicular, Medial cuneiform, Intermediate cuneiform, Lateral cuneiform, Cuboid (tarsal bones)

Visual Analogies: Sphenoid bone looks like a bat; ethmoid bone like an iceberg; thoracic vertebra like a giraffe; lumbar vertebra like a moose.