The Skeletal System

Overview of the Skeleton

The human skeleton is a complex framework of approximately 206 bones and associated cartilages. It provides structural support, protects vital organs, and enables movement through its articulations and connections with muscles.

• Axial Skeleton: Forms the longitudinal axis of the body and is primarily structured for protection. It includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Structured for motion, it consists of the bones of the girdles and the upper and lower limbs.

• Major Divisions:

  • Skull: 22 bones (8 cranial, 14 facial)

  • Vertebral Column: 33 vertebrae (cervical, thoracic, lumbar, sacrum, coccyx)

  • Thoracic Cage: 12 pairs of ribs, sternum, part of vertebral column

  • Pectoral Girdle: Clavicle and scapula

  • Pelvic Girdle: Two pelvic bones and the sacrum

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

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

Bone Markings

Bones display various surface features known as bone markings, which serve as sites for muscle, ligament, and tendon attachment, as well as passages for nerves and blood vessels.

• Depressions: Allow blood vessels and nerves to travel along a bone or provide a place for articulation.

• Openings: Permit access for delicate structures to pass through bones.

• Projections: Sites for ligament and tendon attachment or articulation with other bones.

Skull Anatomy

Structure and Divisions of the Skull

The skull is the most complex structure in the skeleton, composed of cranial and facial bones. In adults, all skull bones except the mandible are united by immovable joints called sutures.

• Cranial Bones (8): Frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid

• Facial Bones (14): Maxilla (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal concha (2), vomer, mandible

Major Cavities of the Skull

The skull contains several important cavities that house and protect sensory organs and the brain.

• Cranial Cavity: Encloses the brain, divided into the cranial vault (calvaria) and cranial base. The base contains the anterior, middle, and posterior cranial fossae, which support the brain.

• Orbits: House the eyeballs and associated structures.

• Nasal Cavity: Contains sensory receptors for smell and forms the first part of the respiratory tract.

• Oral Cavity: Surrounds the teeth and tongue, forming the entry to the digestive tract.

• Paranasal Sinuses: Air-filled spaces that lighten the skull and enhance voice resonance.

Detailed Anatomy of Skull Bones

Each cranial and facial bone has unique features and articulations. The following tables summarize the main bones and their characteristics.

Views of the Skull

Understanding the skull requires examining it from multiple perspectives: anterior, lateral, posterior, superior, and inferior views. Each view highlights different anatomical features and bone articulations.

Special Cavities and Sinuses

• Orbit: Formed by seven bones (frontal, maxilla, zygomatic, sphenoid, ethmoid, lacrimal, palatine) and houses the eyeball and associated structures.

• Nasal Cavity: Formed by several bones and lined with mucous membranes. The nasal septum divides the cavity into two fossae. Deviated septum can affect breathing.

• Paranasal Sinuses: Found within the frontal, sphenoid, ethmoid, and maxillary bones. They lighten the skull and enhance voice resonance. Infections can cause sinusitis.

• Oral Cavity: Houses the teeth, tongue, and some salivary glands. The roof is formed by the hard palate, the walls by the maxillae and mandible, and the floor/posterior wall by soft tissues.

Fetal Skull and Fontanels

The fetal skull contains membranous areas called fontanels, which allow for flexibility during birth and accommodate brain growth. These areas ossify by 18–24 months of age.

• Anterior Fontanel: Between frontal and parietal bones

• Posterior Fontanel: Between parietal and occipital bones

• Sphenoid and Mastoid Fontanels: At the sides of the skull

Forensic Skull Anatomy

Skull features can be used to estimate sex, age, and sometimes ethnic heritage in forensic investigations.

• Sex Differences: Male skulls have a sloped forehead, prominent supraorbital ridge, 90-degree mandibular angle, and larger mastoid process. Female skulls have a straight forehead, less prominent features, and a wider mandibular angle.

• Age Estimation: Based on suture closure and dental development.

Hyoid Bone

The hyoid bone is a small, C-shaped bone in the neck that does not articulate with any other bone. It serves as an attachment point for muscles involved in swallowing and speech.

The Vertebral Column

Structure and Curvatures

The vertebral column, or spine, consists of 33 vertebrae divided into cervical, thoracic, lumbar, sacral, and coccygeal regions. It supports the body, protects the spinal cord, and provides attachment points for ribs and muscles.

• Cervical (7): Neck region

• Thoracic (12): Articulate with ribs

• Lumbar (5): Lower back

• Sacrum (5 fused): Articulates with pelvic bones

• Coccyx (3–5 fused): Tailbone

Spinal curvatures include primary (thoracic and sacral, present at birth) and secondary (cervical and lumbar, develop after birth) curves.

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature (C or S shaped); may be congenital, neuromuscular, or idiopathic.

• Lordosis: Exaggerated cervical and lumbar curvatures; also called swayback.

• Kyphosis: Exaggerated thoracic curvature; also called humpback.

Structure of a Typical Vertebra

All vertebrae share common features:

• Body (Centrum): Weight-bearing region

• Vertebral Foramen: Passage for the spinal cord

• Pedicles and Laminae: Form the vertebral arch

• Articular Processes: Form joints with adjacent vertebrae

• Transverse and Spinous Processes: Sites for muscle attachment

Regional Differences in Vertebrae

Study Boost: Mnemonics

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

• Virgil Is Now Making My Pet Zebra Laugh: Vomer, Inferior nasal conchae, Nasal, Mandible, Maxilla, Palatine, Zygomatic, Lacrimal (facial bones)

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

Additional info: For further study, refer to diagrams and models to visualize the three-dimensional relationships of bones and their markings.