The Skeletal System

Overview and Structural Divisions

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. It consists of approximately 206 bones and associated cartilages, organized into two main divisions: the axial and appendicular skeletons.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage. Its primary function is protection of vital organs.

• Appendicular Skeleton: Comprises the bones of the girdles and limbs, structured for motion and manipulation.

Major Components of the Skeleton

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

• Vertebral Column: 33 vertebrae, including cervical, thoracic, lumbar, sacral, and coccygeal regions.

• Thoracic Cage: 12 pairs of ribs, sternum, and part of the vertebral column, protecting thoracic organs.

• Pectoral Girdle: Clavicle and scapula, supporting the upper limb.

• Pelvic Girdle: Two pelvic bones and sacrum, supporting the lower limb.

• Upper and Lower Limbs: Arm (humerus), forearm (radius and ulna), thigh (femur), leg (tibia and fibula), hands and feet (metacarpals/metatarsals and phalanges).

Bone Markings

Bone markings are surface features that serve as sites for muscle, ligament, and tendon attachment, or as passages for nerves and blood vessels.

• Depressions: Allow passage of vessels/nerves or articulation between bones.

• Openings: Permit access and protect delicate structures.

• Projections: Sites for attachment or articulation.

Skull Structure

Cranial and Facial Bones

The skull is composed of cranial bones (protecting the brain) and facial bones (forming the face).

• Cranial Bones: Frontal, parietal, sphenoid, ethmoid, temporal, occipital.

• Facial Bones: Nasal, lacrimal, palatine, zygomatic, inferior nasal concha, vomer, maxilla, mandible.

Skull Cavities and Sinuses

The skull contains several cavities:

• Cranial cavity: Houses the brain.

• Orbits: Contain the eyes.

• Nasal cavity: First part of the respiratory tract.

• Oral cavity: Houses teeth and tongue.

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

Key Cranial Bones and Their Features

• Frontal Bone: Forms the forehead and superior part of the orbit.

• Parietal Bones: Form the superior and lateral walls of the cranial vault.

• Occipital Bone: Forms the posterior part of the skull and base.

• Temporal Bones: Form the lateral walls; contain structures for hearing.

• Sphenoid Bone: Central bone with 'bat-like' shape; forms part of the cranial base.

• Ethmoid Bone: Located deep in the anterior skull; forms part of the nasal cavity and orbit.

Key Facial Bones and Their Features

• Nasal, Lacrimal, Zygomatic Bones: Form the bridge of the nose, medial orbit, and cheekbones.

• Palatine Bones: Form part of the hard palate.

• Mandible: Lower jaw; only movable skull bone.

• Maxillae: Upper jaw; forms part of the orbit, nasal cavity, and hard palate.

• Vomer: Forms the inferior part of the nasal septum.

Skull Views and Sutures

• Anterior View: Shows frontal, nasal, maxilla, mandible, and other facial bones.

• Lateral View: Shows temporal, sphenoid, zygomatic, and mandible.

• Posterior/Superior/Inferior Views: Highlight occipital, parietal, and sutures.

Internal and Disarticulated Skull

• Midsagittal Section: Reveals internal anatomy, including nasal septum and sinuses.

• Disarticulated Skull: Demonstrates how individual bones fit together.

Orbit and Nasal Cavity

• Orbit: Formed by seven bones; houses the eye and associated structures.

• Nasal Cavity: Formed by several bones; lined with mucous membranes; divided by the nasal septum.

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within the frontal, sphenoid, ethmoid, and maxillary bones. They lighten the skull and enhance voice resonance.

Fetal Skull and Fontanels

The fetal skull contains membranous areas called fontanels, which provide flexibility during birth and ossify by 18–24 months.

• Anterior Fontanel: Between frontal and parietal bones.

• Posterior Fontanel: Between parietal and occipital bones.

• Sphenoid and Mastoid Fontanels: Located at the temples and junctions of cranial bones.

Forensic Skull Anatomy

Skull features can help determine sex and age.

• Sex Differences: Male skulls have sloped foreheads, prominent supraorbital ridges, 90-degree mandibular angles, and larger mastoid processes.

• Age and Ethnicity: Size, suture appearance, and teeth provide clues.

Hyoid Bone

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

Vertebral Column

Structure and Curvatures

The vertebral column consists of 33 vertebrae, divided into cervical, thoracic, lumbar, sacral, and coccygeal regions.

• Primary Curvatures: Thoracic and sacral, present during fetal development.

• Secondary Curvatures: Cervical and lumbar, develop after birth.

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature, C or S shaped.

• Lordosis: Exaggerated cervical/lumbar curvature (swayback).

• Kyphosis: Exaggerated thoracic curvature (humpback).

Structure of Vertebrae

• Body (Centrum): Weight-bearing structure.

• Vertebral Foramen: Passage for spinal cord.

• Pedicles and Laminae: Form the vertebral arch.

• Articular, Transverse, and Spinous Processes: Sites for muscle attachment and articulation.

Regional Vertebrae

• Cervical Vertebrae: Smallest, with transverse foramina; C1 (Atlas) and C2 (Axis) are specialized.

• Thoracic Vertebrae: Larger, heart-shaped bodies, articulate with ribs.

• Lumbar Vertebrae: Largest, kidney-shaped bodies, thick spinous processes.

Sacrum and Coccyx

• Sacrum: Five fused vertebrae, forms posterior pelvic wall.

• Coccyx: Four fused vertebrae, forms the tailbone.

Thoracic Cage

Structure and Function

The thoracic cage consists of the sternum, 12 pairs of ribs, and thoracic vertebrae. It protects the heart, lungs, and major vessels.

• Sternum: Manubrium, body, xiphoid process.

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

Pectoral Girdle and Upper Limb

Pectoral Girdle

• Clavicle: S-shaped bone, connects sternum and scapula.

• Scapula: Triangular bone, forms shoulder joint.

Humerus, Radius, and Ulna

• Humerus: Only bone of the arm, forms shoulder and elbow joints.

• Radius and Ulna: Forearm bones, articulate at radioulnar joints.

Pelvic Girdle and Lower Limb

Pelvis and Pelvic Bones

• Pelvic Girdle: Sacrum and two pelvic bones (ilium, ischium, pubis).

• Pelvis: Greater (false) and lesser (true) pelvis, pelvic inlet and outlet.

Femur, Patella, Tibia, and Fibula

• Femur: Largest bone, forms hip and knee joints.

• Patella: Sesamoid bone in quadriceps tendon.

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

• Fibula: Lateral, non-weight-bearing bone.

Ankle and Foot

• Tarsals: Seven short bones, including talus and calcaneus.

• Metatarsals and Phalanges: Form the foot and toes.

• Arches: Medial, lateral, and transverse arches support the foot.

Study Boost: Mnemonics

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

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

• Breakfast at 7, Lunch at 12, Dinner at 5: Vertebrae numbers (7 cervical, 12 thoracic, 5 lumbar).

Additional info:

• Tables and images were recreated and expanded for clarity and completeness.

• Some anatomical context was inferred to ensure self-contained study notes.