Module 7.1 Overview of the Skeletal System

Structure of the Skeletal System and Skeletal Cartilages

The skeletal system consists of about 206 bones and their associated cartilages, which work together to protect and support the body. The skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial skeleton: Forms the body’s longitudinal axis and encloses body cavities that protect underlying organs. It includes:

  • The skull

  • The vertebral column

  • The thoracic cage

• Appendicular skeleton: Consists of the bones of the pectoral girdle, upper limb, pelvic girdle, and lower limb. This group is primarily suited for movement, support, and muscle attachment.

  • The pectoral girdle

  • The upper limb

  • The pelvic girdle

  • The lower limb

Bone Markings

Bones have a number of surface features, including depressions, openings, and projections:

• Depressions: Pathways for blood vessels and nerves or sites for articulations.

• Openings: Enclose delicate structures and allow them to travel through bones.

• Projections: Sites where bones articulate or where ligaments and tendons attach.

Module 7.2 The Skull

Overview of Skull Structure

The skull is composed of 22 bones, divided into two groups: cranial bones and facial bones.

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

• Facial bones (14): Maxilla (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal conchal bones (2), mandible (1), vomer (1).

All bones of the skull (except the mandible) are fused at immovable joints called sutures.

Cavities within the Skull

• Orbits: Cavities that encase the eyeball, lacrimal gland, and associated structures.

• Nasal cavity: Formed by several bones and lined with mucous membranes. Includes:

  • Posterior wall: Sphenoid body and pterygoid processes.

  • Lateral wall: Ethmoid and perpendicular plate of palatine, inferior nasal conchae.

  • Floor: Palatine and maxilla.

  • Nasal septum: Perpendicular plate of ethmoid and vomer, hyaline cartilage.

• Paranasal sinuses: Frontal, sphenoid, ethmoid, and maxillary bones. These sinuses communicate with the nasal cavity and help warm, humidify, and filter air.

• Oral cavity: Contains the tongue, teeth, and most salivary glands; first part of the digestive tract.

Fetal Skull Bones

• Fetal skull bones are not fused; soft spots called fontanels allow flexibility during birth.

• Fontanels and unfused sutures enable the skull to fit through the birth canal.

Module 7.3 The Vertebral Column and Thoracic Cage

Overview of the Vertebral Column

The vertebral column, or spine, is composed of 33 bones called vertebrae. Vertebrae are classified by location and structure:

• 7 cervical vertebrae (neck)

• 12 thoracic vertebrae (articulate with ribs)

• 5 lumbar vertebrae (lower back)

• 5 sacral vertebrae (fused to form sacrum)

• 3-5 coccygeal vertebrae (fused to form coccyx)

Spinal Curvatures

• Cervical curvature: Concave, develops as infant lifts head.

• Thoracic curvature: Convex, present at birth.

• Lumbar curvature: Concave, develops as child begins to walk.

• Sacral curvature: Convex, present at birth.

• Scoliosis: Abnormal lateral curvature.

• Lordosis: Exaggerated cervical/lumbar curvature.

• Kyphosis: Exaggerated thoracic curvature (hunchback).

Structure of Vertebrae

• Body (centrum): Primary weight-bearing surface.

• Vertebral foramen: Large opening for spinal cord.

• Vertebral arch: Formed by pedicles and laminae.

• Transverse processes: Lateral projections for muscle attachment.

• Spinous process: Posterior projection for muscle attachment.

The seven cervical vertebrae are the smallest and have unique features:

• C1: The atlas

• C2: The axis

Thoracic and Lumbar Vertebrae

• Thoracic vertebrae: Articulate with ribs, have costal facets.

• Lumbar vertebrae: Largest, support most weight.

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

• Coccyx: Three to five fused vertebrae, final segment.

Intervertebral Discs

There are 23 intervertebral discs, which are fibrocartilage pads between vertebrae that absorb shock and bind the column together.

• Nucleus pulposus: Soft inner jelly-like substance, shock absorber.

• Anulus fibrosus: Outer ring of fibrocartilage.

Thoracic Cage

• Sternum: Flat bone forming the anterior thoracic cage, consists of:

  • Manubrium

  • Body

  • Xiphoid process

• Ribs: 12 pairs, curved into a C-shape, attach to thoracic vertebrae.

• Intercostal spaces: Spaces between ribs, important for breathing and movement.

• Ribs are grouped into:

  • True ribs

  • False ribs

• Costal margin: Formed by costal cartilage of ribs 7-10.

• Structure of ribs includes:

  • Head

  • Tubercle

  • Shaft (body)

Module 7.4 Bones of the Pectoral Girdle and Upper Limb

Pectoral Girdle

The pectoral girdle consists of the clavicle and scapula, supporting the 30 bones of the upper limb.

• Clavicle (collarbone):

  • Sternal end

  • Acromial end

  • Conoid tubercle

• Scapula:

  • Coracoid process

  • Subscapular fossa

  • Glenoid cavity

  • Spine

  • Acromion at the acromioclavicular joint (AC)

Humerus

The humerus is the largest and strongest bone of the upper limb, articulating with the scapula and forearm bones.

• Greater tubercle

• Lesser tubercle

• Intertubercular sulcus

• Surgical neck

• Deltoid tubercle

• Radial groove

• Medial and lateral epicondyles

• Articulation features:

  • Capitulum

  • Trochlea

  • Lateral radial fossa

  • Medial coronoid

  • Olecranon fossa

Forearm Bones: Radius and Ulna

• Radius:

  • Radial head

  • Radial tuberosity

  • Ulnar notch

  • Radial styloid process

• Ulna:

  • Trochlear notch

  • Olecranon

  • Radial notch

  • Styloid process

Wrist and Hand Bones

• Carpals: Eight short bones arranged in two rows.

  • Proximal row: Scaphoid, lunate, triquetrum, pisiform

  • Distal row: Trapezium, trapezoid, capitate, hamate

• Metacarpals: Five long bones, numbered I-V from lateral to medial.

• Phalanges: Fourteen bones in the fingers, each with a base, body, and head. Each finger has proximal, middle, and distal phalanges (thumb has only proximal and distal).

Module 7.5 Bones of the Pelvic Girdle and Lower Limb

Pelvic Girdle

The pelvic girdle and lower limb complete the appendicular skeleton. The hipbones (coxal bones) articulate with the sacrum.

• Bowl-shaped pelvis: Composed of sacrum and two coxal bones.

Key Table: Skull Bones Classification

Key Table: Vertebral Column Regions

Key Table: Carpal Bones of the Wrist

Key Definitions

• Articulation: A joint where two bones meet.

• Epiphysis: The end part of a long bone, initially growing separately from the shaft.

• Diaphysis: The shaft or central part of a long bone.

• Foramen: An opening or hole in a bone for nerves and blood vessels.

• Process: A projection or outgrowth of bone.

• Sinus: A cavity within a bone, typically filled with air and lined with mucous membrane.

Key Equations

• Number of bones in adult human skeleton: $206$

• Number of vertebrae in vertebral column:

Examples and Applications

• Example: The atlas (C1) allows the head to nod "yes," while the axis (C2) allows the head to rotate "no."

• Application: Fontanels in the fetal skull are clinically important for assessing hydration and intracranial pressure in infants.