Chapter 7: The Skeletal System: The Axial Skeleton

CHAPTER 7: THE SKELETAL SYSTEM: THE AXIAL SKELETON

Divisions of the Skeletal System

The human skeleton consists of 206 named bones.
These bones are grouped into two principal divisions:
- Axial Skeleton:
- Consists of the bones that lie around the longitudinal axis of the body.
- Components:
  - Skull bones
  - Auditory ossicles (ear bones)
  - Hyoid bone (only bone in the body that does not articulate with any other bone)
  - Ribs
  - Sternum (breastbone)
  - Vertebral column bones
- Appendicular Skeleton:
- Consists of the bones of the upper and lower limbs (extremities), plus the bones forming the girdles connecting the limbs to the axial skeleton.

Divisions of the Skeletal System

TABLE 7.1: The Bones of the Adult Skeletal System

Division of the Skeleton: Axial Skeleton
- Structure:
- Skull: 22 bones
  - Cranium: 8
  - Face: 14
  - Hyoid: 1
  - Auditory ossicles: 6
- Vertebral column: 26
- Thorax: 29 (1 sternum + 28 ribs )
Subtotal for Axial Skeleton: 80 bones
Division of the Skeleton: Appendicular Skeleton
- Pectoral (shoulder) girdles:
- Clavicle: 2
- Scapula: 2
- Upper limbs:
- Humerus: 2
- Ulna: 2
- Radius: 2
- Carpals: 16
- Metacarpals: 10
- Phalanges: 28
- Pelvic (hip) girdle:
- Hip, pelvic, or coxal bone: 2
- Lower limbs:
- Femur: 2
- Patella: 2
- Tibia: 2
- Fibula: 2
- Tarsals: 14
- Metatarsals: 10
- Phalanges: 28
Subtotal for Appendicular Skeleton: 126 bones.
Total in Adult Skeleton: 206 bones.

Divisions of the Axial Skeletal System

Skull
Sternum
Ribs
Vertebrae:
- Cervical vertebrae
- Thoracic vertebrae
- Lumbar vertebrae
Costal cartilages
Sacrum
Coccyx

Types of Bones

Classification of Bones into five types based on shape:
- Long Bones:
- Greater length than width; slightly curved for strength.
  - Examples: Femur, Tibia, Fibula
- Short Bones:
- Cube-shaped; nearly equal in length and width.
  - Examples: Carpals, Tarsals
- Flat Bones:
- Thin, composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue.
  - Examples: Cranial bones, Sternum
- Irregular Bones:
- Complex shapes; cannot be grouped into the previous categories.
  - Examples: Vertebrae, Hip bones
- Sesamoid Bones:
- Protect tendons from excessive wear and tear.
  - Examples: Patellae, Foot, Hand

Bone Surface Markings

Bones have characteristic surface markings that are structural features adapted for specific functions.
Types of Surface Markings:
- Depressions and Openings:
- Allow the passage of blood vessels and nerves or form joints.
- Processes:
- Projections or outgrowths that form joints or serve as attachment points for ligaments and tendons.

Skull

Overview:
- The skull (cranium) consists of 22 bones.
- These bones are grouped into two categories:
- Cranial Bones: 8 bones forming the cranial cavity.
  - Specific Bones:
  - Frontal bone
  - Two Parietal bones
  - Two Temporal bones
  - Occipital bone
  - Sphenoid bone
  - Ethmoid bone
- Facial Bones: 14 bones forming the face.
  - Specific Bones:
  - Two Nasal bones
  - Two Maxillae
  - Two Zygomatic bones
  - Mandible
  - Two Lacrimal bones
  - Two Palatine bones
  - Two Inferior nasal conchae
  - Vomer
General Features:
- Cranial and facial bones protect and support special sense organs and the brain.
- The skull forms several cavities:
- Nasal cavity
- Orbits (eye sockets)
- Paranasal sinuses (lined with mucus membrane)
Sutures:
- Immovable joints that fuse most skull bones together.
Facial Bones Framework:
- Provide support for the entrances to the digestive and respiratory systems.

Skull (Cranial and Facial Bones)

Cranial Bones

Components:
- Frontal Bone
- Parietal Bones
- Temporal Bones
- Occipital Bone
- Sphenoid Bone
- Ethmoid Bone

Facial Bones

Components:
- Nasal Bones:
  - Form the bridge of the nose.
- Maxillae:
  - Form the upper jawbone.
- Zygomatic Bones:
  - Commonly called cheekbones; form the prominences of the cheeks.
- Lacrimal Bones:
  - Form part of the medial wall of each orbit; smallest bone in face.
- Palatine Bones:
  - Form the posterior portion of the hard palate.
- Inferior Nasal Conchae:
  - Form part of the inferior lateral wall of the nasal cavity.
- Vomer:
  - Forms the inferior portion of the nasal septum.
- Mandible:
  - Lower jawbone; the largest, strongest facial bone; the only movable skull bone.

Skull Special Features

The Nasal Septum divides the interior of the nasal cavity into right and left sides:
- Composed of:
- Vomer bone
- Perpendicular plate of the ethmoid bone
Orbits (Eye Socket):
- Union of seven bones.
- Specific Bones:
  - Frontal bone
  - Lacrimal bone
  - Ethmoid bone
  - Zygomatic bone
  - Maxillary bone
  - Palatine bone
  - Sphenoid bone
Foramina & Fossa:
- Foramen: Opening for blood vessels, nerves, or ligaments of the skull.
- Fossa: A shallow depression on the bone surface.

Unique Features of the Skull

Sutures:
- Immovable joints that hold most skull bones together, names include sagittal, coronal, squamous, and lambdoidal sutures.
Paranasal Sinuses:
- Cavities within cranial and facial bones near the nasal cavity.
- Lined with mucous membranes and drain into the nasal cavity.
- Serve as resonating chambers that intensify and prolong sounds.
Fontanels:
- Areas of unossified tissue; at birth, commonly called "soft spots" linking the cranial bones.
- Eventually replaced with bone to form sutures.
- Provide flexibility to the fetal skull for rapid brain growth, allowing the skull to change shape as it passes through the birth canal.

Paranasal Sinuses

Types of Sinuses:
- Frontal Sinus
- Ethmoid Sinus
- Sphenoid Sinus
- Maxillary Sinus

Fontanels

Specific Fontanels:
- Anterior Fontanel: Frontal suture.
- Posterior Fontanel
- Mastoid Fontanel
- Sphenoidal Fontanel

Vertebral Column

Also referred to as the spine, backbone, or spinal column.
Functions:
- Protect the spinal cord.
- Support the head.
- Serve as a point of attachment for the ribs, pelvic girdle, and muscles.
Curvature:
- Curved in different locations to increase strength, maintain balance in the upright position, absorb shocks during walking, and protect vertebrae from fracture.

Vertebral Column Structure

Composition: Series of bones called vertebrae (26 in adults).
- 7 cervical (C₁ - C₇)
- 12 thoracic (T₁ - T₁₂)
- 5 lumbar (L₁ - L₅)
- 1 sacrum (5 fused vertebrae)
- 1 coccyx (4 fused vertebrae)

Intervertebral Discs

Found between adjacent vertebrae.
Functions:
- Form strong joints.
- Permit various movements of the vertebral column.
- Absorb vertical shock.
Vertebrae Components:
- Consist of a body (weight bearing), a vertebral arch (surrounds the spinal cord), and several processes (points of attachment for muscles).

Vertebral Column Regions

Cervical Region (C₁–C₇):
- Atlas (C₁): First cervical vertebra; supports the skull.
- Axis (C₂): Second cervical vertebra; allows side-to-side rotation of the head; has Dens (odontoid process) that replaces the body on Atlas.
Thoracic Region (T₁–T₁₂)
Lumbar Region (L₁–L₅):
- The strongest and largest vertebrae.
Sacrum:
- Triangular bone formed by the union of five sacral vertebrae (S₁–S₅).
Coccyx:
- Triangular in shape; formed by the fusion of usually four coccygeal vertebrae.

Sacrum and Coccyx

Sacrum:
- Body of the first sacral vertebra, transverse ridges (site of vertebral fusion), sacral canal, alae, sacral promontory, median sacral crest, anterior and posterior sacral foramina.
Coccyx: 4 fused vertebrae.

Thorax

The thoracic cage is formed by:
- Sternum
- Ribs
- Costal cartilages
- Thoracic vertebrae
Functions:
- Enclose and protect organs in the thoracic and abdominal cavities.
- Provide support for the bones of the upper limbs.
- Play a role in breathing.

Thorax Components

Sternum:
- "Breastbone" located in the center of the thoracic wall.
- Consists of:
- Manubrium
- Body
- Xiphoid process
Ribs:
- Twelve pairs provide structural support to the sides of the thoracic cavity.
- First seven are true ribs; the rest are false ribs (last two are floating ribs).
Costal Cartilages:
- Contribute to the elasticity of the thoracic cage.