chapter 7
Chapter 7: Axial Skeleton
View of a Skull, Leonardo da Vinci, c. 1489
Overview of Axial Skeleton
Key Components:
Divisions of the Skeletal System
Types of Bones
Bone Surface Markings
Bones of the Axial Skeleton
Skull
Hyoid Bone
Vertebral Column
Thorax
Disorders: Homeostatic Imbalances
Divisions of the Skeletal System
The human skeleton consists of 206 named bones grouped into two principal divisions:
Axial Skeleton
Appendicular Skeleton
Graphic Depiction: The axial skeleton is highlighted in blue, while the appendicular skeleton constitutes the remainder.
Axial Skeleton
Consists of 80 bones that lie around the longitudinal axis of the human body:
Skull bones
Auditory ossicles (ear bones)
Hyoid bone
Ribs
Sternum (breastbone)
Bones of the vertebral column
Appendicular Skeleton
Consists of 126 bones of the upper and lower limbs (extremities) and the bones forming the girdles that connect limbs to the axial skeleton.
Types of Bones
Each of the 206 named bones can be classified into one of 5 main categories based on anatomical characteristics:
Long Bones: Greater in length than in width; often slightly curved for weight bearing.
Examples: femur, tibia, fibula, humerus, ulna, radius, metacarpals, metatarsals, phalanges.
Short Bones: Cube-shaped, include the carpals and tarsals.
Flat Bones: Thin, composed of two nearly parallel plates of compact bone enclosing a layer of spongy bone.
Examples: cranial bones, ribs, sternum, scapulae, clavicles.
Irregular Bones: Complex shapes including vertebrae and some facial bones.
Sesamoid Bones: Vary in number; protect tendons from excessive wear.
Example: Patella.
Can develop fractures due to friction, tension, and stress.
Sutural Bones (Wormian Bones): Not considered one of the main types of bones; small extra bone plates located within the sutures of cranial bones.
Found as isolated examples; unusual but not rare.
Bone Surface Markings
Bones show specific surface markings adapted for particular functions:
Major Types of Surface Markings:
Depressions and Openings: Allow passage of blood vessels and nerves; form joints.
Processes: Projections or outgrowths that form joints; serve as attachment points for ligaments and tendons.
Specific Surface Markings
A process: Any bone projection, large or small.
Spinous Process: A slender projection from a vertebra.
Foramen: An opening in bone through which blood vessels and/or nerves pass.
Condyle: A large, round, articular bony process (e.g., Trochlea and Capitulum of the humerus).
Epicondyle: A bony protuberance above a condyle.
Fossa: A shallow depression in bone.
Tubercle: A small rounded projection.
Tuberosity: A large bony prominence that is not articular.
Meatus: A tube-like canal (e.g., external auditory meatus).
Trochanters: Two very large bony projections on the femur.
Bones of the Axial Skeleton
Comprises 80 bones including:
Skull
Auditory Ossicles
Hyoid Bone
Vertebral Column: Includes sacrum and coccyx.
Ribs
Sternum
Skull
Function: Protects and supports the brain and special sense organs.
Forms the large cranial cavity and several smaller cavities:
Nasal cavity
Orbits (eye sockets)
Paranasal sinuses
Small cavities housing organs involved in hearing and equilibrium.
Cranial Bones
Total: 8 Cranial Bones
Frontal bone (1)
Parietal bone (2)
Temporal bone (2)
Occipital bone (1)
Sphenoid bone (1)
Ethmoid bone (1)
Facial Bones
Total: 14 Facial Bones
Mandible (1)
Maxilla (2)
Zygomatic bones (2)
Nasal bones (2)
Lacrimal bones (2)
Palatine bones (2)
Inferior Nasal conchae (2)
Vomer (1)
Suture and Fontanels
Suture: An immovable joint between bones of the skull.
Fontanels: Soft, mesenchyme-filled spaces between cranial bones in babies that will become sutures in adults.
Paranasal Sinuses
Prominent in:
Frontal bone
Ethmoid bone
Sphenoid bone
Maxillae bones
Functions:
Lined with mucous membranes to humidify and warm the air.
Reduce weight in the skull.
Help resonate the sound of the voice.
Keystone Bone
The sphenoid bone is known as the "keystone" of the cranial floor because it articulates with all other cranial bones, holding them together.
Framework for Facial Muscles
The skull provides a framework for:
Attachment of muscles that move various parts of the head.
Attachment for muscles that produce facial expressions.
Facial bones provide the framework of the face and support entrances to the digestive and respiratory systems.
Hyoid Bone
Description: U-shaped bone that does not articulate with any other bone.
Function:
Supports the tongue.
Provides attachment for some tongue muscles and muscles of the pharynx and neck.
Vertebral Column
Composed of a series of bones called vertebrae.
Vertebrae Components:
Body (weight-bearing)
Vertebral arch (pedicle plus lamina)
Several processes containing articular surfaces and muscle attachment sites.
Cervical Vertebrae: 7 in the neck region (C1-C7).
Thoracic Vertebrae: 12 that articulate with the ribs (T1-T12).
Lumbar Vertebrae: 5 in the lower back (L1-L5).
Sacrum and Coccyx: Single bones resulting from the fusion of several vertebrae.
Vertebral Features
Each vertebra has:
A large central hole (vertebral foramen) for spinal cord passage.
Intervertebral foramen formed for exiting spinal nerves.
Intervertebral Discs: Tough fibrocartilage between adjacent vertebrae; absorbs shock and permits movement of the spine.
Normal Curvatures of the Spine
Viewed from the front, the vertebral column appears straight; viewed from the side, it has four slight bends (normal spinal curvatures).
Curvatures Described:
Cervical and lumbar curves: Convex (bulging out).
Thoracic and sacral curves: Concave (cupping in).
Primary Curves: Curvature matches that of the fetus.
Cervical Vertebrae
Atlas (C1): Holds up the head; named after Atlas in Greek mythology who supported the world.
Axis (C2): Provides pivot; allows head to turn on the neck.
Atlanto-Occipital Joint: Allows for nodding "yes."
Atlanto-Axial Joint: Allows for shaking head "no."
Thoracic Vertebrae and Ribs
Each of the 12 thoracic vertebrae articulate with a rib to form the posterior part of the thoracic cage.
Lumbar Vertebrae
Characteristics: Bear greater loads; stouter than cervical and thoracic vertebrae.
Sacrum and Coccyx
Sacrum: Starts as 5 separate vertebrae that fuse around age 17.
Coccyx (Tailbone): Starts as 4 separate vertebrae that begin to fuse between ages 20 and 30.
Thorax
Thoracic Cage: Skeletal portion of the thorax formed by:
Thoracic vertebrae
Sternum
Ribs and their costal cartilages
Functions:
Encloses and protects organs in thoracic and superior abdominal cavities.
Provides support for bones of the upper limbs.
Plays a role in breathing.
Sternum
Sternum (Breastbone): Located anteriorly in the center of the thoracic wall.
Consists of the manubrium, body, xiphoid process.
Ribs
Total: 12 pairs of ribs providing structural support to the thoracic cavity:
True Ribs (7 pairs): Attach directly to the sternum via costal cartilage.
False Ribs (5 pairs):
Pairs 8, 9, and 10 attach indirectly to sternum.
Pairs 11 and 12 are floating ribs; do not attach to the sternum.
Disorders: Homeostatic Imbalances
Herniated Disc: Occurs due to trauma or aging; results in the nucleus pulposus leaking out due to a tear in the annulus fibrosus.
Curvature Pathologies:
Scoliosis: Increased lateral curvature.
Kyphosis: Increased thoracic curve (bent forward).
Lordosis: Increased lumbar curve (bent backward).
Spina Bifida: Congenital defect in the vertebral column; laminae do not develop normally, varying in severity:
Spina Bifida Occulta: Minor form.
Severe Spina Bifida with Meningomyelocele: Cyst-like sac protrudes from the spinal column containing meninges, cerebrospinal fluid, and spinal cord/nerves.