Chapter 7: The Axial Skeleton - Comprehensive Notes

Chapter 7: The Skeletal System - The Axial Skeleton

Learning Objectives

• Describe the bones of the axial skeleton.
• Describe the different skull bones: single and paired.
• Explain the construction of the ethmoid and sphenoid bones, including the sella turcica, and their importance.
• Explain the construction of facial bones in relation to their functions.
• Locate and identify the features of the vertebral column and its curvatures.
• Locate and label the features of a typical vertebra and distinguish between cervical, thoracic, and lumbar vertebrae.
• Identify structures of the thoracic cage and distinguish between true and false ribs.

The Axial Skeleton and Homeostasis

• The axial skeleton contributes to homeostasis by protecting many of the body’s organs:
  • Cranium: surrounds the brain.
  • Vertebrae: surround the spinal cord.
  • Ribs: surround the heart and lungs.
• Bones support the storage and release of calcium, an important ion.

Divisions of the Skeletal System

• The human skeleton consists of 206 named bones grouped into two principal divisions:
  • Axial skeleton (80 bones): skull bones, auditory ossicles, hyoid bone, ribs, sternum, vertebrae, and sacrum.
  • Appendicular skeleton (126 bones): bones of the upper and lower extremities and the bones forming the girdles that connect the limbs to the axial skeleton.

Skull: An Overview

• The skull contains 22 bones, excluding the 3 middle ear bones in both ears.
• An anterior view of the skull shows the bones that form the forehead, orbits (eye sockets), nasal cavity, nasal septum, and upper and lower jaws.
• Associated with these bones are various processes, ridges, lines, depressions, and foramina.

Function of Cranial and Facial Bones

• Enclose and protect the brain.
• Attachment sites for head and neck muscles.
• Form the framework of the face.
• Contain cavities for special sense organs.
• Openings for food/air passage.
• Secure the teeth.
• Anchor the facial muscles of expression.

Components of the Skull

• Rounded brain case that houses the brain.
• Facial bones that form the upper and lower jaws, nose, orbits, and other facial structures.

Unique Features of the Skull

Sutures

• Immovable joints found in the skull holding bones together (e.g., temporal and parietal bones).
  • Coronal: between the frontal bone and two parietal bones.
  • Sagittal: between the two parietal bones.
  • Lambdoid: between the parietal bones and occipital bone.
  • Squamous: between the parietal bones and temporal bones.

Paranasal Sinuses

• Mucous membrane-lined cavities in the frontal, maxillary, sphenoid, and ethmoid bones.
• Functions:
  • Resonating chambers to enhance the voice.
  • Increase the surface area of the nasal mucosa to moisten air.

Fontanels

• Areas in a fetus/baby where unossified mesenchyme develops into dense connective tissue.
• Close up through intramembranous ossification by 2 years of age.

Cranial Bones

Frontal Bone

• Forms the forehead, roofs of the eye sockets (orbits), and most of the front part of the cranial floor.
• Frontal sinuses lie deep within the bone.

Parietal and Temporal Bones

• Form the sides and roof of the cranial cavity.
• Separated on top of the skull by the sagittal suture.

Occipital Bone

• Forms the posterior part and most of the cranial base.
• Foramen magnum passes through this bone.
• Occipital condyles are on either side of the foramen magnum, connecting with the first vertebrae.

Sphenoid Bone

• A single, complex bone of the central skull.
• Serves as a “keystone” bone because it joins with almost every other bone of the skull.
• Forms much of the base of the central skull and extends laterally to contribute to the sides of the skull.
• The right and left lesser wings are inside the cranial cavity.
• Sella turcica (“Turkish saddle”) is located at the midline of the middle cranial fossa.
• The hypophyseal (pituitary) fossa is a rounded depression in the floor of the sella turcica that houses the pea-sized pituitary (hypophyseal) gland.

Temporal Bones

• Form the lower sides of the cranium and part of the cranial floor.
• The external auditory meatus (ear canal) is located within these bones.

Ethmoid Bone

• Light spongy bone in the anterior part of the cranial floor between the eye sockets.
• Houses the nasal cavity.
• Contains the nasal conchae that cause turbulence in inhaled air, cleaning the air before it passes into the rest of the respiratory tract.

Facial Bones

Zygomatic Bones

• Two cheekbones that form the prominences of the cheeks and part of the lateral wall and floor of the eye sockets.

Maxillae

• Paired bones that form the upper jawbone.
• Articulates with every bone in the face except the mandible.
• Forms the anterior 3 \\/4 of the hard palate.

Palatine Bones

• Paired L-shaped bones.
• Form the posterior portion of the hard palate, part of the floor and lateral wall of the nasal cavity, and a small portion of the eye sockets.

Mandible

• The lower jawbone, the largest and strongest facial bone and the only movable skull bone other than the middle ear bones (auditory ossicles).
• The condylar process articulates with the mandibular fossa of the temporal bone.

Nasal Bones

• Paired bones forming the bridge of the nose.
• The rest of the nose consists of cartilage.

Vomer

• Triangular bone on the floor of the nasal cavity.
• One of the parts of the nasal septum.

Lacrimal Bones

• Paired smallest bones of the face.
• Found near the tear ducts.

Paranasal Sinuses

• Paired cavities near the nasal cavity.
• Located in the frontal bone, sphenoid bone, ethmoid bone, and maxillae.
• Lined with mucous membranes.
• Four paired sinuses in humans:
  • Maxillary sinuses: the largest, located under the eyes in the maxillary bones.
  • Frontal sinuses: superior to the eyes within the frontal bone.
  • Ethmoid sinuses: formed from several air cells within the ethmoid bone between the nose and eyes.
  • Sphenoid sinuses: located within the body of the sphenoid bone.
• Functions:
  • Decreasing the relative weight of the skull.
  • Increasing the resonance of the voice.
  • Providing a buffer against facial trauma.
  • Insulating sensitive structures from rapid temperature fluctuations in the nose.
  • Humidifying and heating inspired air.
  • Playing a role in immunological defense.

Hyoid Bone

• Does not articulate with any other bone.
• Supports the tongue and provides an attachment site for some muscles of the neck and pharynx.
• Suspended from the styloid processes by ligaments and muscles.
• Located in the neck between the mandible and larynx.

Sutures

• Jointed areas where flat bones come together.
• Types:
  • Coronal: between the frontal bone and two parietal bones.
  • Sagittal: between the two parietal bones.
  • Lambdoid: between the parietal bones and occipital bone.
  • Squamous: between the parietal bones and temporal bones.

Fontanels

• Membrane-filled spaces found between cranial bones in infants.
• Replaced with bone by intramembranous ossification and become sutures.
• Commonly referred to as the “soft spot” on a baby’s head.

Cranial Bones: Principal Foramina of the Skull

• Foramen/Canal - Location - Structures Passing Through
• Carotid canal - Petrous portion of temporal bone - Internal carotid artery, sympathetic nerves for eyes.
• Hypoglossal canal - Superior to base of occipital condyles - Hypoglossal (XII) nerve, branch of ascending pharyngeal blood vessel.
• Infraorbital - Inferior to orbit in maxilla - Infraorbital nerve and blood vessels, branch of maxillary branch of trigeminal (V) nerve.
• Jugular - Posterior to carotid canal between petrous portion of temporal bone and occipital bone - Internal jugular vein; glossopharyngeal (IX), vagus (X), and accessory (XI) nerves.
• Lacerum - Bounded anteriorly by sphenoid bone, posteriorly by petrous portion of temporal bone, medially by sphenoid and occipital bones - Branch of ascending pharyngeal artery.
• Magnum - Occipital bone - Medulla oblongata and its membranes (meninges), accessory (XI) nerve, vertebral and spinal arteries.
• Mandibular - Medial surface of ramus of mandible - Inferior alveolar nerve and blood vessels.
• Mastoid - Posterior border of mastoid process of temporal bone - Emissary vein to transverse sinus, branch of occipital artery to dura mater.
• Mental - Inferior to second premolar tooth in mandible - Mental nerve and vessels.
• Cribriform - Cribriform plate of ethmoid bone - Olfactory (I) nerve.
• Optic canal - Between superior and inferior portions of small wing of sphenoid bone - Optic (II) nerve, ophthalmic blood vessels.
• Ovale - Greater wing of sphenoid bone - Mandibular division of trigeminal (V) nerve.
• Rotundum - Junction of anterior and medial parts of sphenoid bone - Maxillary division of trigeminal (V) nerve.
• Stylomastoid - Between styloid and mastoid processes of temporal bone - Facial (VII) nerve, stylomastoid blood vessels.
• Supraorbital - Supraorbital margin of orbit in frontal bone - Supraorbital nerve and blood vessels.

Vertebral Column

• Also called the spine or backbone.
• Composed of vertebrae.
• Functions:
  • Acts as a strong flexible rod that can rotate and move in certain directions.
  • Encloses and protects the spinal cord.
  • Supports the skull.
  • Point of attachment for ribs, pelvic girdle, and back muscles.

Regions of the Vertebral Column

• Composed of 26 vertebrae divided into 5 regions.
• 7 cervical vertebrae in the neck (C1-C7).
• 12 thoracic vertebrae (T1-T12).
• 5 lumbar vertebrae supporting the lower back (L1-L5).
• 1 sacrum (consists of 5 fused sacral vertebrae) (S1-S5).
• 1 coccyx (consists of 4 fused coccygeal vertebrae).

Vertebral Column Curvature

• Spine curves like a snake.
• Cervical and lumbar curves are convex (bulging out anteriorly).
• Thoracic and sacral curves are concave (bulging out posteriorly).

Structure of Vertebrae

Spinal Canal

• A bony tunnel surrounding the spinal cord.
• Made up of the front (anterior) of the vertebral body, the pedicles on the sides of the vertebral body, and the lamina in the back.
• In the lower back, it contains the spinal cord and the nerve roots of the lower spine.

Facet Joints

• Each vertebra has a paired joint on its right side and a second paired joint on its left side, allowing a connection with the vertebrae above and below it.
• The pair that faces upward is the superior articular facet.
• The pair that faces downward is the inferior articular facet.

Intervertebral Discs

• Located between the bodies of the vertebrae from the second cervical to the sacrum.
• Composed of an outer ring of fibrocartilage (annulus fibrosus) and an inner, soft nucleus (nucleus pulposus) with a layer of hyaline cartilage on the top and bottom of each disc.
• Absorb shock and separate the vertebrae from one another.

Components of a Typical Vertebra

Body

• Thick, disc-shaped front portion of a vertebra.
• The weight-bearing part of a vertebra. Bears about 80 percent of the load while standing and provides an attachment for the discs between the vertebrae.
• The front or anterior section of the vertebral body protects the spinal cord and nerve roots.
• Both the vertebral body and the discs increase in size from the head to the sacrum.

Pedicles and Vertebral Arch

• Extends backwards from the body of the vertebra.
• Formed by two short, thick processes (pedicles) that unite with the flat parts of the arch (laminae), ending with a single sharp, slender projection (spinous process).
• Each vertebra has two cylinder-shaped projections (pedicles) of hard bone that stick out from the back part of the vertebral body, providing side protection for the spinal cord and nerves.

Transverse Processes

• Extend laterally on each side where the lamina and pedicle join.
• The transverse processes are located at right angles to the junction of pedicles and the lamina.
• They provide a place for the back muscles to attach to the spine. For the most part, these are absent in the vertebra of the neck (the cervical spine).

Spinous Process

• Projects from the junction of the laminae.
• Combined with the two transverse processes, these three are points of attachment for muscles to the vertebral column.
• The bumps that can be felt down the back are the spinous processes. They are bony projections that arise at right angles (perpendicular) to the midline of the lamina.
• Each spinous process is attached to the spinous process above and below it by ligaments. Sometimes these processes are absent or bifid in the cervical spine.

Spinal Cord Openings

• The vertebral foramen is the space between the vertebral arch and body that contains the spinal cord.
• All of the vertebral foramen combined forms the vertebral canal.
• The intervertebral foramen is the opening between adjoining vertebrae on both sides of the column and contains a single spinal nerve.

Vertebrae Type Comparison

Cervical Vertebrae

• Unique structures for identification.

Thoracic Vertebrae

• Support the ribs and have special structures for rib head and tubercle attachment.

Lumbar Vertebrae

• The largest and strongest vertebrae.
• No special structures specifically associated with these vertebrae.

Sacrum and Coccyx

• The triangular-shaped sacrum is part of the pelvic girdle and is composed of 5 vertebrae that fuse.
• The coccyx is much smaller than the sacrum but is also triangular in shape and is composed of 4 vertebrae that fuse.

The Thorax

• The thorax is the entire chest region.
• The bones that compose the thoracic cage are the sternum, ribs, and costal cartilages.

Sternum

• Composed of 3 segments:
  1. The upper manubrium
  2. The middle body
  3. The lower xiphoid process
• The sternum articulates with the clavicles and the costal cartilages.

Ribs

• 12 pairs
• Provide structural support to the thoracic cavity.
  • True (vertebrosternal) ribs – first 7 pairs; their cartilage is directly connected to the sternum.
  • False (vertebrochondral) ribs – next 5 pairs; cartilage is indirectly connected to the sternum.
  • Floating (vertebral) ribs – last 2 pairs; these are not connected to the sternum.

Bone Surface Markings

• Bones have characteristic surface markings – structural features adapted for specific functions.
• Two major types of surface markings:
  • Depressions and openings
    • Allow the passage of soft tissues
    • Form joints
  • Processes
    • Projections or outgrowths that form joints
    • Serve as attachment points for ligaments and tendons

Depressions and Openings

• Fissure: Narrow slit between bones for passage of blood vessels or nerves.
  • Example: Superior orbital fissure of sphenoid bone
• Foramen: Hole for passage of blood vessels, nerves, or ligaments.
  • Example: Optic canal of sphenoid bone
• Fossa: Shallow depression
  • Example: Coronoid fossa of humerus
• Sulcus: Furrow on a bone for passage of blood vessel, nerve, or tendon.
  • Example: Intertubercular sulcus of humerus
• Meatus: Tubelike opening.
  • Example: External acoustic meatus of temporal bone

Processes That Form Joints

• Condyle: Rounded projection with a smooth articular surface.
  • Example: Lateral condyle of femur
• Facet: Smooth, flat, slightly concave articular surface.
  • Example: Superior articular facet of vertebra
• Head: Usually rounded articular process supported on a neck.
  • Example: Head of femur

Processes That Form Attachment Points for Connective Tissue

• Crest: Prominent ridge or elongated process.
  • Example: Iliac crest of hip bone
• Epicondyle: Usually roughened projection on a condyle.
  • Example: Medial epicondyle of femur
• Line: Long, narrow ridge or border (less prominent than a crest).
  • Example: Linea aspera of femur
• Spinous process: Sharp, slender projection.
  • Example: Spinous process of vertebra
• Trochanter: Very large projection found only on the femur.
  • Example: Greater trochanter of femur
• Tubercle: Variably sized rounded projection.
  • Example: Greater tubercle of humerus
• Tuberosity: Variably sized projection with rough, bumpy surface.
  • Example: Ischial tuberosity of hip bone

Disorders

• Many disorders may occur that affect the skeleton in one form or another.
• In the vertebral column, a herniated disc may occur due to trauma or is sometimes simply associated with aging.

Abnormal Curves of the Vertebral Column

• At times, the normal curves of the spinal column may become exaggerated.
• Curve-related pathologies include:
  • Scoliosis: increased lateral curvature.
  • Kyphosis: increased thoracic curve (bent forward).
  • Lordosis: increased lumbar curve (bent backwards).

Clinical Connections

Temporomandibular Joint (TMJ) Dysfunction

• Dull pain around ear, tender jaw muscles, clicking noise when opening/closing mouth.
• Caused by improperly aligned teeth, grinding of teeth, trauma to the head, arthritis, etc.
• Treatment includes moist heat or ice, soft foods, and pain relievers.

Deviated Nasal Septum

• Occurs when the septum does not run along the midline of the nasal cavity.
• Caused by trauma to the nose or developmental abnormality.
• May lead to infection, inflammation, congestion, headaches, and nosebleeds.
• May require surgery to fix.

Spina Bifida

• A congenital defect of the vertebral column where the laminae do not develop normally.
• The degrees of this deformity vary from minor (spina bifida occulta) to severe (spina bifida with meningomyelocele).

Fractures of the Vertebral Column

• Most commonly occur at C1, C2, C4-T7, and T12-L2.
• Spinal cord or nerve damage may occur as a result of a fracture.