The Skeletal System: The Axial Skeleton

The Axial Skeleton and Homeostasis

  • The axial skeleton contributes to homeostasis by protecting organs:
    • Cranium: surrounds the brain.
    • Vertebrae: surround the spinal cord.
    • Ribs: surround the heart and lungs.
  • Bones support calcium storage and release.

Divisions of the Skeletal System

  • The human skeleton has 206 named bones.
    • Axial skeleton (80 bones): skull, auditory ossicles, hyoid, ribs, sternum, vertebrae, and sacrum.
    • Appendicular skeleton (126 bones): upper and lower extremities, and girdles connecting limbs to the axial skeleton.

Types of Bones

  • Bones classified into five main types based on shape:
    • Long: Greater length than width.
    • Short: Cube-shaped.
    • Flat: Thin layers of parallel plates.
    • Irregular: Complex shapes.
    • Sesamoid: Shaped like a sesame seed.
  • Sutural bones: Small, extra bone plates located within the sutures of cranial bones
  • Sutures: Jointed areas where flat bones come together

Bone Surface Markings

  • Structural features adapted for specific functions.
    • Depressions and openings: Allow passage of soft tissues and formation of joints.
    • Processes: Projections or outgrowths that form joints and serve as attachment points for ligaments and tendons.

Depressions and Openings

  • Allow passage of soft tissue (nerves, blood vessels, ligaments, tendons) or formation of joints
    • Fissure: Narrow slit between adjacent parts of bones through which blood vessels or nerves pass. Example: Superior orbital fissure of sphenoid bone.
    • Foramen: Opening through which blood vessels, nerves, or ligaments pass. Example: Optic canal of sphenoid bone.
    • Fossa: Shallow depression. Example: Coronoid fossa of humerus.
    • Sulcus: Furrow along bone surface that accommodates blood vessel, nerve, or tendon. Example: Intertubercular sulcus of humerus.
    • Meatus: Tubelike opening. Example: External acoustic meatus of temporal bone.

Processes

  • Projections or outgrowths on bone that form joints or attachment points for connective tissue, such as ligaments and tendons
    • Condyle: Large, round protuberance with a smooth articular surface at end of bone. Example: Lateral condyle of femur.
    • Facet: Smooth, flat, slightly concave or convex articular surface. Example: Superior articular facet of vertebra.
    • Head: Usually rounded articular projection supported on neck (constricted portion) of bone. Example: Head of femur.
    • Crest: Prominent ridge or elongated projection. Example: Iliac crest of hip bone.
    • Epicondyle: Typically roughened projection above condyle. Example: Medial epicondyle of femur.
    • Line: Long, narrow ridge or border (less prominent than crest). Example: Linea aspera of femur.
    • Spinous process: Sharp, slender projection. Example: Spinous process of vertebra.
    • Trochanter: Very large projection. Example: Greater trochanter of femur.
    • Tubercle: Variably-sized rounded projection. Example: Greater tubercle of humerus.
    • Tuberosity: Variably-sized projection that has a rough, bumpy surface. Example: Ischial tuberosity of hip bone.

Skull

  • The skull contains 22 bones, excluding the 3 middle ear bones in each ear.
  • Associated with processes, ridges, lines, depressions, and foramina.

Cranial Bones

  • Frontal, Parietal, Temporal, Occipital, Sphenoid, Ethmoid

Facial Bones

  • Vomer, Maxillae, Zygomatic, Mandible

Other Components

  • Nasal Septum
  • Orbit

Principal Foramina of the Skull

  • 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.
  • Olfactory: 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.

Unique Features of the Skull

  • Sutures: Immovable joints holding skull bones together (e.g., coronal, sagittal, lambdoid, squamous).
  • Paranasal sinuses: Mucous membrane-lined cavities in the frontal, maxillary, sphenoid, and ethmoid bones.
    • Resonating chambers to enhance the voice.
    • Increase surface area of the nasal mucosa and help to moisten it.
  • Fontanels: Areas in a fetus/baby where unossified mesenchyme develops into dense connective tissue; close up through intramembranous ossification by 2 years of age.

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.

The Vertebral Column

  • Also known as the spinal column, backbone, or spine.
  • Composed of 26 vertebrae divided into 5 regions.
  • Protects the spinal cord.
  • Curves: cervical, thoracic, lumbar, sacral

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.

Cervical Vertebrae

  • Atlas (C1)
  • Axis (C2)

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 that are 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:
    • The upper manubrium
    • The middle body
    • 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

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 sometimes is simply associated with aging
  • Curve-related pathologies include:
    • Scoliosis (increased lateral curvature)
    • Kyphosis (increased thoracic curve—bent forward)
    • Lordosis (increased lumbar curve—bent backwards)
  • Spina bifida is a congenital defect of the vertebral column where the laminae do not develop normally.
  • Fractures of the vertebral column most commonly occur at C1, C2, C4-T, and T12-L2