Anatomy and Physiology: The Axial Skeleton System

The Axial Skeleton and Homeostasis

• The axial skeleton contributes to homeostasis by:

  • Protecting the body's organs.

    • Cranium: Surrounds the brain.

    • Vertebrae: Surrounds the spinal cord.

    • Ribs: Surround the heart and lungs.

  • Supporting calcium storage and release, which is important for ion balance.

Divisions of the Skeletal System

• The human skeleton has 206 named bones.

Axial vs. Appendicular Skeleton

• 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.

  • Bones forming the girdles connecting limbs to the axial skeleton.

Bones of the Adult Skeletal System

Axial Skeleton

• Skull:

  • Cranial cavity bones: 8

  • Facial bones: 14

• Hyoid bone: 1

• Auditory ossicles: 6

• Vertebral column: 26

• Thorax:

  • Sternum: 1

  • Ribs: 24

• Total bones in axial skeleton: 80

Appendicular Skeleton

• Pectoral girdles:

  • Clavicle: 2

  • Scapula: 2

• Upper limbs:

  • Humerus: 2

  • Ulna: 2

  • Radius: 2

  • Carpals: 16

  • Metacarpals: 10

  • Phalanges: 28

• Pelvic girdle:

  • Hip, pelvic, or coxal bone: 2

• Lower limbs:

  • Femur: 2

  • Patella: 2

  • Fibula: 2

  • Tibia: 2

  • Tarsals: 14

  • Metatarsals: 10

  • Phalanges: 28

• Total bones in appendicular skeleton: 126

• Total bones in adult skeleton: 206

Types of Bones: Shape

• Bones are classified into 5 main types based on shape:

  • Long: Greater length than width. These bones are not just defined by their size but also by their elongated shape. Examples include bones in the limbs such as the femur, tibia, fibula, humerus, ulna, and radius. They primarily function in movement and support.

  • Short: Cube-shaped. These bones are approximately equal in length, width, and height, giving them a cube-like shape. Examples include the carpal bones in the wrist and the tarsal bones in the ankle. They provide stability and support with limited motion.

  • Flat: Thin layers of parallel plates. Flat bones are thin and generally curved, serving to protect internal organs and providing a large surface area for muscle attachment. Examples include the cranial bones (occipital, parietal, frontal), the scapula (shoulder blade), the sternum (breastbone), and the ribs.

  • Irregular: Complex shapes. Irregular bones have complex shapes that do not fit into the other categories. Their shapes are often due to their specific functions and locations. Examples include the vertebrae, hip bones, and certain skull bones like the sphenoid

Types of Bones: Location

• Sutural bones are small, extra bone plates located within the sutures of cranial bones.

• Sutures are the jointed areas where flat bones come together.

Bone Surface Markings

• Bones have characteristic surface markings, which are 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

• Sites allowing the passage of soft tissue (nerves, blood vessels, ligaments, tendons) or formation of joints.

• Fissure:

  • Description: Narrow slit between adjacent parts of bones through which blood vessels or nerves pass.

  • Example: Superior orbital fissure of sphenoid bone.
    FISH-ur

• Foramen:

  • Description: Opening through which blood vessels, nerves, or ligaments pass.

  • Example: Optic canal of sphenoid bone.

  • Plural: foramina
    fō-RĀ-men
    fō-RĀ-min-a

• Fossa:

  • Description: Shallow depression.

  • Example: Coronoid fossa of humerus.

  • Plural: fossae
    FOS-a
    FOS-ē

• Sulcus:

  • Description: Furrow along bone surface that accommodates blood vessel, nerve, or tendon.

  • Example: Intertubercular sulcus of humerus.

  • Plural: sulci
    SUL-kus
    SUL-sī

• Meatus:

  • Description: Tubelike opening.

  • Example: External acoustic meatus of temporal bone.

  • Plural: meati
    mē-Ā-tus
    mē-Ā-tī

Processes

• Projections or outgrowths on bone that form joints or attachment points for connective tissue, such as ligaments and tendons.

Processes that form joints:

• Condyle:

  • Description: Large, round protuberance with a smooth articular surface at end of bone.

  • Example: Lateral condyle of femur.
    KON-dīl
    condylus = knuckle

• Facet:

  • Description: Smooth, flat, slightly concave or convex articular surface.

  • Example: Superior articular facet of vertebra.
    FAS-et
    fa-SET

• Head:

  • Description: Usually rounded articular projection supported on neck (constricted portion) of bone.

  • Example: Head of femur.

Processes that form attachment points for connective tissue:

• Crest:

  • Description: Prominent ridge or elongated projection.

  • Example: Iliac crest of hip bone.

• Epicondyle:

  • Description: Typically roughened projection above condyle.

  • Example: Medial epicondyle of femur.
    epi- = above

• Line:

  • Description: Long, narrow ridge or border (less prominent than crest).

  • Example: Linea aspera of femur.
    linea

• Spinous process:

  • Description: Sharp, slender projection.

  • Example: Spinous process of vertebra.

• Trochanter:

  • Description: Very large projection.

  • Example: Greater trochanter of femur.
    trō-KAN- ter

• Tubercle:

  • Description: Variably-sized rounded projection.

  • Example: Greater tubercle of humerus.
    TOO-ber- kul
    tuber- = knob

• Tuberosity:

  • Description: Variably-sized projection that has a rough, bumpy surface.

  • Example: Ischial tuberosity of hip bone.

Skull

• The skull contains 22 bones, not including the 3 middle ear bones in both ears.
  Associated with these bones are a number of processes, ridges, lines, depressions, and foramina.

Cranial Bones

Frontal Bone

• Frontal Squama: The flat, main part of the frontal bone that forms the forehead.

• Supraorbital Foramen/Notch: A small hole (foramen) or indentation (notch) above each eye socket. It allows the passage of blood vessels and nerves to the forehead.

• Supraorbital Margin: The upper edge of the eye socket, providing protection to the eye.

• Optic Canal: A canal through which the optic nerve passes, connecting the eye to the brain.

• Superior Orbital Fissure: A larger opening behind the eye socket that allows the passage of several nerves and blood vessels to the eye and forehead.

Parietal and Temporal Bones

• Coronal Suture: The joint between the frontal bone and the two parietal bones.

• Zygomatic Arch: The bony arch on the side of the skull formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone; it's the arch you can feel on your cheek.

• Parietal Bone: One of the two bones that form the sides and roof of the skull.

• Temporal Squama: The flat, thin part of the temporal bone.

• Squamous Suture: The joint between the temporal squama and the parietal bone.

• Temporal Bone: The bone on the side of the skull that contains the structures of the ear.

• Zygomatic Process: A projection from the temporal bone that helps form the zygomatic arch.

• Lambdoid Suture: The joint between the parietal bones and the occipital bone at the back of the skull.

• Mastoid Portion: The part of the temporal bone behind the ear.

• Occipital Bone: The bone at the back of the skull.

• External Occipital Protuberance: The bump on the back of the head that you can feel.

• External Acoustic Meatus: The ear canal opening in the temporal bone.

• Mastoid Process: A bony projection behind the ear, serving as an attachment point for muscles.

• Styloid Process: A pointy projection below the ear, serving as an attachment point for muscles and ligaments of the neck.

• Occipital Condyle: Oval structures on the occipital bone that articulate with the first vertebra (atlas).

• Sphenoid Bone: An internal bone that contributes to the base of the skull and eye sockets.

• Ethmoid Bone: An internal bone that contributes to the nasal cavity and eye sockets.

• Lacrimal Bone: A small bone forming part of the eye socket.

• Lacrimal Fossa: A groove in the lacrimal bone that houses the lacrimal sac (part of the tear drainage system).

• Nasal Bone: The bone that forms the bridge of the nose.

• Temporal Process: Projection from the zygomatic bone that helps form the zygomatic arch.

• Mandibular Fossa: A depression on the temporal bone where the mandible (lower jaw) articulates.

• Maxilla: The upper jaw bone.

• Articular Tubercle: A small bump in front of the mandibular fossa that helps with jaw movement.

• Mandible: The lower jaw bone.

Clinical Connection

• Temporomandibular Joint (TMJ) Dysfunction:

  • Symptoms: Dull pain around the ear, tender jaw muscles, clicking noise when opening/closing mouth.

  • Causes: Improperly aligned teeth, grinding of teeth, trauma to the head, arthritis.

  • Treatment: Moist heat or ice, soft foods, pain relievers.

• Deviated Nasal Septum:

  • Description: Septum does not run along the midline of the nasal cavity.

  • Causes: Trauma to the nose or developmental abnormality.

  • Symptoms: Infection, inflammation, congestion, headaches, and nosebleeds.

  • Treatment: May require surgery.

Facial Bones

• Key structures:

  • Frontal bone: Forms the forehead and upper part of the eye sockets.

  • Supraorbital margin: The bony ridge above the eyes in the frontal bone.

  • Sphenoid bone: A bone at the base of the skull behind the eyes.

  • Optic canal: Canal in the sphenoid bone through which the optic nerve passes.

  • Superior orbital fissure: Opening in the sphenoid bone for nerves and blood vessels.

  • Palatine bone: Forms the posterior part of the hard palate and nasal cavity floor.

  • Inferior orbital fissure: Opening between the sphenoid and maxilla bones for nerves and blood vessels.

  • Zygomaticofacial foramen: Small opening on the zygomatic bone for nerves and blood vessels.

  • Zygomatic bone: Cheekbone.

  • Infraorbital foramen: Opening in the maxilla below the eye socket for nerves and blood vessels.

  • Nasal bone: Forms the bridge of the nose.

  • Lacrimal bone: Small bone in the medial wall of the orbit.

  • Orbit: Eye socket.

  • Ethmoid bone: Bone in the nasal cavity between the eyes.

  • Lacrimal fossa: Depression in the lacrimal bone for the lacrimal sac.

  • Maxilla: Upper jaw bone.

  • Perpendicular plate of ethmoid bone: Vertical part of the ethmoid bone that forms part of the nasal septum.

  • Inferior nasal concha bone: One of the three paired nasal conchae in the nasal cavity.

  • Vomer: Forms the posteroinferior part of the nasal septum.

  • Coronal suture: Joint between the frontal and parietal bones.

  • Supraorbital notch: Notch in the supraorbital margin for the passage of nerves and blood vessels.

  • Temporal bone: Bone on the side of the skull.

  • Middle nasal concha: One of the three paired nasal conchae in the nasal cavity.

  • Alveolar process of maxilla: Portion of the maxillary bone that forms the sockets for the upper teeth.

  • Mental foramen: Opening on the mandible for nerves and blood vessels.

  • Alveolar process of mandible: Portion of the mandibular bone that forms the sockets for the lower teeth.

  • Mandible: Lower jawbone.Mandible

• The mandible (lower jawbone) is the largest and strongest facial bone.

• Other than the middle ear bones (auditory ossicles), it is the only moveable skull bone.

Nasal Septum

• NASAL SEPTUM:

  • Perpendicular plate of ethmoid bone

  • Septal nasal cartilage

  • Vomer

  • Horizontal plate of palatine bone

  • Palatine process of maxilla

Orbit

• Key structures:

  • Frontal bone: Forms the upper part of the eye socket.

  • Supraorbital margin: The bony ridge above the eyes in the frontal bone.

  • Sphenoid bone: A bone at the base of the skull behind the eyes.

  • Optic canal: Canal in the sphenoid bone through which the optic nerve passes.

  • Superior orbital fissure: Opening in the sphenoid bone for nerves and blood vessels.

  • Palatine bone: Forms the posterior part of the hard palate and nasal cavity floor.

  • Inferior orbital fissure: Opening between the sphenoid and maxilla bones for nerves and blood vessels.

  • Zygomaticofacial foramen: Small opening on the zygomatic bone for nerves and blood vessels.

  • Zygomatic bone: Cheekbone.

  • Infraorbital foramen: Opening in the maxilla below the eye socket for nerves and blood vessels.

  • Nasal bone: Forms the bridge of the nose.

  • Lacrimal bone: Small bone in the medial wall of the orbit.

  • Ethmoid bone: Bone in the nasal cavity between the eyes.

  • Lacrimal fossa: Depression in the lacrimal bone for the lacrimal sac.

  • Maxilla: Upper jaw bone

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 artery

  • 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

Unique Features of the Skull

• Sutures: Immovable joints in the skull that hold bones together.

  • Examples: Coronal, sagittal, lambdoid, and squamous sutures.

  • Coronal Suture: Located between the frontal and parietal bones.

  • Sagittal Suture: Located between the parietal bones.

  • Lambdoid Suture: Located between the parietal and occipital bones.

  • Squamous Suture: Located between the temporal and parietal bones.

• Paranasal Sinuses: Mucous membrane-lined cavities in the frontal, maxillary, sphenoid, and ethmoid bones.

  • Functions: Act as resonating chambers for enhancing voice and increase the surface area of the nasal mucosa 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 of the Vertebral Column

• Cervical curve with 7 cervical vertebrae

  • A secondary curve (develops after birth).

  • Convex anteriorly.

  • Forms as the infant begins to hold their head upright.

• Thoracic curve with 12 thoracic vertebrae

  • A primary curve (present at birth).

  • Concave anteriorly.

  • Accommodates the thoracic organs.

• Lumbar curve with 5 lumbar vertebrae

  • A secondary curve (develops after birth).

  • Convex anteriorly.

  • Appears when the child begins to stand and walk.

• Sacral Curve with 5 fused sacral vertebrae

  • A primary curve (present at birth).

  • Concave anteriorly.

  • Accommodates the pelvic organs.

• Intervertebral Disc between vertebrae

  • Located between the bodies of adjacent vertebrae, from C2 to the sacrum.

  • Functions to absorb shock and permit movement of the vertebral column.

  • Absence of intervertebral discs between the sac

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.

• Functions: Absorb shock and separate the vertebrae from one another.

Vertebrae

• Vertebrae from each region, in addition to containing common structures, have unique structures that help to identify which type they are.

Cervical Vertebrae

• Atlas (C1)

• Axis (C2)

• Typical cervical vertebra

• Dens of 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.

Comparison of Cervical, Thoracic, and Lumbar 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.

• 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).

• Spina bifida is 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-T, and T12-L2.

  • Spinal cord or nerve damage may occur as a result of a fracture.