The Axial Skeletal System

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The Axial Skeletal System
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Divisions of the Skeletal System

• Humans are born with approximately 300 bones
  which fuse to 206 bones as adults.
• There are 2 main divisions of the skeletal
  system axial skeleton and appendicular skeleton.

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Divisions of the Skeletal System

• Axial skeleton forms the vertical axis of the
  body.
• 80 bones skull (22), vertebral column (26),
  ribcage (25), auditory ossicles (6), and hyoid
  (1)
• Appendicular skeleton forms the arms, legs and
  the girdles
• Girdles attach the arms and legs to the axial
  skeleton
• 126 bones pectoral girdle (4), arms (60), legs
  (60) and pelvic girdle (2)

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Skull

• Superior end of the vertebral column
• Composed of flat and irregular shaped bones
• Large hollow space within the skull is called the
  cranial vault or cranial cavity.
• Functions to
• Surround and protect the brain
• Be points of attachment for the facial muscles
  (landmarks)

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Divisions of the Skull

• Cranial division consists of 8 flat bones that
  form a protective box around the brain.
• Help to form the cranial vault (cavity)
• Frontal (1) forms the anterior portion of the
  cranial cavity
• Forms the superior orbits of the eyes and forms
  the forehead

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Frontal Bone Landmarks

• Supraorbital margin a thickened ridge of bone
  found superior to the orbit of the eye.
• Just deep to the eyebrow and more prominent on
  the lateral portion
• Point for muscle attachment (PFMA)
• Supraorbital foramen a small opening found on
  the medial aspect of the supraorbital margin.
• Can feel it best inferior to the margin
• Allows blood vessels and nerves to enter the
  frontal bone

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Frontal Bone Landmarks

• Frontal sinus A hollow space found within the
  frontal bone, superior and medial to the
  supraorbital margin.
• Can only be seen with a sagittal cut
• ¼ inch superior to the eyebrows
• House mucus and macrophages for trapping and
  destroying foreign particles.

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Frontal Bone Landmarks

• Frontal sinus A hollow space found within the
  frontal bone, superior and medial to the
  supraorbital margin.
• Can only be seen with a sagittal cut
• ¼ inch superior to the eyebrows
• House mucus and macrophages for trapping and
  destroying foreign particles.

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Parietal Bones (2)

• Form the lateral walls and the superior portion
  of the cranium.
• Landmarks
• Temporal fossa A large, shallow depression that
  begins on the parietal bone and extends to the
  frontal bone.
• PFMA

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Temporal Bones (2)

• Form the inferior lateral walls and a portion of
  the floor of the cranium.
• Articulate with the mandible (lower jaw) to form
  the temporomandibular joint (TMJ)
• Temporal bone landmarks
• Mastoid process A large, blunt projection found
  posterior to the external auditory meatus.
• Bump behind the ear
• PFMA

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Temporal Bone Landmarks cont..

• Styloid process A thin, sharp projection found
  inferior and medial to the external auditory
  meatus.
• Covered with muscle so it is more difficult to
  identify
• PFMA
• Zygomatic process A thin, flat projection found
  anterior to the external auditory meatus.
• PFMA
• External auditory meatus The external ear canal
• Opening through which the auditory nerve runs.

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Temporal Bone Landmarks cont..

• Mandibular fossa A shallow depression found
  inferior and slightly anterior to the external
  auditory meatus
• This forms an articulation with the mandible
• Easy to see inferiorly if the mandible is removed

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Occipital Bone (1)

• Forms the posterior wall and the floor of the
  cranium.
• The spinal cord passes through this as it exits
  the cranial vault.
• Occipital bone landmarks
• External occipital protuberance A prominent
  midline projection found on the superior surface.
• Where the occipital bone turns to form the
  horizontal part.

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Occipital Bone Landmarks cont..

• Superior nuchal line Two curved ridges that
  extend laterally from the external occipital
  protuberance.
• PFMA
• Inferior nuchal line Two curved ridges that
  extend laterally from the external occipital
  protuberance, inferior to the superior nuchal
  line.
• PFMA
• Foramen magnum A large opening in the inferior
  surface of the occipital bone that allows the
  spinal cord to exit the cranial cavity.
• Largest foramen in the body.

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Occipital Bone Landmarks cont..

• Occipital condyles Paired oval-shaped
  projections found lateral to the foramen magnum.
• Form an articulation with the 1st bone of the
  spine (atlas)

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Sphenoid Bone (1)

• Forms the anterior floor of the cranial cavity.
• Also forms a portion of the lateral walls of the
  cranial cavity.
• Forms the posterior wall of the orbits of the
  eyes.
• The keystone bone for the cranium because it
  articulates with all other cranial bones.
• Shape resembles a bat with outstretched wings
  when viewed superiorly.

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Sphenoid Bone Landmarks

• Greater wing The larger, inferior projection of
  the sphenoid that forms a portion of the floor
  and the lateral walls of the cranium.
• Also forms the posterior wall of the orbits of
  the eyes.
• Lesser wing The smaller, superior projection of
  the sphenoid bone located posterior to the
  frontal bone.

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Sphenoid Bone Landmarks cont..

• Sella turcica A small, saddle-like depression
  found between the greater and lesser wings that
  surrounds and protects the pituitary gland.
• Pituitary is an important endocrine gland
• 3 parts to the sella turcica
• Tuberculum sellae The anterior portion of the
  sella turcica.
• Closest to the lesser wing.

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Sphenoid Bone Landmarks cont..

• 3 parts to the sella turcica continued
• Hypophyseal fossa The seat of the saddle.
• Where the pituitary gland resides
• Dorsum sellae The posterior portion of the sella
  turcica.
• Closer to the greater wing

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Ethmoid Bone (1)

• The small bone located anterior to the sphenoid
  bone in the middle of the frontal bone.
• Forms a small portion of the anterior floor of
  the cranium.
• Also forms a small portion of the medial wall of
  the eye orbits.
• Also forms the superior portion of the nasal
  septum.

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Ethmoid Bone Landmarks

• Cribriform plate Paired projections found
  lateral to the crista galli.
• Has small openings called the olfactory foramina.
• Olfactory foramina A series of small openings
  found within the cribriform plate that allow
  nerves from the olfactory epithelium to pass
  directly into the brain.
• These nerves give us our sense of smell.

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Ethmoid Bone Landmarks cont..

• Crista galli A small triangular projection found
  in the center of the ethmoid bone.
• Near the front of the cranial cavity.
• Point of attachment for the meninges (protective
  coverings of the brain).
• Perpendicular plate A small vertical projection
  arising from the inferior surface of the ethmoid
  bone.
• Forms the superior portion of the nasal septum.
• Articulates with the vomer (facial bone).

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Ethmoid Bone Landmarks cont..

• Superior and middle nasal conchae Two thin,
  scroll-shaped projections found lateral to the
  perpendicular plate
• The middle nasal conchae is inferior to the
  superior nasal conchae.
• These increase surface area of the nasal
  passageways
• Ethmoidal cells Air spaces found within the
  lateral masses of the ethmoid bone.
• Small sinuses

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Sutures

• Fibrous joints found between the bones of the
  cranium.
• There are 4 major sutures
• Coronal unites the frontal bone and both
  parietal bones
• Sagittal unites the two parietal bones on the
  superior midline of the skull
• Lambdoid unites the two parietal bones to the
  occipital bone.
• Squamous (2) unite the parietal and temporal
  bones on the lateral sides of the skull

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The Facial Division

• A group of 14 irregular bones that serves as
  points of attachment for muscles of the face.
• Nasal (2) form the bridge of the nose.
• Rectangular shaped bones
• PFMA
• Maxillae (2) Form the upper jaw.
• Articulate with every face bone except the lower
  jaw.
• Form part of the floors of the orbits, lateral
  walls and floor of the nasal cavity, and most of
  the hard palate (bony roof of the mouth).

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Maxillary Landmarks

• Infraorbital foramen Small openings found
  inferior to the orbits of the eyes.
• Allows passage of blood vessels and nerves.
• Palatine process a lateral projection that forms
  one half of the anterior portion of the hard
  palate.
• Typically the 2 processes unite during weeks
  10-12 of embryo development. If not, cleft
  palate will result. This negatively impacts
  speech and swallowing.

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Maxillary Landmarks cont

• Maxillary sinuses a series of small spaces
  within the maxillae.
• Empty into the nasal cavity.

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Zygomatic Bone (2)

• Form the prominence of the cheeks.
• Also form part of the lateral wall and floor of
  each orbit.
• Articulate with the frontal, maxilla, sphenoid
  and temporal bones.
• Zygomatic bone landmarks
• Temporal process a thin, flat projection arising
  from the lateral, posterior surface of the
  zygomatic bone.
• Articulates with the zygomatic process of the
  temporal bone.

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Zygomatic Bone Landmarks cont..

• Zygomatic arch created by the articulation of
  the temporal process of the zygomatic bone and
  the zygomatic process of the temporal bone.

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Lacrimal Bones (2)

• The smallest bones of the facial division.
• Resemble the shape and size of a fingernail
• Posterior and lateral to the nasal bones and form
  part of the medial wall of each orbit.
• Lacrimal bone landmark
• Lacrimal fossa a small vertical groove formed
  with the maxilla, that helps drain fluid away
  from the eye.
• Houses a lacrimal sac that gathers tears and
  passes them into the nasal cavity.

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• Palatine Bones (2) L shaped bones that form
  the posterior portion of the hard palate.
• The parts that make-up the hard palate are called
  horizontal plates.
• Inferior Nasal Conchae (2) scroll shaped bones
  that form a portion of the inferior, lateral
  walls of the nasal cavity.
• Increase surface area and help filter air along
  with the superior and middle nasal conchae of the
  ethmoid bone.

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• Vomer (1) a triangular bone that forms a portion
  of the posterior floor of the nasal cavity.
• Articulates with the perpendicular plate of the
  ethmoid bone to form the inferior portion of the
  bony nasal septum.
• Mandible (1) the largest bone of the facial
  division.
• Except for the ossicles, it is the only moveable
  skull bone.

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Mandibular Landmarks

• Mandibular body a triangular bone that forms a
  portion of the posterior floor of the nasal
  cavity.
• Ramus the short, vertical portion of the
  mandible.
• Angle the area where the ramus and the body of
  the mandible meet.
• Coronoid process a small triangular projection
  found on the superior anterior portion of the
  ramus.

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Mandibular Landmarks cont

• Condylar process a small rounded projection
  found on the superior posterior portion of the
  ramus.
• Articulates with the mandibular fossa to create
  the temporomandibular joint (TMJ).
• Mental foramen small openings found in the
  anterior surface of the body of the mandible that
  allow blood vessels and nerves to enter the
  mandible.
• Alveoli sockets for teeth

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Auditory Ossicles

• The 6 smallest bones in the human body.
• Located medial to the eardrum.
• Connected by synovial joints.
• Function to transfer sound waves from the eardrum
  to the inner ear.
• The bones are as follows
• Malleus- attaches to the eardrum and is commonly
  called the hammer.
• Incus- middle bone that is commonly called the
  anvil.
• Stapes- Smallest bone and is commonly called the
  stirrup.

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Hyoid Bone

• Located superior to the larynx (voice box).
• U shaped
• The only bone that does not articulate with
  another bone.
• Suspends from the styloid processes by ligaments
  and muscles.
• Often fractured during strangulation.
• Functions to support the tongue.

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The Vertebral Column

• Also called the spine, backbone or spinal column.
• Consists of 33 (children) or 26 (adults) bones
  called vertebrae.
• Functions to protect the spinal cord, support the
  head, and serve as attachment points for the
  ribs, pelvis, back muscles and arm muscles.

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Vertebrae

• Vary in size, shape and detail but have many
  similarities.
• Consist of 3 main parts vertebral body,
  vertebral arch and several processes.
• Vertebral body the thickened anterior portion of
  a vertebra.
• Holds the intervertebral disc and contains
  foramina for the entrance of blood vessels.
• Intervertebral discs pads of fibrocartilage that
  help hold the vertebrae in place.
• Compress throughout the day due to weight and
  water loss. This compression does not change
  height as we age.

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Vertebrae continued

• Vertebral arch located posterior to the
  vertebral body.
• Forms the vertebral foramen with the vertebral
  body.
• The vertebral arch consists of the pedicles and
  the laminae.
• Pedicles the shorter anterior portions of the
  vertebral arch.
• Laminae the longer posterior portions of the
  vertebral arch.

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Vertebrae continued

• Vertebral foramen the opening formed by the
  vertebral body and the vertebral arch.
• Contains the spinal cord, adipose tissue, areolar
  connective tissue and blood vessels.
• Processes bony projections that arise from the
  vertebral arch.
• Transverse processes paired lateral projections
  that arise from the vertebral arch.
• Spinous process the single posterior projection
  that arises from the vertebral arch.
• Both of these are PFMA

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Cervical Vertebrae

• The first 7 vertebrae (C1-C7).
• Smaller bones than other vertebrae but larger
  vertebral foramen.
• Also have 2 transverse foramen through which the
  vertebral artery, vein and nerve fibers pass.
• C2-C6 have a branching spinous process.
• C1 is called the atlas.
• Supports the skull.
• It lacks a body and a spinous process.

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Cervical Vertebrae cont

• C2 is called the axis.
• Has a body and a peglike process called the dens
  or the odontoid process.
• The dens makes a pivot on which the atlas and
  head rotate.
• C7 is called the vertebra prominens.
• Has a large spinous process that is not branched
  and can be felt at the back of the neck.

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Thoracic Vertebrae

• The next 12 vertebrae (T1-T12).
• Have facets (flat surfaces) where they articulate
  with the 12 rib pairs.
• Movement of these vertebrae are most limited
  because the ribs attach to the sternum anteriorly.

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Lumbar Vertebrae

• The next 5 vertebrae (L1-L5).
• The largest and strongest of the bones of the
  spine.
• Spinous processes are thick and broad.

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Sacrum

• The next vertebra in the adult vertebral column.
• A triangular shaped bone formed from the fusion
  of 5 bones. This starts around age 16 and is
  usually completed by age 30.
• Auricular surfaces large, ear-shaped roughened
  surfaces found on the lateral aspect of the
  sacrum.
• This articulates with the hip bones of the pelvis.

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Coccyx

• The last vertebra in the adult vertebral column.
• The tailbone that is formed by the fusion of 4
  vertebrae that occurs between the age of 20-30.
• In males, it points anteriorly and in females it
  points inferiorly.

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ACT-UP
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ACT-UP

1. What region of the vertebral column might be most
   affected by osteoporosis?
2. Why?
3. What would we expect to see (on the outside) when
   looking at a person suffering from osteoporosis?

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Thorax

• Includes the sternum, ribs and the bodies of the
  thoracic vertebrae.
• Sternum located along the anterior midline of
  the thorax.
• Also known as the breastbone
• Consists of 3 parts that fuse by age 25 and the
  points of fusion can be seen as transverse
  ridges.
• If thoracic surgery is necessary, the sternum may
  be cut along the midline.

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Regions of the Sternum

• Manubrium the superior portion of the sternum
• Articulates with the clavicles (collarbones) and
  the costal cartilages of the 1st-2nd rib pairs.
• Sternal body the intermediate portion of the
  sternum.
• Articulates directly or indirectly with the
  costal cartilages of the 2nd-10th rib pairs.
• Xiphoid process the inferior portion of the
  sternum.
• Where some abdominal muscles attach.

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Ribs

• 12 pairs of flat bones that form a protective
  cage around the heart and the lungs.
• Increase in length from 1-7 and then decrease
  from 7-12.
• Each rib pair articulates posteriorly with its
  corresponding thoracic vertebra.
• Costal cartilage elongated pads of hyaline
  cartilage used to attach the ribs to the sternum.
• Allows the ribcage to be more elastic and limits
  fracturing from blows to the chest.

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Types of Ribs

• True ribs Rib pairs 1-7
• Their costal cartilages attach directly to the
  sternum.
• False ribs Rib pairs 8-12
• Their costal cartilages do not attach directly to
  the sternum. Rib pairs 8-10 have cartilages that
  attach to the cartilage of 7 (which attaches to
  the sternum.
• Floating ribs Rib pairs 11-12
• The costal cartilages do not attach to the
  sternum at all.

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