Chapter 6 - Axial Skeleton

Components of Axial Skeleton:
The skull that contains 22 bones.
The bones associated with the skull including the ear ossicles and hyoid bones.
The vertebral column that consists of 24 vertebrae, 1 sacrum and 1 coccyx. 
The thoracic cage with 24 ribs and one sternum.
Functions of the Axial Skeleton
     The axial skeleton forms a framework for the organs of the ventral and dorsal body cavities. The bones form an extensive surface area for muscle attachment. Many bones contain red marrow that is involved in hemopoiesis. 
Skull
     The skull contains 22 bones of which 8 form the cranium that encloses and protects the brain suspended in the cranial cavity and 14 facial bones that protect and support the digestive and respiratory tracts. 
     The skull bones, with a few exceptions, are connected by immovable joints called sutures. Some of the major sutures are:
Lambdoid suture - between the occipital and parietal bones. Sutural bones are often found here. 
Sagittal suture - between the parietal bones. 
Coronal suture - between the frontal and parietal bones. 
Squamous suture - between the parietal and temporal bones. 
Frontonasal suture - between the frontal and nasal bones.
Bones of the Cranium
Occipital Bone
     The occipital bone contributes to the posterior, lateral and inferior surfaces of the cranium. The following features are found on this bone:
foramen magnum - large circular opening that connects the cranial and spinal cavities. 
occipital condyles - processes that articulate with the first cervical vertebra. 
external occipital protuberance - midline bump on the external surface. 
jugular notch - together with the temporal bone forms the jugular foramen. 
hypoglossal canal - openings superior to the occipital condyles through which the hypoglossal nerves pass. 
Parietal Bones
     These bones contribute to the superior and lateral surface of the cranium. 
Frontal Bone
     The frontal bones have frontal and orbital parts.
Frontal (Squamous) Part - the part of the frontal bone that underlies the forehead. 
frontal (metopic) suture - suture between the frontal bones that usually disappears by age 8. 
supraorbital margin - edge of the frontal part that forms the superior border of the orbits. 
supraorbital foramen or notch - opening or notch in the middle of the supraorbital margin. 
superciliary arches - thickened ridges over the supraorbital margins that supports the eyebrows. 
Orbital Part - the part that forms the roof of the orbit. 
lacrimal fossa - depression on the inferior surface of the orbital part that accommodates the lacrimal gland.
Temporal Bones
     The temporal bones contribute to the lateral and inferior walls of the cranium. The temporal bone can be divided into three parts:
Squamous part
     The squamous part forms the lateral surface that borders the squamous suture. Features found here include:
zygomatic process - forms the inferior margin of the squamous part. This process with the temporal process of the zygomatic bone forms the zygomatic arch (cheekbone).
mandibular fossa - depression on the inferior base of the zygomatic process that articulates with the mandible.
articular tubercle - elevation anterior to the mandibular fossa.
Tympanic part
     This part is the region that surrounds the external acoustic meatus or external auditory canal. This passageway ends as a tympanic membrane.
Petrous part
     This is the largest most massive part of the temporal bone that houses the senses of hearing and balance. Features on this part are:
mastoid process - bulge posterior and inferior to the external acoustic meatus. Contains mastoid sinuses.
styloid process - sharp process near the mastoid process to which ligaments and tendons attach.
stylomastoid foramen - opening between the base of the styloid and mastoid processes through which the facial nerve passes.
jugular fossa - depression on the temporal bone that together with the jugular notch of the occipital bone forms the jugular foramen.
carotid canal - passageway by which the internal carotid artery penetrates the skull and reaches the brain.
foramen lacerum - jagged opening between the temporal and occipital bones. This is not an opening in the living skull as it is closed by hyaline cartilage.
internal acoustic meatus - canal on the medial side of the petrous part through which nerves and vessels supplying the inner ear and the facial nerve pass.
Sphenoid
     This complex bone articulate with every other cranial bone. Although large, most of the sphenoid is hidden by more superficial bones. The sphenoid can be divided into the following parts:
Body
     The body is the central portion of the sphenoid. A prominent feature of the body is the sella turcica ("Turkish saddle") that forms a bony enclosure around the pituitary gland. The sella turcica can be divided into the following parts:
hypophyseal fossa - depression in which the pituitary gland rests ("seat of saddle").
tuberculum sellae - anterior border of the sella turcica ("horn of saddle").
optic groove - groove directly anterior to the tuberculum sellae.
optic canals - openings at either end of the optic groove through which the optic nerves pass.
dorsum sellae - posterior border of the sella turcica ("back of saddle").

To the left is a picture of a Turkish saddle. 

Lesser Wings
     Wing-like extensions anterior to the sella turcica.
Greater Wings
    Large wing-like processes that extend laterally from the body. The greater wings act as a brace that strengthens the sides of the skull.
superior orbital fissure - irregular slit-like opening between the lesser and greater wings.
     The greater wings near their attachment to the body have the following openings:
foramen rotundum - a round opening.
foramen ovale - an oval opening.
foramen spinosum - opening where the greater wings form a sharp process called the sphenoidal spine.
Pterygoid Processes
     Extending downward on either side of the body are processes called the pterygoid processes. These processes form plates that permit muscle attachment. Pterygoid canals are present at the base of these processes.
Ethmoid
     This irregular bone forms part of the orbital complex, floor of the cranium, roof of the nasal cavity, and part of the nasal septum. Features of this bone include:
cribriform plate - superior surface of the ethmoid perforated by olfactory foramina through which the olfactory nerves pass.
crista galli - prominent ridge in the middle of the cribriform plate and an attachment point for the falx cerebri.
lateral masses - entire bony masses attached on either side of the cribriform plate. The lateral masses include:
superior nasal conchae and middle nasal conchae - scroll-like medial projections into the nasal cavity.
ethmoidal labyrinth - cells in the interior of the lateral masses that open into the nasal cavity.
perpendicular plate - bony partition underneath the cribriform plate that forms part of the nasal septum.
Cranial Fossae
     When looking into the open skull from above, the interior contours of the cranium form curving depressions at three levels. These depressions are the cranial fossae and include:
anterior cranial fossa - formed by frontal, ethmoid and lesser wings of sphenoid.
middle cranial fossa - formed by body and greater wings of sphenoid, temporal bone up to the petrous ridge and parietal bones.
posterior cranial fossa - formed primarily by occipital bone with a contribution from temporal and parietal bones. 
Bones of the Face
Maxillae 
     The largest of the facial bones forms together the upper jaw. Features of the maxillae include:
alveolar processes - horseshoe-shaped oral margin that contains the teeth.
inferior orbital fissure - jagged, slit-like opening between the maxilla and sphenoid in the orbit. 
infraorbital foramen - opening that penetrates the orbital rim. 
maxillary sinus - largest sinuses in the skull. Lightens the maxilla superior to the teeth and produces mucus that drains into the nasal cavity. 
palatine process - horizontal process that forms the hard palate.
incisive canals - openings through the palatine process behind the central incisors (front teeth). 
Palatine Bones
     These bones are small and L-shaped bones. It has a horizontal plate and perpendicular plate. Features associated with this bone include:
greater palatine foramen - opening between the horizontal plate and palatine process of maxilla. 
lesser palatine foramen - smaller openings behind the greater palatine foramen. 
Nasal Bones 
     Small paired bones that articulate with frontal bones and frontal process of maxillae. 
Inferior Nasal Conchae
     Paired scroll-like bones on the lateral wall of the nasal cavity. 
Zygomatic Bones 
     The zygomatic bones form the lateral rim of the orbit. Features include:
temporal process - articulates with the zygomatic process of the temporal bone to form the zygomatic arch. 
zygomaticofacial foramen - opening on anterior surface of zygomatic bone. 
Lacrimal Bones
     The smallest bones of the skull located in the medial wall of the orbit. Features include:
Lacrimal Groove - depression that leads to the ...
Vomer
     A bone that forms the inferior portion of the nasal septum. 
Mandible
     The mandible forms the lower jaw. The entire bone can be divided into a horizontal body and ascending rami. The ramus meets the body at the angle. Features of the ramus include:
condylar processes - processes that form the temporomandibular joint (TMJ) with the temporal bone. 
coronoid processes - processes on which the temporalis muscle inserts.
mandibular notch - depression between the coronoid and condylar processes. 
mylohyoid line - ridge on medial side of ramus serves as attachment for the mylohyoid muscle.
mandibular foramen - opening for blood vessels and nerves for lower teeth. 
     Features on the body include:
mental foramina - openings for nerves and blood vessels for the lower lip and chin. 
alveolar process - horseshoe-shaped part that contains the roots of the lower teeth. 
Orbital Complex
     The orbits are the bony recesses that enclose and protect the eyes. The orbital complex consists of the bones that form the orbit and include the maxilla, lacrimal bone, ethmoid, palatine bone, sphenoid, frontal bone and zygomatic bone.
Nasal Complex
     The nasal complex includes all the bones that form the nasal cavity and paranasal sinuses.
Paranasal Sinuses
     The paranasal sinuses are air-filled chambers that are connected to the nasal cavity. These sinuses are found in the sphenoid, ethmoid, frontal bones and maxillae. Functions of the sinuses include:
1. Lighten the skull.
2. Produce mucus.
3. Resonate during sound production.
Hyoid Bone
     The only bone in the body that does not articulate directly with another. It lies inferior to the skull suspended by the stylohyoid ligament. Features include:
body - central rectangular part of the bone.
greater horns - large processes that extend on either side of the body.
lesser horns - smaller processes that extend on either side of the body.
     The hyoid provides an attachment surface for muscles and ligaments that stabilize and move the tongue and larynx.
Skulls of Infants
     The skull bones develop from the fusion of multiple ossification centers. Until fusion, the separate bones are attached by fibrous connective tissue. These fibrous regions form the "soft spots" on the newborn's head are are called fontanels.