Comprehensive Study Notes on the Skeletal System and Axial Skeleton

Definition of Bones as Organs: Bones are considered structural organs because they are composed of two or more distinct tissue types working together.
Skeletal System Composition: The skeletal system is a comprehensive organ system consisting of:
- Bones
- Cartilage
- Ligaments
- Tendons
Mass Contribution: The skeletal system accounts for approximately $20\%$ of total human body weight.
Primary Bone Types by Shape: Bones come in various shapes and sizes, organized into four primary classifications:
- Long bones
- Short bones
- Flat bones
- Irregular bones
Primary Systemic Divisions: The skeletal system is divided into two major divisions:
- Axial skeletal system
- Appendicular skeletal system

Total Bone Counts:
- The adult human skeleton usually consists of $206$ named bones.
- The axial skeleton comprises $80$ bones that form the vertical central axis of the body.
Anatomical Regions Included: The axial skeleton encompasses the bones of the:
- Head
- Neck
- Vertebral column
- Ribs
- Sternum
Major Subdivisions of the Axial Skeleton: The axial skeleton has three major subdivisions:
- Skull
- Thoracic cage
- Vertebral column

General Composition: The skull consists of $22$ bones in total, divided into cranial bones and facial bones.
Developmental Features and Ossification:
- At birth, fibrous membranes connect the growing skull bones in areas that have not yet undergone complete intramembranous ossification.
- Fontanels: Known as the "soft spots" of the infant skull, these fibrous membranous areas undergo intramembranous ossification later in life to become rigid bone sutures.
Sutures of the Skull: Immovable, interlocking fibrous joint structures that connect the various bones of the skull:
- Coronal Suture: Located superiorly/laterally along the coronal plane.
- Squamous Suture: Connects the temporal bone to the parietal bone laterally.
- Lambdoid Suture: Interlocks to connect the occipital bone and the parietal bones.
Inferior Cranial Features: An inferior view of the skull reveals numerous foramina and structural features that allow passage for nerves, blood vessels, and other connective tissues and organs to enter and exit the skull.

Cranial Bones ( $8$ Bones Total): Form the cranium to enclose and protect the brain.
- Frontal bone ( $1$ )
- Parietal bones ( $2$ )
- Temporal bones ( $2$ )
- Sphenoid bone ( $1$ )
- Ethmoid bone ( $1$ )
- Cranial bones are held together by bone sutures involving the parietal bone, frontal bone, occipital bone, and temporal bone.
Facial Bones ( $14$ Bones Total): Form the facial skeleton and provide attachment surfaces for muscles that allow facial expressions and movement.
- Maxilla ( $2$ )
- Palatine bones ( $2$ )
- Zygomatic bones ( $2$ )
- Lacrimal bones ( $2$ )
- Nasal bones ( $2$ )
- Vomer ( $1$ )
- Inferior nasal conchae ( $2$ )
- Mandible ( $1$ )
Anatomical Landmarks of the Jaw and Mastication:
- Key structural features of the jaw and facial section include the teeth, mandible, coronoid process, mental foramen, and mandibular condyle.
- Together with skeletal muscles and connective tissues, these jaw bones are essential for mechanical digestion (the chewing of food).

Anatomical Definition: Paranasal sinuses are air-filled cavities located within specific cranial and facial bones.
Mucous Membrane Lining: They are lined with mucous membranes that are directly continuous with and connect to the nasal cavity.
Locations of Paranasal Sinuses:
- Frontal sinus
- Ethmoid sinus
- Sphenoidal sinus
- Maxillary sinus
Physiological Functions:
- Reduce the overall weight of the skull.
- Increase the sound resonance of the voice.

Vertebrate Classification: Humans are vertebrates due to the presence of a central vertebral column (backbone), relying on a sturdy internal frame centered on a prominent spine.
Extent and Bone Count:
- Extends from the skull to the pelvis.
- The adult vertebral column consists of $26$ bones.
Intervertebral Discs: The irregularly shaped vertebrae are cushioned by pads of fibrocartilage known as intervertebral discs.
Structural Features of a Typical Vertebra:
- Body: A drum-shaped structure positioned toward the anterior portion of the bone.
- Vertebral Arch: Formed around the vertebral foramen by the pedicles, laminae, and spinous process.
- Laminae: Two plates arising from the pedicle that fuse together posteriorly to create the single spinous process.
- Vertebral Foramen: The central opening through which the spinal cord passes, collectively forming the continuous vertebral canal.
- Transverse Processes: Project anteriorly and posteriorly between the pedicles and laminae.
Sacrum:
- A triangular bone structure located at the base of the vertebral column.
- Composed of $5$ fused vertebrae.
- Features the sacral hiatus as its key attachment landmark.

Total Curvatures: The adult vertebral column features $4$ distinct curves for flexibility and structural support.
Primary Curvatures (Concave Anteriorly):
- Thoracic Curvature: Located in the chest region.
- Sacral Curvature: Located in the sacrum region.
Secondary Curvatures (Convex Anteriorly):
- Cervical Curvature: Located in the neck region.
- Lumbar Curvature: Located in the lower back region.
Magnetic Resonance Imaging (MRI) Context: Cervical vertebrae, thoracic vertebrae inferior to the cervical region, lumbar vertebrae, and the terminal fused sacral vertebrae can be visually distinguished via MRI scans.

Components: The thoracic cage consists of the ribs, sternum, thoracic vertebrae, and costal cartilage.
Functions:
- Supports the pectoral girdle and upper limbs.
- Plays an active role in breathing.
- Protects vital organs, soft tissues, and viscera within the thoracic and upper abdominal cavities (specifically protecting the heart and lungs).
Subdivisions of the Sternum:
- Manubrium: Superior portion.
- Body: Central main portion.
- Xiphoid Process: Inferior tip.
Anatomy and Classification of Ribs:
- Total Count: $24$ total ribs, organized into $12$ bilateral pairs located on the left and right sides of the midline.
- Structural Features of an Individual Rib: Head, neck, shaft, costal groove, and sternal (anterior) end.
- True Ribs: The first $7$ pairs of ribs (ribs $1$ through $7$ ); classified as true ribs because they connect directly to the sternum via costal cartilage.
- False Ribs: The remaining $5$ pairs of ribs (ribs $8$ through $12$ in descending order); classified as false ribs because they do not reach the sternum directly.