The Skeletal System: The Axial Skeleton Study Notes on the Axial Skeleton

Divisions of the Skeletal System

• The human skeleton consists of a total of $206$ named bones.
• Bones are grouped into two principal divisions: the Axial skeleton and the Appendicular skeleton.
• Axial Skeleton: Consists of bones that lie around the longitudinal axis of the human body. It includes a subtotal of $80$ bones:
  • Skull ($22$ bones total): Divided into the Cranium ($8$ bones) and Face ($14$ bones).
  • Hyoid ($1$ bone): Located in the neck.
  • Auditory ossicles ($6$ bones): Ear bones.
  • Vertebral column ($26$ bones).
  • Thorax ($25$ bones): Includes the Sternum ($1$ bone) and Ribs ($24$ bones).
• Appendicular Skeleton: Consists of bones of the upper and lower limbs (extremities) and the girdles that connect them to the axial skeleton. It includes a subtotal of $126$ bones:
  • Pectoral (shoulder) girdles ($4$ bones total): Clavicle ($2$) and Scapula ($2$).
  • Upper limbs ($60$ bones total): Humerus ($2$), Ulna ($2$), Radius ($2$), Carpals ($16$), Metacarpals ($10$), and Phalanges ($28$).
  • Pelvic (hip) girdle ($2$ bones): Hip, pelvic, or coxal bone.
  • Lower limbs ($60$ bones total): Femur ($2$), Patella ($2$), Fibula ($2$), Tibia ($2$), Tarsals ($14$), Metatarsals ($10$), and Phalanges ($28$).

Types of Bones

Bones are classified into five major types based on their shape:

• Long Bones: Characterized by having a greater length than width and being slightly curved for strength. Examples include the Femur, tibia, fibula, humerus, ulna, radius, and phalanges.
• Short Bones: Cube-shaped and nearly equal in length and width. Examples include the Carpal and tarsal bones.
• Flat Bones: Thin bones composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue. Examples include Cranial bones, the sternum, ribs, and scapulae.
• Irregular Bones: Have complex shapes and cannot be grouped into other categories. Examples include the Vertebrae, hip bones, some facial bones, and the calcaneus.
• Sesamoid Bones: These protect tendons from excessive wear and tear. Examples include the Patellae and certain bones in the foot and hand.
• Sutural Bones: Small bones located specifically within the sutures of cranial bones.

Bone Surface Markings: Depressions and Openings

Surface markings are structural features adapted for specific functions such as passage of soft tissues or joint formation.

• Fissure: A narrow slit between adjacent parts of bones through which blood vessels or nerves pass. Example: Superior orbital fissure of the sphenoid bone.
• Foramen: An opening (hole) through which blood vessels, nerves, or ligaments pass. Example: Optic foramen of the sphenoid bone.
• Fossa: A shallow depression (trench). Example: Coronoid fossa of the humerus.
• Sulcus: A furrow along a bone surface that accommodates a blood vessel, nerve, or tendon (groove). Example: Intertubercular sulcus of the humerus.
• Meatus: A tubelike opening or passageway. Example: External auditory meatus of the temporal bone.

Bone Surface Markings: Processes

Processes are projections or outgrowths that either form joints or serve as attachment points for connective tissues (ligaments and tendons).

Processes that Form Joints:

• Condyle: A large, round protuberance at the end of a bone (knuckle-like). Example: Lateral condyle of the femur.
• Facet: A smooth, flat articular surface. Example: Superior articular facet of a vertebra.
• Head: A rounded articular projection supported on the neck (constricted portion) of a bone. Example: Head of the femur.

Processes that Form Attachment Points:

• Crest: A prominent ridge or elongated projection. Example: Iliac crest of the hip bone.
• Epicondyle: A projection located above a condyle. Example: Medial epicondyle of the femur.
• Line (linea): A long, narrow ridge or border (less prominent than a crest). Example: Linea aspera of the femur.
• Spinous process: A sharp, slender projection. Example: Spinous process of a vertebra.
• Trochanter: A very large projection. Example: Greater trochanter of the femur.
• Tubercle: A small, rounded projection (knob-like). Example: Greater tubercle of the humerus.
• Tuberosity: A large, rounded, usually roughened projection. Example: Ischial tuberosity of the hip bone.

The Skull (Cranium)

The skull consists of $22$ bones and includes various cavities: the large cranial cavity, the nasal cavity, the orbits (eye sockets), paranasal sinuses, and small cavities housing hearing and equilibrium organs.

Cranial Bones ($8$ total): Form the cranial cavity to protect the brain.

• Frontal Bone: Forms the forehead and the supraorbital margin.
• Parietal Bones ($2$): Form the sides and roof of the cranial cavity.
• Temporal Bones ($2$): Form the lateral aspects and floor of the cranium; contain features like the external auditory meatus, mastoid process, and styloid process.
• Occipital Bone: Forms the posterior part and most of the base of the cranium; contains the foramen magnum and occipital condyles.
• Sphenoid Bone: Situated at the middle part of the base of the skull (the "keystone" bone). Contains the sella turcica (housing the hypophyseal fossa), optic foramina, and pterygoid processes.
• Ethmoid Bone: Located on the midline in the anterior part of the cranial floor, medial to the orbits. It is a major superior supporting structure of the nasal cavity and contains projections called conchae (turbinates) lined with mucous membranes to humidify air and trap particles. Key structures include the crista galli and cribriform plate.

Facial Bones ($14$ total): Form the face framework and support entrances to digestive and respiratory systems.

• Nasal Bones ($2$): Form the bridge of the nose.
• Maxillae ($2$): Form the upper jawbone and most of the hard palate (separating the nasal and oral cavities).
• Zygomatic Bones ($2$): Commonly called cheekbones; form the prominences of the cheeks.
• Lacrimal Bones ($2$): Form part of the medial wall of each orbit; include the lacrimal fossa.
• Palatine Bones ($2$): Form the posterior portion of the hard palate.
• Inferior Nasal Conchae ($2$): Form part of the inferior lateral wall of the nasal cavity.
• Vomer: Forms the inferior portion of the nasal septum.
• Mandible: The lower jawbone; the largest and strongest facial bone and the only movable skull bone.

Unique Features of the Skull

• Sutures: Immovable joints that hold most skull bones together. Major sutures include the Coronal, Sagittal, Lambdoid, and Squamous sutures.
• Paranasal Sinuses: Cavities within bones near the nasal cavity. They produce secretions that drain into the nasal cavity and serve as resonating chambers that intensify and prolong sounds. Locations include the frontal, ethmoid, sphenoid, and maxillary bones.
• Fontanels: Areas of unossified tissue ("soft spots") linking cranial bones at birth. They provide flexibility to the fetal skull for passage through the birth canal and are eventually replaced by bone to become sutures. Types include:
  • Anterior fontanel.
  • Posterior fontanel.
  • Anterolateral fontanels.
  • Posterolateral fontanels.
• Nasal Septum: Divides the interior of the nasal cavity into right and left sides. Damage to the septum is often what occurs in a "broken nose."

Hyoid Bone

• The hyoid bone is unique because it does not articulate with any other bone.
• It supports the tongue and provides attachment sites for muscles of the tongue, neck, and pharynx.
• It functions to keep the larynx (voice box) open at all times.

The Vertebral Column

Also known as the spine, backbone, or spinal column, the vertebral column consists of a series of vertebrae ($26$ in adults).

Functions:

• Protects the spinal cord.
• Supports the head.
• Serves as an attachment point for ribs, pelvic girdle, and muscles.

Normal Curves:

• Curves (Cervical, Thoracic, Lumbar, Sacral) increase column strength, maintain balance, absorb shocks during walking, and protect vertebrae from fracture.
• A fetus has a single curve; adults have four.

Abnormal Curves:

• Scoliosis: Lateral curvature.
• Kyphosis: Exaggerated thoracic curvature.
• Lordosis: Exaggerated lumbar curvature.
• Regions of the Vertebral Column:
• Cervical Region: $7$ vertebrae ($C1-C7$). $C1$ is the Atlas; $C2$ is the Axis (featuring the dens).
• Thoracic Region: $12$ vertebrae ($T1-T12$). Articulate with the ribs.
• Lumbar Region: $5$ vertebrae ($L1-L5$). Support the lower back and attach large back muscles.
• Sacrum: A triangular bone formed by the union of $5$ fused sacral vertebrae ($S1-S5$). Serves as a strong foundation for the pelvic girdle.
• Coccyx: Triangular bone formed by the fusion of usually $4$ fused coccygeal vertebrae.

Vertebral Structure and Intervertebral Discs

Intervertebral Discs:

• Located between adjacent vertebral bodies.
• Composed of an outer ring called the annulus fibrosus.
• Functions: Form strong joints, permit movement, and absorb vertical shock.

Typical Vertebral Components:

• Body: Ventral, weight-bearing portion.
• Vertebral Arch: Surrounds the spinal cord; composed of pedicles and laminae.
• Processes: Serve as attachment points for muscles (Spinous, Transverse, Articular).

Structural Comparison of Vertebrae:

• Cervical: Small body; one vertebral and two transverse foramina; slender/bifid spinous processes ($C2-C6$); thick intervertebral discs relative to body size.
• Thoracic: Larger body; long/thick spinous processes projecting inferiorly; present articular facets/demifacets for ribs; thin intervertebral discs relative to body size.
• Lumbar: Largest, massive body; short/blunt spinous processes projecting posteriorly; absent articular facets for ribs; medial/lateral direction of articular facets.

The Thorax

The thoracic cage provides structural support for the thoracic cavity and protects vital organs.

Components of the Thoracic Cage:

• Sternum ("Breastbone"): Located in the center of the thoracic wall. It consists of three parts:
  1. Manubrium: Superior portion, includes the suprasternal notch and clavicular notches.
  2. Body: Middle portion.
  3. Xiphoid Process: Inferior portion.
• Ribs: $12$ pairs that provide support to the sides of the thoracic cavity (Total of $24$ bones). Features include the head, neck, tubercle, costal angle, and costal groove.
• Costal Cartilages: Contribute to the elasticity of the thoracic cage.
• Thoracic Vertebrae: Provide posterior support and articulation points for the ribs.

Functions of the Thorax:

• Enclose and protect organs in the thoracic and abdominal cavities.
• Provide support for upper limb bones.
• Play a critical role in breathing.
• Intercostal spaces are the gaps between the ribs occupied by muscles and nerves.