The Skeletal System: The Axial Skeleton Study Notes

Divisions of the Skeletal System

The adult human skeleton is composed of $206$ named bones, which are organized into two primary divisions based on their location and function:

• Axial Skeleton: This division consists of $80$ bones positioned along the body's longitudinal axis. It includes the skull bones ($8$ cranial and $14$ facial), auditory ossicles ($6$), hyoid bone ($1$), the vertebral column ($26$), and the thorax (composed of $1$ sternum and $24$ ribs).
• Appendicular Skeleton: This division consists of $126$ bones, including the upper and lower limbs ($60$ bones per side/set including phalanges) and the pectoral and pelvic girdles that anchor the limbs to the axial skeleton. Specific counts include the pectoral girdles ($2$ clavicles, $2$ scapulae), upper limbs ($2$ humeri, $2$ ulnae, $2$ radii, $16$ carpals, $10$ metacarpals, $28$ phalanges), pelvic girdle ($2$ hip/coxal bones), and lower limbs ($2$ femurs, $2$ patellae, $2$ fibulae, $2$ tibiae, $14$ tarsals, $10$ metatarsals, $28$ phalanges).

Classification of Bones by Shape

Bones are classified into six distinct categories based on their morphology:

• Long Bones: Characterized by having a greater length than width and a slight curvature for strength. Examples include the femur, tibia, fibula, humerus, ulna, radius, and phalanges.
• Short Bones: Mostly cube-shaped, being nearly equal in length and width. Examples include carpal and tarsal bones.
• Flat Bones: Thin structures formed by two parallel plates of compact bone enclosing a layer of spongy bone. Examples include cranial bones, the sternum, ribs, and scapulae.
• Irregular Bones: Possess complex shapes that do not fit other categories. Examples include vertebrae, hip bones, certain facial bones, and the calcaneus.
• Sesamoid Bones: Small bones located within tendons to protect them from wear and tear. The most prominent examples are the patellae.
• Sutural Bones: Small, variable bones found within the sutures between cranial bones.

Bone Surface Markings

These structural features are adapted for specific functions and are divided into two main types:

1. Depressions and Openings These allow the passage of nerves, blood vessels, and ligaments, or help form joints:

• Fissure: A narrow slit between adjacent bone parts (e.g., superior orbital fissure).
• Foramen: A hole/opening for passageway (e.g., optic foramen).
• Fossa: A shallow depression or trench (e.g., coronoid fossa).
• Sulcus: A furrow or groove along a surface (e.g., intertubercular sulcus).
• Meatus: A tube-like opening or passageway (e.g., external auditory meatus).

2. Processes Projections that either form joints or provide attachment points for connective tissues:

• Condyle: A large, round protuberance at the end of a bone (e.g., lateral condyle).
• Facet: A smooth, flat articular surface (e.g., superior articular facet).
• Head: A rounded projection supported by a constricted neck (e.g., head of the femur).
• Crest: A prominent ridge or elongated projection (e.g., iliac crest).
• Epicondyle: A projection above a condyle (e.g., medial epicondyle).
• Line (Linea): A long, narrow ridge, less prominent than a crest (e.g., linea aspera).
• Spinous Process: A sharp, slender projection (e.g., spinous process of a vertebra).
• Trochanter: A very large projection (e.g., greater trochanter).
• Tubercle: A small, rounded projection (e.g., greater tubercle of the humerus).
• Tuberosity: A large, rounded, and typically roughened projection (e.g., ischial tuberosity).

The Skull

The skull consists of $22$ bones protecting the brain and special sense organs. It contains several cavities: the cranial cavity, nasal cavity, orbits, paranasal sinuses, and hearing/equilibrium housing.

Cranial Bones ($8$) These form the cranial cavity and protect the brain:

• Frontal Bone: Forms the forehead.
• Parietal Bones ($2$): Form the sides and roof of the cavity.
• Temporal Bones ($2$): Form the lateral aspects and floor of the cranium.
• Occipital Bone: Forms the posterior base of the cranium.
• Sphenoid Bone: Located at the middle part of the skull base.
• Ethmoid Bone: Midline structure anterior to the cranial floor; contains conchae to increase nasal surface area for humidifying air and trapping particles.

Facial Bones ($14$) These form the framework of the face and support respiratory/digestive entrances:

• Nasal Bones ($2$): Form the bridge of the nose.
• Maxillae ($2$): Form the upper jaw and most of the hard palate (separating nasal and oral cavities).
• Zygomatic Bones ($2$): Cheekbones forming the cheek prominences.
• Lacrimal Bones ($2$): Medial walls of the orbits.
• Palatine Bones ($2$): Posterior portion of the hard palate.
• Inferior Nasal Conchae ($2$): Inferior lateral walls of the nasal cavity.
• Vomer: Inferior portion of the nasal septum.
• Mandible: The lower jawbone; the largest, strongest, and only movable bone in the skull.

Unique Skull Features

• Sutures: Immovable joints holding skull bones together (e.g., Coronal, Sagittal, Lambdoid, Squamous).
• Paranasal Sinuses: Mucous membrane-lined cavities that lighten the skull and act as resonating chambers for sound.
• Fontanels: "Soft spots" of unossified tissue in fetuses/infants that allow the skull to flex during birth and later ossify into sutures.
• Nasal Septum: Divides the nasal cavity. A "broken nose" typically refers to damage here.

The Hyoid Bone

The hyoid bone is unique because it does not articulate with any other bone. It supports the tongue, providing attachment for tongue, neck, and pharyngeal muscles, and keeps the larynx open.

The Vertebral Column

The spinal column protects the spinal cord and supports the head. In adults, it consists of $26$ vertebrae:

• Cervical ($7$): Neck region. $C1$ is the Atlas; $C2$ is the Axis.
• Thoracic ($12$): Posterior to the thoracic cavity; articulate with ribs.
• Lumbar ($5$): Support the lower back.
• Sacrum ($1$): Formed by the fusion of $5$ sacral vertebrae ($S1-S5$).
• Coccyx ($1$): Formed by the fusion of approximately $4$ coccygeal vertebrae.

Curvature and Abnormalities Adults have four normal curves: cervical, thoracic, lumbar, and sacral. These help balance, strength, and shock absorption. Exaggerated curves include:

• Kyphosis: Exaggerated thoracic curve.
• Lordosis: Exaggerated lumbar curve.
• Scoliosis: Lateral curvature.

Vertebral Structure Vertebrae generally consist of a weight-bearing body, a vertebral arch surrounding the spinal cord, and processes for muscle attachment. Intervertebral discs are located between bodies to absorb shock and permit movement.

The Thorax

The thoracic cage consists of the sternum, ribs, costal cartilages, and thoracic vertebrae. It protects thoracic/abdominal organs and supports the upper limbs.

• Sternum: The "breastbone," consisting of the manubrium, body, and xiphoid process.
• Ribs: $12$ pairs that provide structural support to the sides of the cavity.
• Costal Cartilages: Contribute to the elasticity of the thoracic cage.