The Skeletal System: Axial Gross Anatomy

The Skeletal System: Axial Gross Anatomy

Overview of the Skeletal System

The human skeleton is divided into two main divisions: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of the bones of the limbs and girdles that attach them to the axial skeleton.

• Axial Skeleton: Skull, vertebral column, thoracic cage

• Appendicular Skeleton: Limbs and girdles (pectoral and pelvic)

Bone Markings

Types and Functions of Bone Markings

Bone markings are specific features on bones that serve as sites for muscle, ligament, and tendon attachment, as well as passages for nerves and blood vessels, and points of articulation for joints.

• Sites of muscle or ligament attachment: tuberosity, crest, trochanter, line, tubercle, epicondyle, spine

• Sites involved in joints: head, condyle, ramus

• Passages for nerves and blood vessels: meatus, sinus, fossa, foramen

Definitions:

• Tuberosity: Large rounded projection; may be roughened

• Crest: Narrow ridge of bone; usually prominent

• Trochanter: Very large, blunt, irregularly shaped process (only on femur)

• Line: Narrow ridge of bone; less prominent than a crest

• Tubercle: Small rounded projection or process

• Epicondyle: Raised area on or above a condyle

• Spine: Sharp, slender, often pointed projection

• Head: Bony expansion carried on a narrow neck

• Condyle: Rounded articular projection

• Ramus: Armlike bar of bone

• Meatus: Canal-like passageway

• Sinus: Cavity within a bone, filled with air and lined with mucous membrane

• Fossa: Shallow, basinlike depression in a bone

• Foramen: Round or oval opening through a bone

Bones of the Axial Skeleton

Major Components

The axial skeleton consists of the skull, vertebral column, and thoracic cage. These structures protect vital organs and provide support for the body.

• Skull: Cranium and facial bones

• Vertebral Column: Cervical, thoracic, lumbar vertebrae, sacrum, coccyx

• Thoracic Cage: Sternum, ribs, and costal cartilages

The Skull

Cranium and Facial Bones

The skull is composed of two sets of bones: the cranium (which encloses the brain) and the facial bones (which form the structure of the face). The cranium consists of 8 bones, while the face is formed by 14 bones.

• Cranial Bones (8): Frontal, Parietal (2), Occipital, Temporal (2), Sphenoid, Ethmoid

• Facial Bones (14): Nasal (2), Maxillae (2), Zygomatic (2), Mandible, Lacrimal (2), Palatine (2), Inferior nasal conchae (2), Vomer

Major Sutures of the Skull

Sutures are immovable joints that connect the flat bones of the cranium. The four main sutures are:

• Coronal suture: Between frontal and parietal bones

• Sagittal suture: Between the two parietal bones

• Squamous suture: Between temporal and parietal bones

• Lambdoid suture: Between parietal and occipital bones

The Sphenoid Bone

The sphenoid bone is a complex bone located at the base of the skull. It is considered the "keystone" of the cranial floor because it articulates with all other cranial bones.

• Key features: Sella turcica, greater and lesser wings, pterygoid processes, optic canal, superior orbital fissure

The Ethmoid Bone

The ethmoid bone forms part of the anterior cranial fossa, the nasal septum, and the walls of the nasal cavity and orbits. It contains several important structures:

• Cribriform plate: Contains olfactory foramina for passage of olfactory nerves

• Crista galli: Attachment for the brain's dura mater

• Perpendicular plate: Forms part of the nasal septum

• Superior and middle nasal conchae: Increase surface area in the nasal cavity

Facial Bones and Markings

The facial bones provide the framework for the face, house the special sense organs, and provide openings for air and food passage. The mandible is the only moveable bone of the skull (excluding the ossicles of the middle ear).

• Mandible: Body, ramus, coronoid process, condylar process, mandibular notch, alveolar process, mandibular foramen, mental foramen

• Maxillae: Alveolar process, infraorbital foramen

• Palatine: Horizontal plate, perpendicular plate

Bones of the Nasal Cavity and Paranasal Sinuses

The nasal cavity is formed by several bones and contains structures that help warm, moisten, and filter air. The paranasal sinuses are air-filled spaces within certain skull bones that lighten the skull and enhance voice resonance.

• Functions of nasal conchae: Increase surface area, warm and moisten air, trap particles

• Paranasal sinuses: Located in the frontal, sphenoid, ethmoid, and maxillary bones

The Fetal Skull

At birth, the bones of the skull are not fully ossified and are separated by fibrous membranes called fontanels. These allow for compression during childbirth and brain growth during infancy.

• Fontanels: Anterior, posterior, sphenoidal (anterolateral), mastoid (posterolateral)

• Closure: Fontanels close by about 2 years of age

• Bone count: Babies have about 300 bones; adults have 206 due to fusion

The Vertebral Column

Regions and General Structure

The vertebral column, or spine, is composed of 26 bones in adults and provides the main support for the body while protecting the spinal cord. It is divided into five regions:

• Cervical: 7 vertebrae (C1–C7)

• Thoracic: 12 vertebrae (T1–T12)

• Lumbar: 5 vertebrae (L1–L5)

• Sacrum: 5 fused vertebrae

• Coccyx: 3–5 fused vertebrae

Cervical Vertebrae (C1–C7)

Cervical vertebrae are characterized by small bodies, bifid spinous processes, and transverse foramina. The first two cervical vertebrae, atlas (C1) and axis (C2), have unique features that allow for head movement.

• Atlas (C1): No body or spinous process; articulates with occipital condyles for "yes" motion

• Axis (C2): Has dens (odontoid process) for "no" motion

Thoracic Vertebrae (T1–T12)

Thoracic vertebrae have heart-shaped bodies, long spinous processes, and facets for rib articulation. The vertebral foramen is circular.

• Costal facets: For articulation with ribs

Lumbar Vertebrae (L1–L5)

Lumbar vertebrae are the largest and strongest, with thick bodies for weight bearing and short, blunt spinous processes. The vertebral foramen is triangular.

Sacrum and Coccyx

The sacrum is formed by the fusion of five vertebrae and articulates with the hip bones. The coccyx, or tailbone, is formed by the fusion of 3–5 small vertebrae and serves as an attachment site for ligaments and muscles.

Intervertebral Discs and Ligaments

Intervertebral discs are composed of an outer ring of fibrocartilage (anulus fibrosus) and an inner gel-like core (nucleus pulposus). They absorb shock and allow flexibility. Ligaments and muscles provide additional support to the vertebral column.

Disorders of the Spine

Abnormal spinal curvatures can be congenital or acquired. Common disorders include:

• Scoliosis: Abnormal lateral curvature

• Kyphosis: Exaggerated thoracic curvature (hunchback)

• Lordosis: Exaggerated lumbar curvature (swayback)

The Thoracic Cage (Bony Thorax)

Structure and Function

The thoracic cage protects vital organs in the thorax and supports the shoulder girdles and upper limbs. It consists of the sternum, ribs, thoracic vertebrae, and costal cartilages.

• Sternum: Manubrium, body, xiphoid process

• Ribs: 12 pairs; true ribs (1–7), false ribs (8–12), floating ribs (11–12)

• Costal cartilage: Connects ribs to sternum

Table: Classification of Ribs

Type	Rib Numbers	Attachment
True (vertebrosternal)	1–7	Directly to sternum via costal cartilage
False (vertebrochondral)	8–10	Indirectly to sternum via cartilage of rib 7
Floating (vertebral)	11–12	No anterior attachment

Example: The first rib is a true rib, directly attached to the sternum, while the eleventh rib is a floating rib with no anterior attachment.

Additional info: The thoracic cage also plays a role in respiration by allowing expansion and contraction of the lungs.