Axial Skeleton Overview

Skeletal System Organization

The skeletal system is divided into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central axis of the body and provides support and protection for the brain, spinal cord, and thoracic organs.

• Axial Skeleton: Includes the skull, thoracic cage, and vertebral column.

• Appendicular Skeleton: Comprises the pectoral and pelvic girdles, and the upper and lower appendages.

• Articulations/Joints: Points where bones meet, allowing for movement and flexibility.

Bone Structure

General Bone Anatomy

Bones are complex organs composed of several types of tissues. Their structure is adapted for support, protection, and movement.

• Long Bones: Consist of a diaphysis (shaft) and epiphyses (ends), with an articular surface for joint formation.

• Sectional Anatomy: Includes compact bone (dense outer layer), spongy bone (porous inner layer), and the medullary cavity (central cavity containing bone marrow).

Skull

Cranial Bones

The skull is composed of cranial and facial bones that protect the brain and form the structure of the face.

• Cranium: Made up of 8 bones.

• Paired Bones: Parietal and temporal bones.

• Unpaired Bones: Occipital, frontal, sphenoid, and ethmoid bones.

• Major Sutures: Sagittal, coronal, squamous, and lambdoid sutures connect cranial bones.

• Major Structures:

  • Orbits (eye sockets)

  • Cranial cavity

  • Optic canal

  • Cribriform plate

  • Sella turcica

  • Styloid process

  • Mastoid process

  • Occipital condyle

• Major Foramina: Openings such as the foramen magnum, acoustic meatus (external and internal), jugular foramen, and carotid canal allow passage of nerves and blood vessels.

Facial Bones

Facial bones form the structure of the face and provide attachment points for muscles.

• Paired Bones: Maxilla, nasal, and zygomatic bones.

• Unpaired Bones: Mandible.

Skull - Associated Bones

Additional bones associated with the skull include the auditory ossicles and the hyoid bone.

• Auditory Ossicles: Malleus, incus, and stapes; these small bones are located in the middle ear and are essential for hearing.

• Hyoid Bone: A U-shaped bone in the neck that supports the tongue and is not directly attached to other bones.

Vertebral Column

Vertebrae Classification

The vertebral column is composed of 24 vertebrae, divided into cervical, thoracic, and lumbar regions, plus the sacrum and coccyx.

• Cervical: 7 vertebrae (C1–C7)

  • C1 (Atlas): Supports the skull and allows nodding motion.

  • C2 (Axis): Allows rotation of the head.

• Thoracic: 12 vertebrae (T1–T12)

• Lumbar: 5 vertebrae (L1–L5)

• Sacrum: Fused vertebrae forming the back of the pelvis.

• Coccyx: Tailbone, composed of fused vertebrae.

Vertebral Anatomy

• Body: Main weight-bearing portion.

• Vertebral Arch: Encloses the vertebral foramen.

• Spinous Process: Projects posteriorly for muscle attachment.

• Transverse Process: Projects laterally for muscle and ligament attachment.

• Vertebral Foramen: Canal through which the spinal cord passes.

• Superior/Inferior Articular Processes: Form joints between adjacent vertebrae.

Thoracic Cage

Sternum

The sternum is a flat bone located in the center of the chest, forming the anterior portion of the thoracic cage.

• Manubrium: Upper portion of the sternum.

• Body: Middle and largest part.

• Xiphoid Process: Small, cartilaginous lower tip.

Ribs

The ribs are curved bones that form the sides of the thoracic cage and protect the thoracic organs.

• True Ribs: First 7 pairs, directly attached to the sternum via costal cartilage.

• False Ribs: Next 5 pairs, not directly attached to the sternum.

• Floating Ribs: Last 2 pairs of false ribs, not attached to the sternum at all.

• Costal Cartilage: Hyaline cartilage that connects ribs to the sternum.

• Rib Anatomy: Includes the head, neck, tubercle, and shaft.

• Tubercle: Articulates with the transverse process of the vertebra.

Additional Info

• MRB Sandwich: Refers to the anatomical relationship between muscle, rib, and bone in the thoracic cage (Additional info: This term is not standard; inferred to mean the layering of muscle, rib, and bone in the thoracic wall).

Summary Table: Axial Skeleton Components