Human Skeleton | AXIAL SKELETON
Summary
This video provides a comprehensive overview of the human axial skeleton, focusing on the skull and vertebral column. It details the bones and regions of the skull, including the cranium (frontal, parietal, temporal, occipital bones), facial bones (zygomatic, maxilla, mandible), and sutures. The vertebral column is explained by its five regions (cervical, thoracic, lumbar, sacral, coccygeal), highlighting the unique structures and functions of the atlas and axis vertebrae. It also covers the general anatomy of a vertebra (centrum, vertebral arch, processes, facets) and provides methods for distinguishing different types of vertebrae.
Key Insights
The atlas (C1) lacks a centrum and has large articulating facets for the skull.
The atlas, the first cervical vertebra, is unique as it lacks a vertebral body (centrum) and has very large, superior articulating facets to support the skull. It also has the largest vertebral foramen.
The axis (C2) features the odontoid process for head rotation.
The axis, the second cervical vertebra, has a prominent upward projection called the odontoid process (dens). This process fits into the atlas's large foramen, allowing for head rotation (shaking head 'no'). It also has a forked spinous process.
Sections
Introduction to the Axial Skeleton
The axial skeleton consists of the skull and vertebral column.
The axial skeleton is the main focus of this video, with the appendicular skeleton to be covered in a follow-up. It is composed of two primary parts: the skull and the vertebral column.
The skull is a complex structure made of multiple fused bones.
The skull is a collective term for all the bones in this region. It is divided into sections with different names, and many smaller bones fuse together as a person ages.
Bones of the Skull
The cranium protects the brain and includes specific cranial bones.
The cranium is the dome-shaped part of the skull that encases the brain. Its major bones are the frontal bone (forehead), parietal bones (top and sides), temporal bones (sides near temples), and occipital bone (back).
Facial bones include the zygomatic, maxilla, and mandible.
The zygomatic bone forms the cheekbone and eye socket (orbit). The maxilla, or upper jaw, is a fixed bone. The mandible, or lower jaw, is the movable part of the jaw.
Sutures are fused joints in the adult skull that were open in infancy.
Sutures are the wavy lines visible on the cranium, indicating where bones have fused together over time. These sutures were flexible in infants, allowing for passage through the birth canal.
Regions of the cranium often correspond to regions of the brain.
The names of the cranial bones also indicate the general areas of the brain located beneath them. For example, the frontal bone covers the frontal lobe.
The foramen magnum is a critical opening at the base of the skull.
This large opening, typically found in the occipital bone, is where the spinal cord connects to the brainstem and exits the skull.
The Vertebral Column
The vertebral column has five distinct regions.
The vertebral column is divided into cervical (7 vertebrae), thoracic (12 vertebrae), lumbar (typically 5, but can vary), sacral (5 fused vertebrae forming the sacrum), and coccygeal (4 fused vertebrae forming the coccyx or tailbone).
The sacrum and coccyx are fused vertebrae.
The sacral and coccygeal bones are formed from multiple vertebrae that fuse together into single, larger bones, hence they are often counted as one bone each (or 5 and 4 units respectively).
Structure of a Typical Vertebra
Key features of a vertebra include the centrum and various processes.
A typical vertebra includes the Centrum (body), which supports body weight; the Spinous Process (posterior projection); Transverse Processes (lateral projections); and Articular Processes/Facets, which articulate with other vertebrae or ribs.
The vertebral arch and foramen protect the spinal cord.
The vertebral arch, along with the centrum, encloses the vertebral foramen (or neural canal), a large opening through which the spinal cord passes. Some texts refer to this as the neural arch and neural canal.
Additional structures like laminae and pedicles form the vertebral arch.
The lamina forms the back walls of the vertebral arch, while the pedicles connect the arch to the centrum. These parts join to form the complete vertebral arch.
Distinguishing Specific Vertebrae
The atlas (C1) lacks a centrum and has large articulating facets for the skull.
The atlas, the first cervical vertebra, is unique as it lacks a vertebral body (centrum) and has very large, superior articulating facets to support the skull. It also has the largest vertebral foramen.
The axis (C2) features the odontoid process for head rotation.
The axis, the second cervical vertebra, has a prominent upward projection called the odontoid process (dens). This process fits into the atlas's large foramen, allowing for head rotation (shaking head 'no'). It also has a forked spinous process.
Cervical vertebrae generally have smaller centra and large vertebral foramina.
Beyond C1 and C2, cervical vertebrae have relatively small centra and large vertebral foramina compared to their size, reflecting their position closer to the head and need for spinal cord space.
Thoracic vertebrae have facets for rib articulation.
Thoracic vertebrae are identifiable by the presence of facets on their transverse processes and bodies, which articulate with the ribs. They typically have medium-sized centra and a more circular vertebral foramen.
Lumbar vertebrae have large centra and smaller vertebral foramina.
Lumbar vertebrae are characterized by their large, kidney-shaped centra to support significant body weight and relatively small vertebral foramina. Their spinous processes are short and thick.
Centrum size increases down the vertebral column, while the vertebral foramen decreases.
A general trend observed is that the size of the vertebral centrum increases from cervical to lumbar regions to bear more weight, while the vertebral foramen (spinal canal) generally decreases in size as it moves further from the brain.
Transverse process shape helps differentiate thoracic and lumbar vertebrae.
Thoracic vertebrae often have transverse processes that angle backwards giving a slight 'V' shape, while lumbar vertebrae tend to have shorter, thicker transverse processes oriented more laterally, resembling a 'T' shape.
Terminology Recap
Key terms include axial skeleton, skull, vertebral column, cranium, and facial bones.
Recap of major components: Axial skeleton (skull + vertebral column), cranium (braincase), facial bones (zygomatic, maxilla, mandible), sutures (skull joints), foramen magnum (spinal cord opening).
Vertebral regions and parts are cervical, thoracic, lumbar, sacral, coccygeal.
Recap of vertebral regions and their characteristics: Cervical (neck, includes atlas/axis), Thoracic (ribs attach), Lumbar (lower back), Sacral (fused), Coccygeal (tailbone, fused). Key vertebral parts: Centrum, spinal process, transverse process, articular facets, vertebral foramen.
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