11.1
Overview of the Axial Skeleton
The skeleton is subdivided into two major divisions—the axial skeleton and the appendicular skeleton. The axial skeleton forms the vertical, central a…
The axial and appendicular skeletons are the two divisions of the skeletal system.
The skull, the thoracic cage, and the vertebral column form the axial skeleton, which provides support, protection, and upright posture.
The skull contains eight cranial bones enclosing the brain and fourteen facial bones providing a framework for the face.
In addition, six auditory ossicles– the small bones in the middle ear involved in hearing and the single suspended hyoid bone supporting the voice box are associated with the skull.
From the base of the skull extends the vertebral column, containing twenty-six vertebrae, which protect the spinal cord, separated by intervertebral discs.
The vertebral column begins with seven cervical vertebrae, followed by twelve thoracic and five lumbar vertebrae, and ends with the sacrum and the coccyx.
The thoracic cage, which protects the heart and the lungs, consists of twelve pairs of ribs with their costal cartilage and the sternum.
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Q1: What are the three main components of the axial skeleton?
The axial skeleton consists of the skull, vertebral column, and thoracic cage. These three structures form the central vertical axis of the body and protect vital organs including the brain, spinal cord, heart, and lungs. Together, they provide structural support and enable upright posture.
Q2: How many bones make up the adult skull and what is their function?
The adult skull contains 22 individual bones, with 21 immobile bones united into a single unit and the mandible as the only moveable bone. The skull protects the brain, supports facial structures, forms the nasal cavity, and encloses the eyeballs. Associated bones include the hyoid bone and three auditory ossicles in each middle ear.
Q3: What is the structure and purpose of the vertebral column?
The vertebral column consists of 24 vertebrae plus the sacrum and coccyx, each separated by intervertebral discs. This flexible column supports the head, neck, and body while protecting the spinal cord that passes through openings in the vertebrae. The vertebral column is subdivided into five regions, with vertebrae named and numbered in ascending order from top to bottom.
Q4: What bones and structures comprise the thoracic cage?
The thoracic cage consists of 12 pairs of ribs with their costal cartilages and the sternum, the flattened bone of the anterior chest. The ribs are anchored posteriorly to the 12 thoracic vertebrae. This protective structure encloses and shields the heart and lungs from injury.
Q5: What role do the hyoid bone and auditory ossicles play in the skeletal system?
The hyoid bone is a U-shaped bone suspended below the mandible that serves as an attachment site for muscles of the jaw, larynx, and tongue. The auditory ossicles are three small bones in the middle ear—the malleus, incus, and stapes—involved in hearing. Both structures are associated with the skull but not directly connected to it.
Q6: How many bones are in the axial skeleton and what percentage of the total skeleton do they represent?
The axial skeleton of an adult consists of 80 bones, including the skull, vertebral column, and thoracic cage. This represents the central structural framework of the body. In contrast, the appendicular skeleton contains 126 bones, making the axial skeleton approximately 39 percent of the total skeletal system.
Q7: How does the axial skeleton protect the body's vital organs?
The skull protects the brain and houses the middle and inner ear structures. The vertebral column protects the spinal cord through openings in each vertebra. The thoracic cage protects the heart and lungs. Together, these three axial components create a protective framework for the body's most critical organs and systems.