The axial skeleton is the central core of the human body housing and protecting its vital organs.
The axial skeleton has two functions.
The first it to support and protect the organs in the dorsal and ventral cavities.
The second being that it creates a surface for the attachment of muscles.
The axial skeleton comprises of the following bones:
• skull
• vertebral column
• sacrum
• ribs
• sternum
More specifically, the axial skeleton consists of the following 80 bones:
• 29 bones in the skull – 8 cranial and 14 facial bones and then also 6 auditory ossicles and the Hyoid Bone
• 25 bones of the thorax – the sternum and 24 ribs
• 26 bones in the vertebral column – 24 vertebrae, the sacrum and the coccyx
Skull
The skull rests on the upper end of the vertebral column. It is divided into two parts, namely, the skull and the cranium.
The skull is composed of 22 bones that are fused together except for the mandible. These 21 fused bones are separate in children to allow the skull and brain to grow, but fuse to give added strength and protection as an adult. The mandible remains as a movable jaw bone and forms the only movable joint in the skull with the temporal bone. The bones of the inferior and anterior portion of the skull are known as facial bones and support the eyes, nose, and mouth.
The bones of the superior portion of the skull are known as the cranium and protect the brain from damage. The cranium is formed by a number of flat and irregular bones. The 8 bones of the cranium are:
• 1 x Ethmoid Bone
• 1 x Frontal Bone
• 1 x Occipital Bone
• 2 x Parietal Bones
• 1 x Sphenoid Bone
• 2 x Temporal Bones
The 14 facial bones are:
• 2 x Inferior Nasal Conchae
• 2 x Lacrimal Bones
• 1 x Mandible
• 2 x Maxillae
• 2 x Nasal Bones
• 2 x Palatine Bones
• 1 x Vomer
• 2 x Zygomatic Bones
There are 26 bones that form the vertebral column of the human body. Twenty four separate vertebrae extend downwards from the occipital bone of the skull. They are named by region:
• Cervical (neck) – 7 vertebrae
• Thoracic (chest) – 12 vertebrae
• Lumbar (lower back) – 5 vertebrae
• Sacrum – 5 fused vertebra
• Coccyx (tailbone) – 1 vertebra
With the exception of the singular sacrum and coccyx, each vertebra is named for the first letter of its region and its position along the superior-inferior axis. For example, the most superior thoracic vertebra is called T1 and the most inferior is called T12.
Characteristics of a typical vertebrae
A typical vertebra consists of a body and a vertebral arch, which has several processes (articular, transverse, and spinous) for articular and muscular attachments. Between the body and the arch is the vertebral foramen: the sum of the vertebral foramina constitutes the vertebral canal, which houses the spinal cord. In addition to the transverse and spinous processes, which serve as short levers, the 12 thoracic vertebrae are connected by joints with paired, long levers, namely the ribs.
The body
This is the broad, flattened, largest part of the vertebrae. When the bodies are stacked, one on top of each other in the vertebral column, the flattened surface of each vertebrae articulate with the surfaces of the adjacent vertebrae. However, the bodies of the vertebrae are separated from each other by intervertebral discs, there is no direct bone-to-bone contact.
The body is mainly spongy bone and red marrow, but the margins of the upper and lower surfaces consist of a ring of compact bone, the vertebral end-plates.
The vertebral (neural) arch
This encloses a large vertebral foramen. It lies behind the body and forms the posterior and lateral wall of the vertebral foramen. The vertebral arch consists of right and left pedicles (plates of bone) which form the lateral walls of the vertebral foramen. The laminae are form the posterior walls of the vertebral foramen.
The transverse processes emerge laterally at the junction of the pedicles and laminae, and the spinous process proceeds posteriorly from the union of the laminae. The superior and inferior articular processes project vertically from the vertebral arches on each side and bear articular facets. When vertebra are in their anatomical position, notches between adjacent pedicles form intervertebral foramina, each of which typically transmits neural structures..
Region-specific vertebral characteristics
Cervical vertebrae
These are the smallest vertebrae. The transverse processes have a foramen through which a vertebral artery passes upward towards the brain. The first two cervical vertebrae, namely the atlas and axis, are atypical.
The atlas (C1), which has neither body nor spinous process, consists of two lateral masses connected by a short anterior and a longer posterior arch. Each lateral mass presents upper and lower facets, for the occipital condyle of the skull and for the axis, respectively.
The axis (C2) is characterized by a small projection, the dens (or odontoid process), which projects upward from the body and articulates with the anterior arch of the atlas. The dens is anchored to the occipital bone and is limited behind by the transverse ligament of the atlas. The head pivots on this joint.
The 7th cervical vertebrae (C7) is also known as the the vertebra prominens. It has a long spinous prominence in a swollen turburcle, which is easily felt at the base of the neck.
Thoracic vertebrae
The 12 thoracic vertebrae are larger than the cervical vertebrae. This is as a result that this section of the vertebral column has to support more body weight. The bodies and transverse processes have facets for articulation with the ribs.
Lumber vertebrae
These are the largest of the vertebrae as they support the weight of the upper body. They have extensive spinous processes for attachment of the muscles of the lower back.
Sacrum
This consists of five (sometimes six) vertebrae are fused in the adult to form the sacrum, which can be felt below the “small of the back.” The sacrum articulates above with L5, laterally with the hip bones, and inferiorly with the coccyx. It has a roughly triangular appearance with a pelvic and dorsal surface, a lateral mass on each side, and a base and apex.
Coccyx
The vertebrae (usually four) below the sacrum are fused in the adult to form the coccyx, which resembles a miniature sacrum in shape.
Sternum and Ribs (Thoracic cage)
The thoracic cage is formed by the sternum anteriorly, 12 pairs of ribs forming the lateral bony cages and the 12 thoracic vertebrae.
Sternum
The sternum, or breastbone, is a thin, knife-shaped bone located along the midline of the anterior side of the thoracic region of the skeleton.
The manubrium is the uppermost section and articulates with the clavicles at the sternoclavicular joints and with the first two pairs of ribs.
The body or middle portion provides attachment to the ribs.
The xiphoid process is the inferior tip of the sternum. It provides attachment to the diaphragm, muscles of the abdominal wall the linea alba.
The sternum connects to the ribs by thin bands of cartilage called the costal cartilage.
Ribs
There are 12 pairs of ribs that together with the sternum form the rib cage of the thoracic region. The first seven ribs are known as “true ribs” because they connect the thoracic vertebrae directly to the sternum through their own band of costal cartilage. Ribs 8, 9, and 10 all connect to the sternum through cartilage that is connected to the cartilage of the 7th rib, so we consider these to be “false ribs.” Ribs 11 and 12 are also false ribs, but are also considered to be “floating ribs” because they do not have any cartilage attachment to the sternum at all.
Due to the arrangement of the ribs and the quantity of cartilage present in the rib cage, it is a flexible structure that can change its shape and size during breathing.