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The skeletal system: The axial skeleton

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This article, the third in a four-part series, explores the axial skeleton. Part four will look at the appendicular skeleton. There are a total of 206 bones in the human adult body (Tortora and Grabowski, 2003), which are divided into two main parts (Fig 1). The axial skeleton consists of the bones of the skull, vertebral column, sternum and ribs, and has a total of 80 bones. The appendicular skeleton consists of bones of the pectoral girdle, pelvic girdle and limbs, and has a total of 126 bones. The axial skeleton consists of:

  • Skull (cranium) - eight;
  • Skull (facial) - 14;
  • Auditory ossicles (three in each ear - not shown) - six;
  • Hyoid (neck bone, aids speech - not shown) - one;
  • Vertebral column - 26;
  • Thorax (sternum) - one;
  • Thorax (ribcage) - 24.

Bones can be classified on the basis of their shape (Table 1), with some bone shapes related to either the axial or the appendicular skeleton. The spine The vertebral column (also called the backbone, spine or spinal column) consists of a series of 33 irregularly shaped bones (during childhood), called vertebrae. These 33 bones are divided into five categories depending on where they are located in the vertebral column (Fig 2). Over time some smaller bones fuse together, resulting in a total of 26 vertebrae in adults (Bogduk, 2005; Tortora and Grabowski, 2003).

  • Cervical vertebrae - the first seven bones. Located at the top of the vertebral column, these bones form the neck and support the head.
  • Thoracic vertebrae - the next 12 bones. These form the rear anchor of the ribcage. They are larger than cervical vertebrae and increase in size from top to bottom.
  • Lumbar vertebrae - five bones. These are the largest bones in the vertebral column. These vertebrae support most of the body’s weight and are attached to many of the back muscles.
  • The sacrum - a triangular bone just below the lumbar vertebrae, it forms the back wall of the pelvic girdle. In children, it consists of four or five sacral vertebrae that fuse together in adulthood.
  • The coccyx - this is the bottom of the spinal column. It consists of three to five bones that are fused together in adults. Many muscles connect to the coccyx.

The skull The skull has two sets of bones - eight cranial and 14 facial (the main ones are shown in Fig 3). Together these form a large cranial cavity, which protects the brain. The skull also supports the structure of the face and contains the eye sockets, nasal bones, temporal bone protecting the structures of the ear, and teeth and jaws to allow biting and chewing (Waugh and Grant, 2006). Sternum and ribs The sternum is a long, flat bone in the middle of the thorax. It consists of the manubrium, the gladiolus and the xiphoid process. It supports the clavicles and connects with the cartilages of the first seven pairs of ribs. Most people have 12 pairs of ribs, which surround the thoracic cavity and protect the lungs, heart and other thoracic organs (some people have one more or one less pair). The ribs are connected to the vertebrae. The eighth to 10th pair connect to the cartilage of the pair above at the front, while the lowest two pairs are ‘floating’. The elasticity of the ribcage allows for breathing. The sternum There are three areas (from top to bottom):

  • Manubrium;
  • Body of sternum - gladiolus;
  • Xiphoid process.

Between the manubrium and body of the sternum is the sternal angle (a cartilaginous joint which acts like a hinge). It is at this point that the sternum can move anteriorly and posteriorly to allow the lungs to expand and contract with breathing (Marieb, 2003). Between the body and the xiphoid process is the xiphisternal joint, which is commonly used as a landmark when initiating cardiac compressions in basic life support. The sternum is composed of highly vascular cancellous tissue, which is covered by a thin layer of compact bone. This is thickest in the manubrium between the articular facets for the clavicles. At the top of the sternum is the suprasternal notch,
a large visible dip where the sternum joins the neck. The ribs Most people have 12 pairs of ribs, although some have a 13th, known as the cervical ribs - the rib remnants of the seventh neck of vertebrae - and some only have 11 pairs. The ribs form the thoracic cage, which protects the lungs, heart and other thoracic organs. They start at the thoracic vertebrae and curve towards the front of the body. The seven upper pairs of ribs attach directly to the sternum with hyaline cartilage. The lower five pairs are called ‘false’ ribs - the bottom two sets are ‘floating’ with no attachment to the sternum (although they are embedded in the lateral muscle wall). The three sets of ribs between these and the upper seven are attached with costal cartilage to rib set seven, and are therefore indirectly connected to the sternum (Marieb, 2003). The spaces between the ribs are known as the intercostal spaces.

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