The Axial Skeleton
The axial skeleton forms the central axis of the body. It consists of the skull, the vertebral column, the ribs and the sternum or breastbone.
The skull consists of 28 different bones (including the ossicles of the ear). The bones of the skull can be divided into two main groups: the cranium which encloses and protects the brain and the facial bones.
The cranium consists of eight flat bones which are rigidly attached to each other with dentate sutures (joints with teeth-like protrusions). They envelop and protect the brain. The frontal bone forms the forehead and portions of the eye sockets (or orbits). The occipital bone, at the base of the skull contains a large opening, called the foramen magnum, through which the spinal cord passes. On each side of the opening is the occipital condyle, - two round protuberances, - by means of which the skull articulates with the first neck (or cervical) vertebra (the atlas). The organs of hearing are situated in the temporal bone, one on each side. The openings leading into these organs can also be seen on each side.
The Facial Bones
The facial skeleton consists of fourteen irregular bones, which are all (with the exception of the lower jawbone) firmly attached to the cranium by means of sutures. They include the nasal bones, the two jawbones and the cheek bones. The lower jaw articulates with the temporal bone part of the cheek bone, just in front of the ear. This allows for the necessary movement of the lower jaw when food is bitten off and chewed. Both upper and lower jaws have alveolar pockets into which teeth fit.
The teeth are embedded in sockets in the ridges of the upper and lower jaw bones. Three regions can be distinguished in a tooth:The root which are embedded in the alveolar pocket of the jaw. The root is firmly attached to the jaw by a surrounding layer of cement and strong connective tissue. The neck is the area where the root(s) and crown meet. The crown projects above the gum. It is covered with a hard, white layer of enamel. The largest part of the tooth consists of dentine which is a harder substance than ordinary bone. The dentine surrounds the cavity which extends from the root to the crown. Blood capillaries and nerves enter the cavity at a small opening in the tip of the root.
There are four types of teeth:
- Incisors are chisel-like teeth with sharp cutting edges found at the front of the jaws. They are used for biting off pieces of food.
- Canines (eye teeth) are sharp, pointed, cone-shaped teeth which are slightly longer than the other teeth. They flank the incisors and are used for biting, tearing and ripping.
- Premolars are situated behind the canines. They have flat surfaces with two pointed protuberances (cusps), which are used for chewing and grinding the food.
- Molars are larger and better developed than the premolars. They are found at the back of the mouth, behind the premolars. They have broad, flat surfaces with 3 pointed protuberances (cusps) and are also used for chewing and grinding the food. They usually have 2 to 3 roots.
Initially a human has a temporary set, or a milk set of 20 teeth with a tooth formula 2.1.2 over 2.1.2. After shedding the temporary set humans get a permanent set of 32 teeth with a tooth formula 18.104.22.168 over 22.214.171.124.
The vertebral column forms the central part of the skeleton. It supports the skull and protects the spinal cord. It also serves as attachment for the ribs, the pectoral and pelvic girdles. The vertebral column consists of separate bones, the vertebrae. The different vertebrae are arranged above each other. Because the separate vertebrae are attached to each other by means of fibrous cartilaginous discs they form a flexible column. Each vertebra has articular surfaces above and below, which allow articulation movement between them.
The vertebral column of 33 vertebrae is divided into five regions according to their position and structure. The five regions consist of: Seven cervical (neck) vertebrae, Twelve thoracic (chest) vertebrae, Five lumbar vertebrae, Five fused sacral vertebrae, and Four fused vertebrae.
Structure of a Typical Vertebra.
A typical vertebra consists of the centrum, a neural arch, a neural spine, two transverse processes and four articular processes with articulating surfaces. The centrum is the front part (anterior) and consists of a solid piece of spongy bone encircled by a layer of compact bone. The upper and lower surfaces are flat and rough and provide attachment for the cartilaginous discs. These surfaces allow a limited degree of movement. The posterior (back) part is called the neural arch. An opening, ( foramen) is formed between the centrum and the neural arch. The spinal cord goes through this opening. The neural canal is formed by the vertebral foramina in the successive vertebrae and it encloses and protects the spinal cord. The neural spine is directed backwards. The two transverse processes project laterally and serve for the attachment of ligaments and muscles. The neural arch has four smaller articular processes with articular surfaces, two on the upper and two on the lower aspect of the vertebra. These articulating surfaces are covered by hyaline cartilage. The spinal nerves leave the vertebral column through the openings between each succeeding pair of vertebrae.
The Cervical Vertebrae
The neck region consists of 7 cervical vertebrae. These are the smallest vertebrae in the vertebral column. The first two cervical vertebrae are known as the atlas and axis. They are specially adapted to support the skull and to enable it to move. They differ from the structure of the typical vertebra in certain respects.
The atlas is the first neck vertebra and supports the skull. It is ring-shaped and has no centrum. A neural spine is absent. The atlas consists of posterior and anterior neural arches and 2 short transverse processes. The spinal foramen (neural canal) is very large. The 2 occipital condyles of the skull fit into the articulating facets on the upper surface of the atlas, on either side of the neural canal. On its lower surface (inferior) surface the atlas has 2 articular surfaces for articulation with the axis.
The axis has a large, strong neural spine. The centrum is small and has become modified to bear the odontoid process (a tooth-like projection) on its upper surface. The odontoid process fits against the facet in the anterior arch of the atlas. This forms a pivot joint or axis, around which the atlas (together with the skull) can rotate, so allowing the head to turn from side to side.
The Thoracic Vertebrae.
There are 12 thoracic vertebrae. The centrum is large and sturdy and the neural spines are long and directed downwards. The long neural spines form an anchorage for the muscles and ligaments that support the head and neck. The head (or capitulum) of each of the first 10 pairs of ribs fits into and articulates with the semi-circular facet which is situated between two successive centra, i.e. between the inferior surface of one and the superior surface of the next centrum. These facets occur on both sides of the centrum. The tubercle of the rib articulates with the facet at the tip of the transverse process.
The Lumbar Vertebrae.
These 5 vertebrae are the largest and strongest in the vertebral column. The transverse processes are very long for the attachment of the powerful back muscle that maintain the posture and flex the spine in movement.
The sacrum is roughly triangular in shape and consists of 5 fused vertebrae. It lies between the hip bones, with which it articulates. Horizontal ridges indicate the divisions between the fused vertebrae. At the ends of these ridges are openings which allow nerves and blood vessels to pass through.
The coccyx consists of 4 fused tail vertebrae which are small and have a relatively simple structure. They do not resemble the structure of a typical vertebra. The muscles of the buttocks are attached to the coccyx.
Twelve pairs of ribs articulate with the 12 vertebrae of the thoracic region. The ribs are flat, narrow bones with a distinctive bow-shaped curve. Each rib consists of a head or capitulum, a small tubercle (which is a short distance back from the head) and the shaft. The head of the rib articulates with the semi-circular articulating facets formed by the centra of two successive thoracic vertebrae. The tubercle fits into and articulates with the articulating facets on the transverse process. The first seven ribs on each side are joined to the breastbone by bars of hyaline cartilage (called costal cartilage in this region). The first seven pairs of ribs are referred to as true ribs. The cartilages of the 8th, 9th and 10th ribs are joined to the costal cartilage of the rib immediately above (i.e. to the costal cartilage of the 7th rib). These three pairs of ribs are known as vertebrochondral ribs. The last two pairs of ribs have free ends which are not attached to the sternum at all. They are floating ribs. The vertebrochondral ribs and the floating ribs are collectively known as false ribs. The ribs (together with their muscles) play an important role in the breathing mechanism of a mammal.
The sternum is a long, flat, dagger-shaped bone. It is about 15 - 18 cm long and is found in the center of the chest region. The broad upper end supports the collar bones. The first seven pairs of ribs are attached to the articulating facets on the sides of the sternum. The 12 thoracic vertebrae, the 12 pair of ribs and the sternum forms the thorax which protects the delicate and vital organs of the thorax, viz. the heart and lungs.