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.
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
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 2.1.2.3 over 2.1.2.3.
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
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.
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.
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.
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.
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