Biology SS2 Second Term
Week 3: Vertebrate Skeleton
Introduction
The skeleton of vertebrates is composed primarily of bone. Cartilage covers articular surfaces between bones and connects the ribs to the sternum. The skeleton is divided into two major parts:
- The axial skeleton includes the skull, mandible, hyoid, ribs, sternum, and vertebrae
- The appendicular skeleton includes the girdle, limb and feet bones
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.
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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, 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 Vertebral Column
Vertebral column is also called back bone or spine and encloses the spinal cord. It is a flexible, curved, vertical rod, and consists of a row of 33 movably articulated ring like bones called vertebrae. Between each of the two bones the space is supplemented by pads of fibro-cartilage called the intervertebral discs. The vertebrae are held together by ligaments which prevent their dislocation, but permit a degree of movement, making the backbone flexible. The adult vertebral column measures 60-70 cm in length.
The vertebrae are grouped and named according to the region they occupy.
- Seven cervical vertebrae form the neck or cervical region.
- Twelve thoracic vertebrae form the back of the thorax or chest.
- Five lumbar vertebrae form the lumbar region or loins.
- Five sacral vertebrae form the sacrum.
- Four caudal vertebrae form the coccyx or tail.
The vertebrae in the three upper regions remain separate or distinct throughout life, and are called the movable vertebrae. Those in the two lower regions, the sacrum and coccyx, are united in the adult to form two bones and are called, the fixed vertebrae.
With the exception of the first two cervical vertebrae, all the movable vertebrae have similar structure; a typical vertebra is a bony ring. Its hole is called the vertebral foramen. The front border of the vertebral foramen is very thick. It is known as the body or centrum.
The remaining boundary of the vertebral foramen is thin. It is termed as the vertebral arch. Each half of a vertebral arch has a vertically narrow side, the pedicel, and a broader hind part, the lamina. The two laminae meet in the midline of the back. The upper and lower margins of the pedicel have concavities called the vertebral notches. When vertebrae are articulated together, adjacent notches form apertures– the intervertebral foramina, for the exit of the spinal nerves. The vertebral arch gives off processes to which the muscles are attached. The processes include a median spinous process and paired articular processes and transverse processes. The spinous process projects back and often also downward from the junction of the laminae. The articular processes of the adjacent vertebrae meet to form synovial joints. They provide limited movement between vertebrae. The vertebral foramina of all the vertebrae when intact form a vertebral canal that encloses the spinal cord.
In between the adjacent vertebrae, there are elastic pads of fibrocartilage- the intervertebral discs. This provides mobility to the vertebrae, check undue friction and take up shocks. Displacement of intervertebral disc is called slip-disc and is dangerous.
The Cervical Vertebrae
The neck region consists of 7 cervical vertebrae. The cervical vertebrae are the smallest of the bones, and except the first and the second, which are peculiar in shape, the cervical vertebrae possess the following characters in common. The first cervical vertebra is called atlas. It is almost ring like. It provides up and down or nodding movement to the skull on it. The second cervical vertebra is termed as axis. Its centrum bears an odontoid process, which allows side to side or turning movement to the atlas and skull together on it.
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The bodies of other cervical vertebrae are small and oblong in shape broader from side to side than from backward. The neural arch is large. The spinous processes are divided or bifid terminally. The transverse processes are perforated by foramina for the passage of the vertebral arteries. Thus this important blood vessel is protected as it passes through the vulnerable region of the neck.
The Thoracic Vertebrae
In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical and lumbar vertebrae; they increase in size going towards the lumbar vertebrae, with the lower ones being a lot larger than the upper. They are distinguished by the presence of facets on the sides of the bodies for articulation with the heads of the ribs, and facets on the transverse processes of all, except the eleventh and twelfth, for articulation with the tubercles of the ribs. By convention, the human thoracic vertebrae are numbered T1-T12, with the first one (T1) located closest to the skull and the others going down the spine towards the lumbar region.
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