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Axial skeleton

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Axial skeleton.png

The Axial skeleton is the portion of the skeletal system that protects and supports the head, neck, and trunk. It is composed of the skull, neck, ribs, and vertebral column. [1] The skull is composed of twenty eight bones, which altogether protect the brain. [1] The vertebral column is formed in five different parts: cervical vertebrae, thoracic vertebrae, lumbar vertebrae, a sacrum, and a coccyx. The main purpose of the vertebral column is to protect the spinal cord. The cervical vertebrae support the head and neck, and the lumbar vertebrae support the upper body, torso and lower back. [1] The thoracic cage, also called the rib cage, protects the internal organs. [1]

Contents

Skull

Human skull and the names of its parts from the side.
The anterior view of the human skull.

As the axial skeleton forms the central axis of the body, it is consisted of skull, the vertebral column, the ribs and the sternum or breastbone.

  • The Skull
    Skull has 28 bones and it can be divided into two different main groups: the cranium and the facial bones. The primary job of the skull is to protect the brain, and it also protects and supports the eyes, ears, and nose.[1]
Bone (Cranial Vault) Number Bone (Face) Number
Parietal 2 Mazilla 2
Temporal 2 Zygomatic 2
Frontal 1 Palatine 2
Sphenoid 1 Lacrimal 2
Occipital 1 Nasal 2
Ethmoid 1 Inferior Nasal Concha 2
Auditory Ossicles 6 Mandible 1
Vomer 1
[1]
  • Cranium

The cranium encloses and protects the brain, therefore, it is thought to be one of the most important bones in the body. It is composed of eight flat bones attached to each other perfectly with dentate sutures. The frontal bone makes the forehead and the eye sockets, and the occipital bone has a large opening called the foramen magnum, which the spinal cord passes. The skull is connected with the cervical vertebra in the occipital condyle, located on each side of the opening. The hearing organs are located in the temporal bone.

  • Facial Bones

There are fourteen irregular-shaped bones in the facial skeleton. These bones are attached to cranium firmly. The bones can be divided into three main sections: nasal bone, the two jawbones and the cheek bones. The lower jar, located in front of the ear, is composed of the temporal bone of the cheek bone. It permits the essential movement of the lower jar when chewing the food.

Initially a human has a temporary set of 20 teeth. After shedding the temporary set, humans get a permanent set of 32 teeth. The teeth are rooted in sockets in the edges of the upper and lower jaw bones. There are three notable regions in a tooth: the root, the neck and the crown. The root is rooted in the alveolar pocket of the jaw and is attached to the jaw through a layer and connective tissue. The neck is the middle part of a tooth, between the root and the crown. The crown grows above the gum and it is covered with the layer of enamel. The dentine, largest part of the tooth and harder substance than bones, surrounds the hole that is extending from the root to the crown. At the top of the root, where this hole is located, the blood capillaries and nerves enter.

Four types of teeth:

Types of teeth Shape Location Uses
Incisors chisel-like teeth with sharp cutting edges front of the jaws used for biting off pieces of food
Canines (eye teeth) sharp, pointed, cone-shaped next to incisors used for biting, tearing and ripping
Premolars flat surfaces with two pointed protuberances (cusps) situated behind the canines used for chewing and grinding the food
Molars broad, flat surfaces with 3 pointed protuberances found at the back of the mouth, behind the premolars chewing and grinding the food
[2]

Vertebral Column

Vertebral Column and its parts shown from the side.
Main Article: Vertebral column

Vertebral column is located in the center of the skeleton so that it attaches the ribs, the pectoral and pelvic girdles. Supporting the skull and protecting the spinal cord, the vertebral column is consisted of vertebrae. The vertebrae are attached to each other and therefore, they form a flexible column because of fibrous cartilaginous discs. The vertebral column is divided into five different sections of thirty three vertebrae. They are seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five fused sacral vertebrae, and four fused vertebrae. The vertebral column is crucial because when spinal cord is injured, the brain can lose the control of the body. It lets the spinal nerves to exit the spinal cord, and it allows the head and the trunk to move.

The purpose of the curved shape of spinal cord is to provide a spring effect while walking or running. However, this shape does not form until two months after the birth. The spinal cord of baby is shaped concavely to fit into its mother's arms. And after two months, baby learns to lift the head, and this is when convex curve develops. At around six months, the baby sits up and it develops the convex curvature of the lumbar vertebrae.

  • Cervical Vertebrae

Having seven bones, the cervical vertebrae are the smallest vertebrae in the vertebral column. Atlas and axis, located in the top section of the cervical vertebrae, support the skull and make it to move. The atlas, ring-shaped, supports the skull as it is located right below the skull. The axis, located below the atlas, having a large and strong neural spine, allows the head to turn from side to side.

  • Thoracic Vertebrae

The thoracic vertebrae are composed of twelve bones.

  • Lumbar Vertebrae

There are five bones forming the lumbar vertebrae. These vertebrae are the largest and strongest in the vertebral column. They give powerful support to back muscles so the spine is flexible. It also supports the upper body, torso, and lower back.

  • The Sacrum

There are five vertebrae in the sacrum. The sacrum is triangular shaped, located between the hip bones. There are openings at the ends of the horizontal ridges, which indicate the divisions between fused vertebrae, to permit the nerves and blood vessels to pass through.

  • The Coccyx

Relatively small and simple, four vertebrae compose the coccyx. Since they are at the bottom and small, they do not resemble the usual vertebra structure. The buttocks muscles are attached to the coccyx. [1] [2]

Thoracic vertebrae from inferior view.
  • Vertebrae

Since the vertebrae rest on top of each other, it bears the weight of the body. Among the bodies of vertebrae the intervertebral disks keep the bodies from rubbing against one another. Surrounded by the vertebral arch, the vertebral foramen form a canal through which the spinal cord travels. The spinal cord is safe because it is surrounded by bones. The movement of the vertebral column occurs from spinous process. This process is located in the bottom of middle of the back. It extends posteriorly and inferiorly as it points down. And the muscles around the process control the movement of the vertebral column. Also, superior articular process and inferior articular process help in movement of the vertebral column. The superior articular process overlaps with the inferior articular process directly above it. Therefore, these two processes add the rigidity to the vertebral column. The intervertebral disks of fibrocartilage hold the bones together by providing cushion, and the superior articular process and inferior articular process balance with disks by providing rigidity with overlapping each other.[1]

Thoracic Cage

Thoracic cage and its two main parts: sternum and ribs.

Also called the rib cage, the thoracic cage protects the heart, lungs and other internal organs. It is essential in breathing since it provides protection.[1]

There are twelve pairs of ribs, and the upper seven pairs are called true ribs since they are attached to sternum. Therefore, the lower five pairs of ribs are called false ribs since they are not directly attached to the sternum. The first three false ribs are attached to a common cartilage which is attached to the sternum. In other words, they attach to the sternum indirectly. The last two false ribs are neither attached to the cartilage nor sternum, and they are called floating ribs.

The sternum, also called the breastbone, is divided into three parts: the manubrium, the body, and the xiphoid process. [1]

Video

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Wile 80
  2. 2.0 2.1 The Axial Skeleton. UWC On-Line Standard 8 Biology Textbook. Web. access on 31 October, 2011. Unknown author.

See Also

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