The Axial Skeleton

The axial skeleton is the central framework of the human body. It includes the skull, the vertebral column (spine), and the thoracic cage (chest or rib cage).

The Skeleton Anterior View
Figure 4-3A. Anterior view of the human skeleton.
Diagram Skeleton
Figure 4-3B. Posterior view of the human skeleton.

a. Vertebral Column (Spine). The vertebral column, or spine, is made up of a vertical series of bony blocks called vertebrae. These vertebrae are joined together in such a way as to form a semiflexible rod. The spine is the central support for the trunk, yet allows trunk movements.

(1) Anatomically and functionally, a typical vertebra (figure 4-4) is constructed of two major parts:

(a) The vertebral body is a drum-shaped cylindrical mass. Its superior and inferior surfaces are flat. Its function is primarily weight-bearing.

(b) The neural arch extends posteriorly, arching over and protecting the spinal cord of the central nervous system. From the neural arch are several processes. These processes serve as attachment areas for the trunk muscles. They also act as levers during various trunk motions.

Trunk Vertebrae
Figure 4-4. A typical vertebra (superior and side views.

(2) The vertebral column has 32-33 vertebrae, one on top of the other. These vertebrae are arranged in regions. The vertebrae of each region have a characteristic shape. The regions are as follows:

(a) Cervical (neck) region, with seven cervical vertebrae.

(b) Thoracic (chest) region, with 12 thoracic vertebrae.

(c) Lumbar (low back) region with five lumbar vertebrae.

(d) The sacrum, which is a bony fusion of five sacral vertebrae.

(e) The coccyx (pronounced COCK-sicks, "tail"), with 3-4 coccygeal vertebrae together.

(3) The vertebrae are held together in two ways:

(a) The intervertebral disc holds the bodies of adjacent vertebrae together. The intervertebral disc is a fibrous ring with a soft center. This disc allows the vertebral bodies to move on one another. This joint between the vertebral bodies is a plane-type joint.

(b) The various parts of adjacent vertebrae are held together by ligaments. A ligament is a dense FCT structure which extends from bone to bone. These ligaments extend along the vertebral column from the base of the skull all the way down to the coccyx.

(4) The spine has four curvatures in the adult human. In the cervical (neck) region and the lumbar (low back) region, the spine curves forward. In the thoracic (chest) region and the sacro-coccygeal (pelvic- sacrum and coccyx) region, the spine curves backwards.

(5) When one examines the back of a person by sight and feel (palpation), certain landmarks are observed.

(a) At the upper shoulder region in the midline, a knob can be seen and felt. This is the tip of the spinous process of the seventh cervical vertebra. Since this is the first vertebra from the top that can be easily palpated, this bony landmark is called the vertebra prominens (the "prominent vertebra").

(b) From the vertebra prominens down to the beginning of the sacrum, one can feel the tip of the spinous process of each vertebra.

b. The Thoracic (Rib) Cage. The rib cage (figure 4-5) forms a protective enclosure for the vital organs contained within the thorax (chest) such as the heart and lungs. It also allows the movements of breathing to take place.

Regions Human Body
Figure 4-5. The human thorax with bones of the shoulder region.

(1) The sternum lies in the midline of the thorax anteriorly. It is made up of three parts: the manubrium at the top, the body as the main part, and the xiphoid process below. On the top of the manubrium is the jugular (sternal) notch, a common landmark. The junction between the manubrium and the body is a joint called the sternal angle. This sternal angle is an important landmark clinically because the second rib attaches to the sternum at this junction. It is just a matter of simple counting after identifying the second rib to know where you are on the thoracic wall.

(2) The rib cage consists of the 12 thoracic vertebrae, 12 pairs of ribs, and the sternum. Each rib is curved laterally from back to front. All 12 pairs of ribs are attached posteriorly to the thoracic vertebrae. The upper six pairs of ribs are attached directly to the sternum by their costal cartilages. The seventh through tenth pairs of ribs are attached indirectly to the sternum through their costal cartilages (by attaching to the costal cartilage of the rib above). Rib pairs 11 and 12 do not attach to the sternum. Instead, they are embedded in the trunk wall muscles.

c. The Skull. The skull (figure 4-6) is the bony framework (skeleton) of the head region. It has two major subdivisions: the cranium which encases and protects the brain and the facial skeleton which is involved with the beginnings of the digestive and respiratory systems. The special sense organs (eyes, ears, etc.) are included and protected within the skull.

Tacing Skeletal Structure Front View
Figure 4-6. The human skull (front and side views).

(1) The bones of the cranium form a spherical case around the brain. With age, the sutures between the cranial bones become more solid. The cranium has a base with several openings for the passage of blood vessels and nerves. The vault (or calvaria) is made up of flat bones arching over and covering the brain.

(2) The facial skeleton consists of bones which surround the nose and the mouth. These are mainly flat and irregular bones. Bones of the facial skeleton also form part of the orbit of each eye.

(3) Certain bones of the skull have air-filled spaces called the paranasal sinuses.

(4) The upper jaw (maxilla) and the lower jaw (mandible) are parts of the facial skeleton which surround the mouth.

(5) The hyoid bone is located at the junction between the head and the neck. It is not articulated directly with the other bones. It is held in place--and moved around--by groups of muscles above and below. The root of the tongue is attached to its upper anterior surface. The larynx is suspended from its inferior surface. These three structures, together, form the hyoid complex. This complex is a functional unit for swallowing.

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