Thorax, Bones of Health Article

Definition

The skeleton of the thorax or chest is a cage that encloses and protects the main organs of respiration and circulation. It has a conical shape, being narrower at the top and broader at the bottom, and longer behind than in front. It consists of the sternum and the ribs.

Description

The bones of the thorax include the sternum, commonly called the breastbone, and the ribs.

The sternum is a narrow, elongated, flattened bone that forms the center of the front of the chest. It consists of three parts: an upper section called the manubrium, a middle section called the body, and a lower section called the xiphoid process that projects down. The junction of the manubrium and body is called the sternal angle. In early life, the xiphoid process is not a bone, but a piece of cartilage. Cartilage is a type of connective tissue containing collagen, a protein substance that forms tough and elastic fibers. It is a softer and more flexible material than bone. As the child grows, the xiphoid process slowly hardens into bone and by adulthood, it has become fused to the body of the sternum. The sides of both manubrium and body are notched so as to attach to seven costal cartilages. These are strips of strong cartilage that prolong into ribs and provide elasticity to the thorax. The upper section of the sternum supports the clavicles (shoulder blades). It contains a notch called the clavicular notch that allows it to articulate with the clavicle. The average length of the adult sternum is about 6.7 in (17 cm), and it is usually somewhat longer in the male than in the female.

The ribs are flexible, long bones that look like arches, and they form a large part of the thoracic skeleton. There are 12 ribs on each side and they are located one below the other in such a way that spaces called inter-costal spaces occur between them. The first seven (1-7) are called the true ribs or the vertebro-sternal ribs. They connect in the back to the vertebral column, and in front to the sternum, through the costal cartilages. The following three ribs (8-10) are called the false ribs or the vertebro-chondral ribs. These ribs have their costal cartilages attached to the cartilage of the true rib above. The last two ribs (11-12) are only attached to the vertebral column and are thus called the floating or vertebral ribs.

All ribs have many structural features in common:

• Head. The head of the rib is the flat surface that connects with the vertebrae in the vertebral column.
• Neck. The neck of the rib is a flattened section that has a length of about 1 in (2.5 cm). It is located between the head and the tubercle. Its inferior surface is flat and smooth and its superior surface is rough for the attachment of ligaments.
• Tubercle. The tubercle is a bony eminence, or growth, that comes right after the neck of the rib. It has two sections, one that serves as a point of attachment to the vertebrae and another that attaches to ligaments.
• Body. The body—or shaft—is the longest part of a typical rib.
• Angle. The angle is the point at which the body of the rib starts to curve, just after the tubercle.
• Costal groove. The costal groove is located on the inner surface of the body of a rib. It provides a seat and protection for the intercostal nerve bundle.

Ribs present some degree of variability. For example, they vary in their angle, the upper ribs being less oblique than the lower. Characteristic features of some special ribs include:

• Rib 1. The first rib is the most curved of all ribs; it is also the broadest, shortest and widest rib. The head is small, rounded, and only has a single bony projection for articulation with the first thoracic vertebra.
• Rib 2. The second rib is much longer than the first, and its body is not flat like that of the other ribs. It has a rough section near its angle for attachment of a large back muscle and also attaches to the sternal angle of the sternum.
• Rib 10. This rib only has one point of attachment to the vertebrae.

The upper opening of the thorax is broader from side to side than from front to back. It is formed by the first thoracic vertebra in the back, the upper section of the sternum in front, and the first rib on either side. It slopes downward and forward, so that the front part of the open ing is on a lower level than the back part. The lower opening of the thorax is formed by the twelfth thoracic vertebra in the back, by the eleventh and twelfth ribs on the sides, and by the costal cartilages of the tenth, ninth, eighth, and seventh ribs in the front. The lower opening is closed by the diaphragm, the thin muscle located below the lungs and heart, that forms the floor of the thorax.