The thorax is commonly known as the chest. In this section we’ll be looking mainly at the musculo-skeletal structures of the thorax, and at its principal blood vessels and nerves. We'll also look at the breast. We will see the lungs and the heart, but only briefly. They’ll be shown fully in Volume Five of this Atlas. We’ll start, as always, with the bones. Then we'll look at the pleural membrane, then at the muscles, then at the blood vessels and nerves.
The bones of the thorax are the thoracic vertebrae, the twelve pairs of ribs, and the sternum. Connecting the upper ten pairs of ribs to the sternum are the costal cartilages.
The first rib is quite small. Like all the ribs, it’s angled downward from back to front. We’ll take a special look at the first rib in a little while.
From the first rib to the third, the thorax widens in the shape of a dome, to about two thirds of its full width. From the third rib to the seventh the thorax widens a little further, in the shape of a cone. From the seventh rib to the twelfth, the thorax narrows slightly, and the ribs become very much shorter.
The sternum, commonly known as the breast-bone, consists of three parts: the manubrium, the body, and the xiphoid process, or xiphisternum.
The manubrium is attached to the body of the sternum by a cartilaginous joint, at which a little movement is possible. There’s a slight angle between the manubrium and the body, the sternal angle, that’s easy to palpate, as is the upper border of the manubrium.
The costal cartilages form a series of flexible, springy links between the ribs and the sternum. The first costal cartilage articulates with the manubrium; the second one articulates with the joint between the manubrium and the body; and the third to the sixth or seventh costal cartilages articulate with the body.
Here’s what the costal cartilages look like in the living body. They’re quite flexible. These are the costo-chondral junctions, where the cartilages join the ribs.
The lowest four costal cartilages, the seventh, eighth, ninth, and tenth, join on to one another in series, forming the costal arch.
The angle between the two costal arches is called the infrasternal angle. The xiphoid process projects downwards in the infrasternal angle, where it can easily be palpated.
The eleventh and twelfth ribs aren’t attached to the costal arch. Since they’re not linked to the sternum, they’re called “floating ribs”.
The ribs, sternum and costal cartilages form an expandable container for the lungs and heart. This large opening, formed on each side by the costal arch and the last two ribs, is called the inferior or lower thoracic aperture. It’s almost completely filled in by the diaphragm, which separates the thorax from the abdomen.
The much smaller opening above, that’s formed by the manubrium, the first ribs, and the first thoracic vertebra, is called the superior or upper thoracic aperture.
Now that we’ve looked at the thorax as a whole, let’s take a look at a typical rib, the sixth rib. The rib is thin and flat, and curved in the form of a spiral.
At the back there are two thickenings, the head and the tubercle, which are separated by the neck. The curvature of the rib is interrupted by this angle, which marks the insertion of the iliocostalis, a back muscle that we’ve seen already.
At the front, the end of the rib is hollowed out, for the attachment of the costal cartilage. The outer aspect of the rib is smoothly curved. Its inner aspect is marked on the underside by this groove, in which the intercostal vessels and nerve run.