The thorax is the region of the body commonly known as the chest between the neck and the abdomen. The thoracic cavity is the hollow in the thorax that is occupied by the thoracic viscera, the heart and its associated vessels in the midline, and the lungs laterally. The thoracic viscera are enclosed by the bony and muscular thoracic cage. The bony components of the cage are the 12 thoracic vertebrae posteriorly, the 12 pairs of ribs and their anterior cartilaginous extensions, the costal cartilages that meet the sternum anteriorly. The intercostal muscles fill the intercostal spaces between the ribs and are involved in ventilation. Another muscle involved in ventilation is the diaphragm, a sheet of muscle that separates the thoracic from the abdominal cavity. If you are not familiar with the basic outline and arrangements of the circulatory and respiratory systems, refer back to Chapters 4 and 5 before reading this section.
A good way to appreciate where these structures lie in relation to each other is to examine their surface anatomy, the position of internal organs related to features that can be observed or palpated (felt) on the surface of the body. Relating surface anatomy to deeper structures is a clinical skill essential not only to the study of the thorax, but also of structures in the head and neck important in dental practice.
In the clinical examination of the living subject, the position of the internal thoracic organs is defined with reference to a set of vertical and horizontal lines running through the surface of bony landmarks. The significant vertical lines are shown in Figure 9.1 as the:
1. Mid-sternal line—in the median plane anteriorly;
2. Mid-clavicular line—through the midpoint of the clavicle;
3. Mid-axillary line—midway between the anterior and posterior axillary folds, formed from skin overlying muscles. If you raise your arm while looking into a mirror, the two folds are obvious; they can also be palpated very easily even with clothes on.
4. Median posterior line—in the median plane anteriorly.
The horizontal position can be defined with reference to the ribs or, less easily, the vertebrae.
Examine Figure 9.1 and yourself, if convenient to do so, at the same time as reading the description. First, run your finger down the anterior midline of your neck. As you do so, you will feel the cartilaginous rings reinforcing the trachea until it disappears behind the sternum. You should now feel the suprasternal notch on the superior margin of the sternum. If you move your fingers laterally from the notch, you will feel the heads of the clavicles on each side as they articulate with the upper part of the sternum. As you can see in Figure 9.1, the first rib lies deep to the clavicle and consequently, is difficult to palpate. It is more practical, therefore, to start rib counting from the second rib. If you return your finger to the midline of the suprasternal notch and run it vertically downwards in the midline, you will encounter the