Regions of the Thorax
The thorax is the superior part of the trunk between the neck and the abdomen. The term chest is commonly used as a synonym for thorax, but our concept of the chest (upper part of the torso) is much more extensive than the thoracic wall and cavity contained within it. The chest is generally conceived as being broadest superiorly owing to the presence of the pectoral, or shoulder girdle with much of its girth accounted for by the associated pectoral and scapular musculature.
Our concept of the well-formed chest is one that narrows inferiorly to the waist and, in adult females, gains further dimension from the breasts. The thoracic cavity and the wall specific to it are actually the opposite of this. They have the shape of a truncated cone, being narrowest superiorly, with the circumference increasing inferiorly, and reaching its maximum at the junction with the abdominal portion of the trunk.
Therefore, the thorax can be defined as consisting of the thoracic cavity, its contents including the primary organs of the respiratory and cardiovascular systems, and the wall that surrounds it. It is the part of the trunk between the neck and abdomen.
The thorax is composed of a ventral and a dorsal surface. Each of these surfaces is partitioned into regions whose names however do not always correspond to the underlying morphological structures. For example, the heart lies within the sternal, mammary and inframammary regions.
Ventral Thoracic Regions
On the anterior surface, the thorax can be divided into several regions, each one being defined by bony landmarks. The regions include:
- Infra-clavicular, which is defined by the lower border of the clavicle, second rib and the lateral border of the manubrium of the sternum. The apex of the lungs are housed in this region.
- Mammary, which contains the breast, parts of the heart on the left side among other underlying structures. It is defined by the third rib superiorly, lateral border of the lower part of the sternum, down to the level of the xiphisternal joint, medially, and by the sixth rib inferiorly.
- Infra-mammary, which is somewhat triangular in shape, bounded by the sixth rib superiorly and the costal margin formed by the costal cartilages of the 6th to 10th ribs medioinferiorly. The apex of the heart runs into this region on the left side.
- Upper sternal, which corresponds to the manubrium of the sternum.
- Lower sternal, which is marked by the body of the sternum and are located between the manubrosternal and xiphisternal joints. It houses a good portion of the heart.
Dorsal Thoracic Regions
On the posterior surface, the thorax is divided into four regions on each half of the midline:
- Suprascapular, the region superior to the spine of scapula.
- Scapular, the part corresponding to the scapula.
- Infrascapular, which is just inferior to the inferior angle of the scapula.
- Interscapular, which spans the part of the thorax between the medial borders of the left and right scapulae. It lies within the bodies of the first and eight thoracic vertebrae.