The thorax, commonly known as the chest, is the upper portion of the trunk situated between the neck and the abdomen. This region acts as a specialized compartment, housing and protecting the primary organs responsible for respiration and circulation. The structure itself is a flexible, bone-and-cartilage enclosure that provides both a secure environment and the necessary mobility for breathing.
Defining the Boundaries
Superiorly, the thorax begins at the base of the neck, marked by the thoracic inlet or superior thoracic aperture. This upper opening is a narrow space bounded by the first pair of ribs, the first thoracic vertebra, and the upper margin of the sternum.
The inferior boundary is a broad, muscular partition known as the diaphragm. This dome-shaped muscle forms the floor of the thoracic cavity, creating a separation between the contents of the chest and the organs of the abdominal cavity below.
The anterior wall of the thorax is formed by the sternum, or breastbone, while the posterior wall is supported by the 12 thoracic vertebrae of the spinal column. The lateral boundaries are made up of the curved structure of the ribs, completing the enclosure.
The Protective Skeletal Structure
The bony framework of the thorax is called the thoracic cage. This cage is composed of 12 pairs of ribs, the sternum at the front, and the 12 thoracic vertebrae in the back. The primary function of this structure is to provide a physical shield for the underlying internal organs and to act as anchor points for the muscles involved in breathing.
The 12 thoracic vertebrae (T1–T12) form the central axis to which the ribs attach posteriorly. Anteriorly, the sternum is an elongated, flattened bone consisting of three parts: the manubrium, the body, and the xiphoid process. The ribs connect to the sternum through flexible costal cartilage, which allows the chest wall to expand and contract during respiration.
Ribs are classified based on their connection to the sternum. The first seven pairs are considered “true ribs” because their costal cartilages attach directly to the sternum. Ribs eight through ten are “false ribs,” as their cartilage connects to the cartilage of the rib immediately above them. The final two pairs, ribs eleven and twelve, are termed “floating ribs” because they do not connect to the sternum or costal cartilage, ending instead in the muscular wall of the back.
Major Internal Components
The space within the protective thoracic cage is the thoracic cavity, which is internally divided into three main compartments. The two lateral compartments are the pleural cavities, and the central compartment is the mediastinum. Each pleural cavity contains a lung, which is enclosed by a double-layered membrane called the pleura.
The mediastinum is located between the two lungs and houses the heart, which is slightly offset to the left and enclosed in a fibrous sac called the pericardium. The mediastinum is a corridor for several other structures traveling between the neck and the abdomen. The trachea, or windpipe, passes through this region to split into the bronchi that enter the lungs.
The esophagus, the muscular tube that transports food from the throat to the stomach, also runs through the posterior section of the mediastinum. This central region accommodates the great blood vessels, including the large ascending and descending aorta, the superior and inferior vena cava, and the pulmonary artery and veins. The thymus gland is also located in the anterior part of the mediastinum.