The Thoracic Cavity

The thoracic cavity (Cavum thoracis) is the second in point of size of the body cavities. In form it is somewhat like a truncated cone, much compressed laterally in its anterior part, and with the base cut off very obliquely.

The dorsal wall or roof is formed by the thoracic vertebræ and the ligaments and muscles connected with them.

The lateral walls are formed by the ribs and the intercostal muscles.

The ventral wall or floor is formed by the sternum, the cartilages of the sternal ribs, and the muscles in connection therewith. It is about one-half as long as the dorsal wall.

The posterior wall, formed by the diaphragm, is very oblique and is strongly convex.

The anterior aperture or inlet (Apertura thoracis cranialis) is relatively small, and of narrow, oval form. It is bounded dorsally by the first thoracic vertebra and laterally by the first pair of ribs. It is occupied by the longus colli muscles, the trachea, œsophagus, vessels, nerves, and the prepectoral lymph glands.

The cavity is lined by the endothoracic fascia and by the pleuræ.

A longitudinal septum, termed the mediastinum (Septum mediastinale), extends from the dorsal wall to the ventral and posterior walls, and subdivides the cavity into two lateral chambers which contain the lungs. Each of these chambers is lined by a serous membrane called the pleura, and is called a pleural cavity (Cavum pleuræ). The mediastinum is, for the most part, not median in position; this is largely due to the fact that the largest organ contained in it, the heart, is placed more on the left side; consequently the right pleural cavity and lung are larger than the left. Practically all the organs in the thorax are in the mediastinal space with the exception of the lungs, the posterior vena cava, and the right phrenic nerve. The part in which the heart and the pericardium are situated, together with that above it, is usually called the middle mediastinal space; the parts before and behind this are termed the anterior and posterior mediastinal spaces respectively.