Human Anatomy - Kotsan I. Y. 2009

Anatomy of the Axial Skeleton
The Thorax as a Whole

In the human Skeleton, The thoracic cage resembles an irregular cone with a truncated apex, its base directed downward (Fig. 27). The Structure of the rib cage comprises anterior, posterior, and two lateral walls that transition seamlessly into one another, as well as superior and inferior openings, or apertures.

The anterior wall is formed by the Sternum, costal cartilages, and the anterior ends of the Ribs. The posterior wall consists of the thoracic vertebrae and the posterior ends of the ribs, while their middle sections form the lateral walls. The superior thoracic aperture (apertura thoracis superior) is bounded by the first thoracic vertebra, the first ribs, and the superior margin of the manubrium of the sternum. It transmits the Esophagus, Trachea, major Blood Vessels, nerves, and other structures.

The inferior thoracic aperture (apertura thoracis inferior) is bounded by the twelfth thoracic vertebra, the ends of the eleventh and twelfth pairs of ribs, the costal arches, and the xiphoid process. This opening is sealed by a muscular partition known as the Diaphragm. The right and left costal margins form the infrasternal angle (angulus infrasternalis).

The shape and dimensions of the thoracic cage depend on age, sex, physical activity, social factors, and individual anatomical variations. Depending on body type, three main shapes of the rib cage are distinguished: conical, flat, and cylindrical. In individuals with a brachyorphic (hypersthenic) body type and well-developed musculature, the chest is conical in shape. Its lower portion is significantly wider than the upper, the ribs have a minimal downward slope, the infrasternal angle is obtuse, and the difference between the anteroposterior and transverse diameters is relatively small. In individuals with a dolichomorphic (asthenic) body type, the rib cage is flat—notably compressed in the anteroposterior direction—with ribs steeply inclined downward and an acute infrasternal angle. The mesomorphic (normosthenic) body type features a cylindrical rib cage, which is intermediate in form between the other two types. In women, the thoracic cage is shorter, more rounded, and narrower in its lower portion, whereas in men it more closely resembles the typical conical shape. In newborns, the anteroposterior diameter of the chest exceeds the transverse diameter, whereas in the elderly, the chest becomes flatter and longer due to reduced Muscle tone and the downward Displacement of the anterior rib ends. Diseases and occupation also influence the shape of the thoracic cage. For instance, in Rickets, the chest acquires a "Pigeon chest" deformity, where the sternum protrudes sharply forward due to an increased anteroposterior dimension. Trumpet players and glassblowers typically develop a broad, prominent chest, and so forth.

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Fig. 27. Thoracic cage (anterior view)

1 — superior thoracic aperture, 2 — infrasternal angle, 3 — inferior thoracic aperture