The thoracic skeleton is an osseo-cartilaginous cage, containing and protecting the principal organs of respiration and circulation. It is conical in shape, being narrow above and broad below, flattened from before backward, and longer behind than in front. It is somewhat reniform on transverse section on account of the projection of the vertebral bodies into the cavity.
The posterior surface is formed by the twelve thoracic vertebræ and the posterior parts of the ribs. It is convex from above downward, and presents on either side of the middle line a deep groove, in consequence of the lateral and backward direction which the ribs take from their vertebral extremities to their angles. The anterior surface, formed by the sternum and costal cartilages, is flattened or slightly convex, and inclined from above downward and forward. The lateral surfaces are convex; they are formed by the ribs, separated from each other by the intercostal spaces, eleven in number, which are occupied by the Intercostal muscles and membranes.
The upper opening of the thorax is reniform in shape, being broader from side to side than from before backward. It is formed by the first thoracic vertebra behind, the upper margin of the sternum in front, and the first rib on either side. It slopes downward and forward, so that the anterior part of the opening is on a lower level than the posterior. Its antero-posterior diameter is about 5 cm., and its transverse diameter about 10 cm. The lower opening is formed by the twelfth thoracic vertebra behind, by the eleventh and twelfth ribs at the sides, and in front by the cartilages of the tenth, ninth, eighth, and seventh ribs, which ascend on either side and form an angle, the subcostal angle, into the apex of which the xiphoid process projects. The lower opening is wider transversely than from before backward, and slopes obliquely downward and backward, it is closed by the diaphragm which forms the floor of the thorax.
The thorax of the female differs from that of the male as follows: 1. Its capacity is less. 2. The sternum is shorter. 3. The upper margin of the sternum is on a level with the lower part of the body of the third thoracic vertebra, whereas in the male it is on a level with the lower part of the body of the second. 4. The upper ribs are more movable, and so allow a greater enlargement of the upper part of the thorax.