The Thorax as a whole is barrel-shaped, narrower above than below, and compressed antero-posteriorly. The inlet, or superior aperture, is kidney-shaped, its plane oblique, sloping downwards and forwards. The lower aperture is curved, and slopes upwards along the twelfth rib to the tip of the eleventh, along the cartilages of the tenth, ninth, eighth, and seventh ribs to the ensiform cartilage. This aperture is closed by the diaphragm.

Fig. 33. - Thorax.