The skeleton of the thorax comprises the thoracic vertebræ dorsally, the
ribs and their cartilages laterally, and the sternum ventrally. The
=thoracic cavity= (Cavum thoracis) resembles in shape an irregular
truncated cone; it is compressed laterally, especially in front, and the
dorsal wall or roof is much longer than the ventral wall or floor. The
=anterior aperture= (Apertura thoracis cranialis) is bounded by the
first thoracic vertebra dorsally, the first pair of ribs and their
cartilages laterally, and the manubrium sterni ventrally. The =posterior
aperture= (Apertura thoracis caudalis) is bounded by the last thoracic
vertebra, the last pair of ribs, the costal arches, and the anterior
part of the xiphoid cartilage.
It may be noted here that the diaphragm (which forms the partition
between the thoracic and abdominal cavities) does not follow the
costal arches in its posterior attachment, so that the posterior ribs
enter also into the formation of the abdominal wall.