Thorax

The thorax is the part of the trunk located between the neck and the abdomen. It is composed of a musculoskeletal wall that encloses the thoracic cavity, which houses the primary organs of the circulatory and respiratory systems.

The thoracic wall forms a bony and muscular cage that protects the vital organs.

• Dorsal boundary: The thoracic vertebrae.

• Ventral boundary: The sternum (sternal region).

• Lateral boundaries: The ribs, costal cartilages, and intercostal muscles, forming the costal region.

• Cranial boundary (Cranial thoracic aperture): This opening is formed by the first pair of ribs, the first thoracic vertebra, and the sternal manubrium.

• Caudal boundary (Caudal thoracic aperture): The opening is almost entirely closed by the diaphragm, a dome-shaped muscle that separates the thorax from the abdomen.

The internal space defined by the thoracic wall is the thoracic cavity. It is divided by serous membranes (the pleura) into several compartments.:

• Pleural cavities: There is a right and a left pleural cavity, each containing a lung. In dogs and cats, these cavities communicate through multiple fenestrations in the mediastinum, which explains why pneumothorax or pleural effusion is often bilateral.

• Mediastinum: This is the central partition located between the two pleural cavities. It contains the heart, major vessels, trachea, oesophagus, thymus, and lymph nodes.

The thorax varies considerably among species. Dogs and cats have a deep and narrow thorax, generally with 13 pairs of ribs and a fenestrated mediastinum, allowing pleural communication. Ruminants have a deep but broader thorax, with 13 pairs of ribs (cattle) and a non-fenestrated mediastinum. The horse has a very deep and long thorax, typically with 18 pairs of ribs and a non-fenestrated mediastinum. Finally, humans have a broad thorax that is flattened in the anteroposterior direction, adapted to bipedalism, with 12 pairs of ribs and a non-fenestrated mediastinum.