Pleura: Serous membrane lining thoracic cavity & covering lungs.

Two layers:
1. Parietal pleura (outer, lines wall) — has 3 named regions:
- Costal pleura (on ribs)
- Diaphragmatic pleura (on diaphragm)
- Mediastinal pleura (on midline)
2. Visceral (Pulmonary) pleura — covers lung surface.

Pleural cavity: Potential space between layers; small amount of fluid; NEGATIVE pressure keeps lungs expanded.

Reflections (lines where parietal pleura turns from wall to organs):
- Cupula pleurae (cranial dome, in front of 1st rib).
- Costodiaphragmatic line of pleural reflection (most important): Caudo-ventral border of pleural sac. From 8th costochondral junction → 11th rib midway → 13th rib head.

Mediastinum: Median partition between the two pleural sacs.

In OX & SHEEP: COMPLETE mediastinum — right & left pleural cavities do NOT communicate.
In HORSE & DOG: FENESTRATED (incomplete) — small holes connect them.

PLEURA — SHRINK-WRAP OF THE LUNGS

The pleura is a thin, slippery serous membrane that surrounds each lung. Like a balloon wrapped twice around your fist, it has two layers with a tiny space between them.

TWO LAYERS:
• Parietal pleura — the OUTER layer; lines the inside of the chest wall and the diaphragm. It is divided by location:
– Costal pleura: on the ribs and intercostal muscles.
– Diaphragmatic pleura: on the diaphragm.
– Mediastinal pleura: on the median wall (mediastinum).
• Visceral (Pulmonary) pleura — the INNER layer; tightly stuck to the lung surface; dips into the fissures between lobes.

The two layers meet at the hilus (root) of the lung where bronchi and vessels enter.

PLEURAL CAVITY:
Between the two layers is a thin film of pleural fluid (a few millilitres) which lubricates lung movement. The pressure inside this cavity is slightly NEGATIVE (less than atmospheric); this 'suction' is what keeps the lung stretched against the chest wall. If air leaks in (pneumothorax), the lung collapses.

PLEURAL REFLECTIONS (the lines where parietal pleura turns):
• Cupula pleurae: Dome-shaped extension of the pleura that bulges past the 1st rib into the base of the neck. Important — a stab in the lower neck can pierce the pleura!
• Cranial line of reflection: From cupula along the sternum.
• Costo-diaphragmatic line of reflection: The most clinically important. It marks the caudo-ventral border of the pleural sac. In the ox, it runs roughly from the 8th costochondral junction → middle of the 11th rib → head of the 13th rib. The lung itself does NOT extend this far back — there is a 'reserve' space (costodiaphragmatic recess) into which the lung expands during deep inspiration.

This line is the key landmark for paracentesis thoracis (see separate entry).

MEDIASTINUM — the wall between the two halves:
The mediastinal pleurae from the right and left sides come together in the median plane to form a thin wall called the mediastinum. It contains all the chest organs except the lungs: heart, great vessels, trachea, oesophagus, thymus, lymph nodes, nerves, thoracic duct.

Divided by position into:
• Cranial mediastinum — in front of the heart (contains thymus, cranial vena cava, brachiocephalic trunk, oesophagus, trachea).
• Middle mediastinum — contains the heart and pericardium.
• Caudal mediastinum — between heart and diaphragm (oesophagus, descending aorta, caudal mediastinal LN).

COMPLETE vs INCOMPLETE MEDIASTINUM — VERY IMPORTANT:
• OX, SHEEP, GOAT, PIG: Mediastinum is COMPLETE — there are no holes, so the right and left pleural cavities are totally separate. Result: If air or pus enters one side (pneumothorax/empyema), the OTHER side stays normal — only one lung is affected.
• HORSE & DOG: Mediastinum is FENESTRATED (incomplete) — has thin areas with holes, so right and left cavities communicate. Result: If one side has pneumothorax, BOTH lungs collapse → very serious.

SPECIAL FOLD: Plica venae cavae — a fold of pleura on the right side enclosing the caudal vena cava as it travels from diaphragm to heart. The accessory lobe of the right lung sits in this fold.