The question is about identifying the nature of the substance that fills the pleural cavity, which is the space between the lungs and the chest wall. Let's explore this in detail.

The pleural cavity is a fluid-filled space that surrounds the lungs. It is located between the two layers of the pleura, a serous membrane that lines the thoracic cavity and covers the lungs. The main function of the pleural cavity and its fluid is to allow smooth movement of the lungs during breathing, reducing friction with the chest wall.

Let's evaluate the given options:

• Liquid: The pleural cavity contains pleural fluid, which is a thin layer of liquid. This fluid acts as a lubricant, allowing the layers of the pleura to glide effortlessly over each other during the respiratory movements. This option correctly describes the nature of the substance in the pleural cavity.
• Dry air: If the pleural cavity were filled with dry air, it would lead to separation of the pleurae, causing a condition known as pneumothorax, which disrupts normal lung function.
• Humid air: Similar to dry air, humid air is not present in the pleural cavity. The presence of air in the pleural space, whether dry or humid, also results in pneumothorax.
• Fatty muscle: This option is incorrect, as fatty tissue or muscle does not fill the pleural cavity. The cavity is exclusively meant to contain pleural fluid.

Therefore, the correct answer is Liquid, as the pleural cavity is filled with pleural fluid. The fluid ensures that the lungs can move smoothly and remain operational during the breathing cycle.