Final answer:
Depression of the ribs decreases the lateral dimensions of the thoracic cavity, leading to a decrease in volume and an increase in pressure which is responsible for exhalation.
Step-by-step explanation:
Depression of the ribs decreases the lateral dimensions of the thoracic cavity, meaning the answer to the question is 1) decreases.
During the process of breathing, the external and internal intercostal muscles play a significant role. These muscles are located between the ribs and their contraction causes the rib cage to change shape, thereby affecting the volume-pressure ratio within the pleural cavities. When the external intercostal muscles contract, the thoracic cavity's volume increases as the rib cage expands outward and the diaphragm moves downward. This leads to a decrease in pressure inside the thoracic cavity, allowing air to flow into the lungs. Conversely, when the ribs are depressed, the thoracic cavity volume decreases, and the pressure inside increases, leading to exhalation.