Final answer:
During inhalation, the thoracic cavity expands due to muscle contraction, leading to decreased air pressure in the thorax and causing air to enter the lungs to equalize pressure. This statement is true.
Step-by-step explanation:
During inhalation, the thoracic cavity becomes larger due to the contraction of the diaphragm and intercostal muscles. This expansion leads to decreased air pressure inside the thorax compared to the outside atmosphere. As a result, air rushes into the lungs to balance the pressure difference, known as a pressure gradient. Therefore, the statement that the thoracic cage expands and the air pressure within the thorax decreases, creating a slight vacuum during inhalation, is True.
The process of normal expiration is the opposite; it's passive and results from the elasticity of lung tissue and relaxation of muscles, leading to increased pressure within the lungs that propels air out.