Which of the following is defined as the expansion of the chest or thorax?

The correct answer is defined as inspiration, which is the process of taking air into the lungs, leading to the expansion of the chest or thorax. During inspiration, the diaphragm contracts and the intercostal muscles also contract, pulling the ribcage outward and upward. This action increases the thoracic cavity's volume, resulting in a decrease in internal pressure relative to atmospheric pressure. Consequently, air is drawn into the lungs as they expand.

Understanding this process is crucial in veterinary medicine, as proper lung function is essential for animal health, and the mechanics of breathing are foundational for assessing respiratory conditions. The other options, while related to breathing and respiratory function, do not specifically refer to the expansion of the chest itself.