Final answer:
The scalene muscles are responsible for elevating the first and second ribs during forced inhalation, enlarging the thoracic cavity and contributing to the expanded lung volume needed for high oxygen demand activities.
Step-by-step explanation:
The muscles that increase the thoracic cavity by elevating the first and second ribs during forced inhalation are the scalene muscles. During forced inhalation, the scalene muscles of the neck contract to lift the thoracic wall, thereby increasing lung volume. In contrast to normal breathing, which involves mainly the diaphragm and the external intercostal muscles, forced breathing or hyperpnea, like during exercise or singing, requires the additional action of accessory muscles like the scalenes. These muscles help expand the rib cage, facilitating a larger influx of air to meet the increased oxygen demand.
Not only do the scalene muscles contribute to forced inhalation, but other muscles like the diaphragm and external intercostal muscles are also essential in this process. The diaphragm flattens, increasing the volume inside the pleural cavities, and the contraction of the external intercostal muscles aids in pushing the ribs outward, both contributing to lower intra-alveolar pressure and allowing air to flow into the lungs. The intercostal muscles play a role in both inspiration and expiration, altering the volume-pressure ratio for effective pulmonary ventilation.