Final answer:
During inspiration, the diaphragm and external intercostal muscles are primarily involved in expanding the thoracic cavity. The serratus anterior, not the serratus posterior, also assists with breathing by stabilizing the scapula. Accessory muscles such as the pectoralis minor contribute to the elevation of the ribs.
Step-by-step explanation:
The process of inspiration, or inhaling, involves several muscles that work together to increase the size of the thoracic cavity. The main muscle of inspiration is the diaphragm, which contracts and flattens to increase the volume within the pleural cavities, leading to decreased pressure and causing air to flow into the lungs.
In addition to the diaphragm, the external intercostal muscles play a crucial role. These 11 pairs of superficial muscles, when they contract, raise the ribcage, thus expanding it to facilitate breathing. Conversely, the 11 pairs of internal intercostal muscles are responsible for expiration as they draw the ribs closer to constrict the ribcage. Despite the user's mention of serratus posterior in the question, it is important to note that the actual muscle contributing to inspiration is the serratus anterior. It originates on the ribs and inserts onto the scapula, assisting indirectly with respiration by holding the scapula against the thoracic wall and allowing the rib cage to move more freely during breathing. Other accessory muscles of inspiration include muscles such as the pectoralis minor that help elevate the ribs.