Final answer:
The primary muscles of inspiration are the diaphragm and external intercostal muscles, both of which help expand the thoracic cavity. The internal intercostal muscles assist with expiration rather than inspiration.
Step-by-step explanation:
The muscles involved in inspiration are primarily the diaphragm and the external intercostal muscles. When the diaphragm contracts, it flattens and moves downward, thereby increasing the volume of the thoracic cavity and reducing internal pressure, which allows air to flow into the lungs. Similarly, the contraction of the external intercostal muscles elevates the ribs and expands the rib cage, further aiding in inspiration by increasing thoracic volume.
Accessory muscles that can assist in inspiration, especially during forced breathing (such as during exercise), may include muscles like the sternocleidomastoid and the scalene muscles in the neck. Despite the internal intercostal muscles being intercostal, they are primarily involved in expiration, particularly forced expiration, by pulling the ribs together and decreasing the volume of the thoracic cavity.