What muscle forms the undersurface of the thorax and is vital for breathing?

The diaphragm is a dome-shaped muscle that forms the undersurface of the thorax and plays a crucial role in the process of breathing. When the diaphragm contracts, it moves downward and flattens, creating a vacuum effect in the thoracic cavity. This allows air to be drawn into the lungs, facilitating inhalation. During exhalation, the diaphragm relaxes and returns to its domed shape, helping to expel air from the lungs.

Unlike the diaphragm, the intercostal muscles, while important for expanding and contracting the rib cage during breathing, act in conjunction with the diaphragm rather than forming the primary surface that separates the thoracic cavity from the abdominal cavity. The rectus abdominis is primarily responsible for trunk flexion and is involved in processes such as forced exhalation but does not play the primary role in inhalation. The sternocleidomastoid assists with neck movement and can help elevate the thorax during labored breathing but is not the primary muscle involved in the mechanics of normal respiration.