The diaphragm is the primary muscle responsible for inspiration during normal, quiet breathing because its contraction increases the vertical dimension of the thoracic cavity. When the diaphragm contracts, it moves downward, creating negative intrathoracic pressure that allows air to flow into the lungs. This muscle’s unique dome shape and central tendon attachment make it highly efficient for this task. In contrast, other muscles like the intercostals assist mainly during forceful breathing. Surgical procedures involving the thorax must carefully preserve diaphragm function to maintain effective ventilation.

Reference: Gray's Anatomy, Henry Gray, 42nd Edition.