- The muscle that primarily facilitates normal, restful inspiration is the diaphragm.
- During quiet breathing, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity.
- This action reduces the pressure inside the lungs relative to atmospheric pressure, allowing air to flow in passively.

- While the external intercostal muscles assist by elevating the ribs and expanding the chest cavity, their role is secondary to the diaphragm during restful inspiration.

- The sternocleidomastoid and scalene muscles are accessory muscles that become active mainly during forced or labored inhalation, such as during exercise or respiratory distress.

- In summary, the diaphragm is the principal muscle responsible for normal, restful inspiration, making it essential for effective and energy-efficient breathing.

Reference: Gray's Anatomy, 42nd Edition, Respiratory System, Chapter 6 / Page 435