# What is the primary muscle of quiet inspiration?
A. Internal intercostals
B. Diaphragm
C. Abdominal muscles
D. External intercostals
The correct answer is B. Diaphragm.
The diaphragm is the primary muscle responsible for quiet inspiration (normal, resting breathing). During quiet inspiration, the diaphragm contracts, flattens, and moves downward, increasing the volume of the thoracic cavity and creating negative pressure that draws air into the lungs. The external intercostals (D) assist in inspiration by elevating the ribs, but they play a secondary role. The internal intercostals (A) are primarily involved in forced expiration, and the abdominal muscles (C) aid in forced expiration or active breathing, not quiet inspiration.
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