INSPIRATION, in Physics, is understood of that action of the breast, by which the air is admitted within the lungs; in which sense, inspiration is a
branch of respiration, and stands opposed to EXSPIRATION.
This admission of the air depends immediately on its spring or elasticity, at the time when the cavity of the breast is enlarged by the elevation of the thorax and abdomen, and particularly by the motion of the diaphragm downwards: so that the air does not enter the lungs, because they are dilated; but those dilate, because the air enters within them. Nor is it the dilatation of the breast which draws in the air, as is commonly thought, though this is a condition absolutely necessary to inspiration; but an actual intrusion of the air into the lungs. See RESPIRATION.