Frost blisters are pustular swellings due to the up-growth of callus-tissue into cavities caused by the uprising of the superficial cortex under the action of intense cold.
24) are of economic importance, as they contain a vesicant substance used for raising medicinal blisters on the human skin.
The manifold plate is then heavily punched from one side, so that the opposite face protrudes in broken blisters, which are then hammered down until each becomes a centre of wave propagation.
The numerous small blisters or vesicles thus derived coalesce, forming a large sac full of "blister-fluid."
Blisters also cause local dilatation of vessels, but are usually applied to the skin for inflammation in deep-seated parts, such as the lungs, though they also relieve pain in the joints in acute rheumatism.