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As has been pointed out, local application of heat produces relaxation of the tissues, particularly of the voluntary and involuntary muscle fibers, spasm of the skeletal muscles is relieved, the walls of the smaller arteries seem to relax, and the vessels dilate. By reason of these conditions a greater amount of arterial blood flows to the part, bringing oxygen and nutriment. Furthermore, the increased blood flow induced on the venous side carries away in larger degree the products of normal or abnormal metabolism, so that they do not remain as local poisons or irritants but are excreted from the body through natural channels. The heat, as applied from the outside and as conveyed internally to the parts by the rapid arterial flow, accelerates the chemical changes that occur in the tissues.
Numerous conditions occur in various parts of the body in which normal physiologic activity or metabolism is interfered with,
A. BRUCE GILL. HEAT IN SURGICAL AND ORTHOPEDIC CONDITIONS. JAMA. 1936;106(1):40–42. doi:10.1001/jama.1936.02770010010046