_Heat Production._—By watching a muscle when contracting, we see that there is not only a change of shape, but a dilatation of its blood-vessels, that is, more blood passes through a muscle when it is contracting than when it is at rest, and this increased flow continues for some little time after the contraction has ceased; there is also a rise of temperature. Nearly three-fourths of the heat developed in the body is produced in the muscles at the actual moment of muscular contraction. Hence, we learn that the whole body is heated by muscular exercise; the even temperature of the various parts is maintained through the circulation of the blood. This combustion, going on throughout the entire economy, is the source of all force or energy in the body. In every movement, every breath taken, in the change even of a muscle of expression or the conception of a passing fancy, combustion has occurred and potential force has been liberated. _Muscular Fatigue._—The fatigue produced by muscular contraction may be due to the consumption of the readily available material present in the muscle, to the consumption of the supply of oxygen, or to the presence of the products of combustion, and, if Weichardt’s theory is correct, to these must be added the presence of a definite “fatigue toxin.” During repose the internal changes of the tissue manufacture new explosive material out of the comparatively raw material already present in the fiber, and the directly hurtful products of the act of combustion are either carried off, or undergo changes by which they are converted into comparatively inert bodies. A stream of fresh blood may exert its restorative influence, not only by quickening both of these events, but also by carrying off the immediate waste-products, while, at the same time, it brings new raw material. Every movement of the body depends as much upon the proper coördination of the muscles for its accuracy, grace, and force as upon the strength of their contraction, and while the fatigue, of which we are conscious in our own bodies after prolonged or unusual exertion, arises partly from the exhaustion of the motor nerves, it is chiefly from the exhaustion of the central nervous system concerned in the production of voluntary impulses. A man who feels absolutely exhausted may, under excitement, perform a very large amount of work with his already wearied muscles. The will rarely, if ever, calls forth the greatest amount of contraction of which the muscle is capable. =Passive Exercise: Massage.=—Massage has been defined as the systematic manipulation of the surface of the body by the hands of the operator in movements of stroking, pinching, kneading, and striking. The passive movements consist of flexions, extensions, rotations, and other movements of joints and limbs by an operator or machine without the coöperation or resistance of the patient.