A very usual experience is to find that a patient has as a preliminary suffered some injury. This may not be very severe, but it is enough to cause the sufferer to spare the limb that is affected; and unfortunately physicians sometimes put a limb with a minor affection in plaster of Paris or in a splint for a time. The patient's own solicitude with regard to the hurt may cause almost as effective splinting of it as a plaster cast or wooden slat and a bandage. Whenever this happens, the unused muscles lose some of their nutrition. This is due partly to the fact that the circulation is interfered with because the active contraction of muscles, especially of the legs, is depended on by nature to help the venous or return circulation by bringing about compression of the veins. The valves in the veins are so arranged that when the {243} veins are compressed and the blood thus pressed out of them, it cannot move away from the heart but is impelled onward toward the heart. Besides, the sending down of nervous impulses for the active use of muscles increases the size of the arteries to the part by direct action on their walls, and whenever there is failure to send impulses down, the arteries do not carry as much blood as usual, and the nutrition of the part suffers as a result. If this inactive state of the muscles continues for a few days, they will become somewhat flaccid, and after a week or more will actually begin to decrease in size. As a consequence of this, they cannot be used to as good advantage as before, and use of them sets up an achy condition as soon as the limb is set free for use, whether it has been splinted by the physician or by the patient's mind. If the patient is still solicitous, he notices this condition of pain and concludes that it means that the muscles are not yet in the condition of health where they should be used, and he puts on the splints, metaphorical or literal, once more. The muscles grow more flaccid and eventually atrophic as a consequence, until sometimes there will be a difference of more than an inch in the girth of two limbs at the same point, and this atrophy may proceed much farther. It seems almost impossible to believe that men and women could thus make a limb useless, but this is actually what happens rather frequently. The effects of the original injury will pass off in a few days, but the effects of the disuse of the limb may remain for months or even years because of the disturbance of circulation and of nerve impulse. It is probable that the nerves themselves have a trophic or nutritional--that is, vitalizing--influence upon muscles. Some physiologists actually talk of there being {244} trophic fibers in the nerves, though it would seem more reasonable to think that the nerve trophic effect comes from the modification of the circulation to the part.