Accordingly, when the muscles are prepared and in proper action, that is, when they place the joint in a suitable position and duly support or brace it, a sprain very rarely occurs. It is when the muscles are unprepared, when we make a false step, or when the foot encounters an unexpected obstacle, and the weight falls suddenly upon the ligaments in an unfavourable direction, that a sprain occurs. A man jumps from a considerable height, or descends deep steps with a heavy weight upon his back, and no harm results; but he slips off the curb-stone, or treads unwarily upon a piece of orange-peel, or turns his foot hastily, to avoid some object on the ground, and sprains his ankle. In order that they may do their work well, be alert, and maintain good guard, the muscles need to be kept in practice. A person unaccustomed to throw a ball is very liable to sprain his elbow with the sudden jerk which is required for that feat; or if a person takes to tumbling and jumping, without proper training, he will probably suffer for his temerity. Again, common experience tells us that a joint which has been sprained is, for a long time, liable to be sprained again. This is because the part remains tender as well as weak; and the muscles do not brace it steadily and firmly, or come nimbly to its aid when it is in danger. In consequence of the foot, in walking, being placed upon the ground with the toes slanting a little outwards, the _out_er and hinder edge of the heel first touches the ground. Hence this part of the heel of the shoe is usually worn down before the remainder. The ball of the little toe next comes to the ground, and the balls of the other toes follow in quick succession; and it is from the great toe--that is, from the inner side of the foot--that the last impulse is given to propel the body, forwards, over the other foot. In order to give full effect to this final impulse an especial muscle, the “Long Fibular” muscle (I in fig. 13), is provided. The tendon (i) of this muscle passes, behind the outer ankle, beneath the sole of the foot, to the great toe. It has the effect of pressing the ball of the great toe upon the ground, while it raises the outer ankle, and so contributes to throw the weight, across, in the direction of the other foot. Thus the foot revolves upon the ground, from the heel to the balls of the toes, and from the outer edge of the former to the inner edge of the latter; and during the revolution, which has been compared, though the comparison fails in many points, to the revolving of the segment of a wheel, the ankle is raised and advanced forwards.