The foot is arched both longitudinally and transversely, so as to give it elasticity, and thus break the sudden shock when the weight of the body is thrown upon it. The ankle-joint is a loose hinge, and the great muscles of the calf can straighten the foot out so far that practised dancers walk on the tips of their toes. The knee is another hinge-joint, which allows the leg to bend freely, but not to be carried beyond a straight line in the other direction. Its further forward movement is checked by two very powerful cords in the interior of the joint, which cross each other like the letter X, and are hence called the _crucial ligaments_. The upper ends of the thighbones are almost globes, which are received into the deep cup-like cavities of the haunch-bones. They are tied to these last so loosely, that, if their ligaments alone held them, they would be half out of their sockets in many positions of the lower limbs. But here comes in a simple and admirable contrivance. The smooth, rounded head of the thighbone, moist with glairy fluid, fits so perfectly into the smooth, rounded cavity which receives it, that it holds firmly by _suction_, or atmospheric pressure. It takes a hard pull to draw it out after all the ligaments are cut, and then it comes with a smack like a tight cork from a bottle. Holding in this way by the close apposition of two polished surfaces, the lower extremity swings freely forward and backward like a _pendulum_, if we give it a chance, as is shown by standing on a chair upon the other limb, and moving the pendent one out of the vertical line. The force with which it swings depends upon its weight, and this is much greater than we might at first suppose; for our limbs not only carry themselves, but our bodies also, with a sense of lightness rather than of weight, when we are in good condition. Accident sometimes makes us aware how heavy our limbs are. An officer, whose arm was shattered by a ball in one of our late battles, told us that the dead weight of the helpless member seemed to drag him down to the earth; he could hardly carry it; it "weighed a ton," to his feeling, as he said. In _ordinary walking_, a man's lower extremity swings essentially by its own weight, requiring little muscular effort to help it. So heavy a body easily overcomes all impedimenta from clothing, even in the sex least favored in its costume. But if a man's legs are pendulums, then a short man's legs will swing quicker than a tall man's, and he will take more steps to a minute, other things being equal. Thus there is a natural rhythm to a man's walk, depending on the length of his legs, which beat more or less rapidly as they are longer or shorter, like metronomes differently adjusted, or the pendulums of different time-keepers. Commodore Nutt is to M. Bihin in this respect as a little, fast-ticking mantel-clock is to an old-fashioned, solemn-clicking, upright time-piece.