practised, as M. Jourdain talked prose, for so many years, without
knowing what a marvellous accomplishment they had mastered. We shall
have to begin with a few simple anatomical data.

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