In the little tin soldier at "attention" we have an example of the sym-
metrical balance of these blocks one directly over the other. But this balance
never exists when the body is in action, seldom, indeed, when it is in repose.
The blocks in their relation to each other are limited to the three possible
planes of movement. They may bend forward and back in the sagittal plane,
twist in the horizontal plane or tilt in the transverse plane. As a rule, all three
movements are present and they may be closely approximated by turning and
twisting the three blocks in the little model of lath and wire.
The limitation to the movement of the spine limits the movements of the
three masses or blocks. Such movement as the spine allows the muscles also
allow.