Constructive Anatomy by Bridgman (PDF)

The masses of the head, chest and pelvis are
unchanging.
Whatever their surface form or markings, they
are as masses to be conceived as blocks.
The conception of the figure must begin with the
thought of these blocks in their relation to each
other. They are to be thought of first as one thinks
of the body of a wasp, with only one line connecting
them, or without reference at all to connecting
portions.
Ideally, in reference to gravitation, these blocks
would be balanced symmetrically over each other.
But rarely in fact, and in action never, is this the
case. In their relations to each other they are limited
to the three possible planes of movement.
That is, they may be bent forward and back in the
sagittal plane, twisted in the horizontal plane, or
tilted in the transverse plane. Almost invariably,
in fact, all three movements are present, to different degrees.