rue that the spine has been shortened in many species as
the result of the loss of the tail, but this means only the dropping
off of a part of it and does not greatly alter its fundamental
character.

The human spine, however, differs from that of other animals in respect
to its suitability for the erect posture. Man is the only animal in the
world who can straighten his body and stand perfectly erect. Even the
anthropoid apes when standing on their feet assume a somewhat
oblique position. The vertebral column in animal life was first
developed on the horizontal plane, and so, naturally, when man was
evolved and adopted the erect position, certain modifications of the
spine were necessary. A new strain developed on the vertebral column
which was due to the new position, and so there came about certain
changes in its structure. For one thing the spine became less
flexible and gained in stability, especially in the lower sections.
The sacrum, for instance, is created by the fusing together of several
vertebrae into one bone for the sake of greater strength and stability.
The sacrum in man is much broader than in animals, for it must supply
solidity and strength to the lower part of the spine, thus adapting it
to the vertical position, and in the same way the lower vertebrae
generally are comparatively broader and heavier, gradually decreasing
in size and tapering toward the top of the spine like the trunk of a
tree.

This particular feature of the human backbone is worthy of special
consideration because it is the upper section of the spine, in which
the vertebrae are smaller and tapering, that weakness is most likely to
exist. It is in this upper section of the spine that strength is most
needed in order to preserve it in perfect alignment, and keep the body
properly erect. And it is for this reason, as the reader will
see, that exercises affecting the upper parts of the spine are most
important. Therefore I have given them special attention.

The curves in the human spine are characteristic, illustrating in
another way the modification of the vertebral column that has been
made necessary by the erect position. The new-born baby has a backbone
that is almost straight, and in this respect it bears a strong
resemblance to that of many of the lower animals. The typical human
curves, however, begin to take form as soon as the child learns to sit
up, and they become more marked as h