e learns to walk and run. These
curves are essential to maintaining the balance of the body in the erect
position.

There are really three curves in the human backbone, the cervical curve
being convex, the dorsal concave, and the lumbar convex, when each is
regarded from the forward aspect. If we consider the sacrum and coccyx,
there is really a fourth curve, this being concave, although in animals
generally the coccyx curves backwards and is extended to form the tail.
In some of the lower animals the spine is nearly straight, while in some
cases it virtually forms a complete arch from one end to the other.

These curves of the spine are generally more marked in the civilized
white races than among the black and savage races, and as a rule they
are more pronounced among women than among men. For instance, in
comparing the sexes we find that in a woman the lumbar curve is more
marked and extends slightly higher than in a man, and that the broad
sacrum characteristic of the human race is even wider, being thus
adapted to the broader hips and wider pelvic cavity of the child-
bearing sex.

Now, the maintenance of a strong and erect spine, and especially of the
normal curves of youth is most important. With the weakness of advancing
age the curves, particularly in the upper part of the spine, tend to
become more pronounced. The more accentuated these curves are the
greater is the weakness of the spine and of the muscles of the back that
is indicated. It is said that a man is as old as his spine,
since the deterioration of the spine means the loss of elasticity and
supporting power in the disk-like cartilages between the vertebrae,
and also the loss of strength in the muscles and ligaments of the back
which tend to hold the spinal vertebrae in place. It is usually found
that vigorous old men who are mentally and physically active at eighty
or ninety years are those who have maintained an erect bearing until
late in life, who have kept their spines straight and strong instead of
allowing them to bend over and double up. In other words, the
deterioration of the spine means a general loss of bodily vigor and a
decline in the nervous energy or vitality.

With the flattening down of the cushiony disks or cartilages between the
vertebrae, and also with the dislocation even in the slightest degree
of these vertebrae, there is brought about more or less interference
with the free action of the sp