24 Yoga anatomYFigure 2.3 shows a parasitic worm with a flattened body, called
a platyhelminth, and in it we see the development of a rudimentary
central nervous system. It exhibits a cluster of primitive nerve cells
at the top and two nerve cords running down its length. Worms
are invertebrates, but in their descendants, these rudimentary
nerve cells have evolved into the brain, the spinal cord, and the
dual trunks of the autonomic nervous system. They all require
the corresponding development of a structure that allows for free
movement but is stable enough to offer protection to these vital
yet delicate tissues—in other words, a skeletal spine.
A central nervous system allows for an enormous amount of
flexibility in a vertebrate’s survival activities, and the spine must
thoroughly protect it while still allowing free movement. In sea
creatures such as the fish (figure 2.4), the shape of the spine is
consistent with its environment: water surrounding on all sides,
exerting an equal amount of mechanical pressure from top to
bottom and side to side. As the fish employs its head, tail, and
fins to propel itself through the water, the spinal movements are
oriented in the side-to-side dimension.
This lateral undulation in the spine was preserved even when
aquatic creatures made the enormous evolutionary leap to ter-
restrial life. Figure 2.5 demonstrates that pattern in the amphibious salamander. Even
though its limbs (evolved from fins) are assisting in locomotion, they are not supporting
the weight of the spine off the ground. That development, probably resulting from a need
to orient the eyes to ever more distant food or threats, requires a dramatic reorientation
of the spinal structures.E5267/Kaminoff/fig2.4/417579/alw/pulled-r2E5267/Kaminoff/fig2.5/417580/alw/pulled-r2E5267/Kaminoff/fig2.6/417581/alw/pulled-r2Figure 2.3 A platy-
helminth worm, with
its rudimentary cen-
tral nervous system.Figure 2.4 Fish with a straight spine. Figure 2.5 Lateral movements in both
aquatic and amphibious spines.