tion is erect will stand longer, walk further, and
perform more labor, than an individual whose position is stooping, but
equal in all other respects. The manly port in an erect attitude,
depends chiefly upon the action of the muscles of the back; and it
follows that the fewer the muscles in a state of tension, the less the
draught upon the nervous system, and the less its exhaustion. Another
advantage which attends the erect position is, the trunk and head are
balanced upon the bones and cartilages of the spinal column. If the
body slightly incline forward, the muscles attached to the posterior
side of the spine, by a gentle contraction, will bring it to the
perpendicular, and even incline it backward. This is immediately
removed by a slight contraction of the muscles upon the anterior side
of the spinal column.

199. In the erect position, there is a constant slight oscillation
of the body backward and forward, like the movement of a pendulum;
while, in the stooping posture, the muscles on the posterior side
of the spinal column are kept in a state of continued tension and
contraction, to prevent the body from falling forward. This enfeebles
the muscles of the back, and exhausts the nervous energy, while the
erect position favors their development and power, because there is
an alternate contraction and relaxation of the muscles. Again, in the
stooping position, the lower limbs are curved at the knee. In this
attitude, there is a constant tension of the muscles of the lower
extremities, which produces muscular exhaustion.

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198. Why will a person who stands erect walk further, and perform more
labor, than if he assumed the stooping posture? 199. Why are the
muscles of the back so soon exhausted in the stooping position?

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[Illustration: Fig. 48. 1, A perpendicular line from the centre of the
feet to the upper extremity of the spinal column, where the head rests.
2, 2, 2, The spinal column, with its three natural curves. Here the head
and body are balanced upon the spinal column and joints of the lower
extremities, so that the muscles are not kept in a state of tension. This
erect position of the body and head is always accompanied with straight
lower limbs.]

[Illustration: Fig. 49. 1, A perpendicular line from the centre of the
feet. 2, Represents the unnatural curved spinal column, and its relative
position to the perpendicular, 1. The lower limbs are curv