,
in positions that throw the weight of the body unequally on certain
portions of the spine, they yield to this frequent compression, and a
distortion ensues. The positions taken by young persons, when learning
to write or draw, or to play on the guitar, harp, or piano, and the
position of the body when sleeping on one side, on high pillows, all
tend to produce this effect, by throwing the weight of the body
unequally, and for a length of time, on particular parts of the spine.

[Illustration: Fig. 5.]

MUSCLES.

The muscles are the chief organs of motion, and consist of collections
of fine fibres or strings, united in casings of membrane or thin skin.
They possess an elastic power, like India rubber, which enables them to
extend and contract. The red meat in animals consists of muscles. Every
muscle has connected with it nerves, veins, and arteries; and those
designed to move the bones, are fastened to them by tendons at their
extremities. The muscles are laid over each other, and are separated by
means of membranes and layers of fat, which enable them to move easily,
without interfering with each other.

The figure on page 74, represents the muscles of the arm, as they appear
when the skin and fat are removed. The muscles _a_ and _b_ are attached,
at their upper ends, to the bone of the arm, and by their lower ends to
the upper part of the fore arm, near the elbow joint. When the fibres of
these muscles contract, the middle part of them grows larger, and the
arm is bent at the elbow. The muscle _c_, is, in like manner, fastened,
by its upper end, to the shoulder blade and the upper part of the arm,
and by its lower end to one of the bones of the fore arm, near the
elbow. When the arm is bent, and we wish to straighten it, it is done by
contracting this muscle. The muscles _d_, _d_, are fastened at one end
near the elbow joint, and at the other near the ends of the fingers; and
on the back of the hand are reduced in size, appearing like strong
cords. These cords are called _tendons_. They are employed in
straightening the fingers, when the hand is shut. These tendons are
confined by the ligament or band, _e_, which binds them down, around the
wrist, and thus enables them to act more efficiently, and secures beauty
of form to the limb. The muscles at _f_, are those which enable us to
turn the hand and arm outward. Every different motion of the arm has one
muscle to produce it, and another to restore the limb to it