s natural
position. Those muscles which bend the body are called _flexors_; those
which straighten it, _extensors_. When the arm is thrown up, one set of
muscles is used; to pull it down, another set: when it is thrown
forward, a still different set is used; when it is thrown back,
another, different from the former; when the arm turns in its socket,
still another set is used; and thus every different motion of the body
is made by a different set of muscles. All these muscles are compactly
and skilfully arranged, so as to work with perfect ease. Among them, run
the arteries, veins, and nerves, which supply each muscle with blood and
nervous power, as will be hereafter described. The size and strength of
the muscles depend greatly on their frequent exercise. If left inactive,
they grow thin and weak, instead of giving the plumpness to the figure,
designed by Nature. The delicate and feeble appearance of many American
women, is chiefly owing to the little use they make of their muscles.
Many a pale, puny, shad-shaped girl, would have become a plump, rosy,
well-formed person, if half the exercise, afforded to her brothers in
the open air, had been secured to her, during childhood and youth.

NERVES.

The nerves are the organs of sensation. They enable us to see, hear,
feel, taste, and smell; and also combine with the bones and muscles in
producing motion.

The first engraving, on p. 77, (Fig. 6,) is a vertical section of the
skull, and of the spinal column, or back bone, which supports the head,
and through which runs the spinal cord, whence most of the nerves
originate. It is a side view, and represents the head and spine, as they
would appear, if they were cut through the middle, from front to back.
Fig. 7, exhibits them as they would appear, if viewed from _behind_. In
Fig. 6, _a_, represents the _cerebrum_, or great brain; _b_, the
_cerebellum_, or little brain, which is situated directly under the
great brain, at the back and lower part of the head; _c_, _d_, _e_, is
the spinal marrow, which is connected with the brain at _c_, and runs
through the whole length of the spinal column. This column consists, as
has already been stated, of a large number of small bones, _f_, _f_,
called _vertebrae_, laid one above another, and fastened together by
_cartilage_, or _gristle_, _g_, between them.

[Illustration: Fig. 6.]

[Illustration: Fig. 7.]

Between each two vertebrae, or spinal bones, there issues from the
spine