And each of these microscopic fibres is a series or chain of
elastic cells, which are so minute that one hundred thousand would
scarcely cover a capital O on this page.
[Illustration: Fig. 51.]
[Illustration: Fig. 52.]
The peculiar property of the cells which compose the muscles is their
elasticity, no other cells of the body having this property. At Fig.
51 is a diagram representing a microscopic muscular fibre, in which
the cells are relaxed, as in the natural state of rest. But when the
muscle contracts, each of its numberless cells in all its small fibres
becomes widened, making each fibre of the muscle shorter and thicker,
as at Fig. 52. This explains the cause of the swelling out of muscles
when they act.
Every motion in every part of the body has a special muscle to produce
it, and many have other muscles to restore the part moved to its natural
state. The muscles that move or bend any part are called _flexors_,
and those that restore the natural position are called _extensors_.
[Illustration: Fig. 53]
Fig. 53 represents the muscles of the arm after the skin and flesh are
removed. They are all in smooth silky cases, laid over each other, and
separated both by the smooth membranes that encase them and by layers
of fat, so as to move easily without interfering with each other.
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