[Illustration: Fig. 23.
A view of the under side of the diaphragm.]

THE DIAPHRAGM, or midriff, is the muscular division between the thorax
and the abdomen. It has been compared to an inverted basin, the
concavity of which is directed toward the abdomen. The muscles receive
their nourishment from the numerous blood-vessels which penetrate their
tissues. The voluntary muscles are abundantly supplied with nerves,
while the involuntary are not so numerously furnished. The color of the
muscles is chiefly due to the blood which they contain. They vary in
size according to their respective functions. For example, the functions
of the heart require large and powerful muscles, and those of the eye,
small and delicate ones. There are between four hundred and sixty and
five hundred muscles in the human body.

[Illustration: Fig. 24.
A representation of the superficial layer of muscles on the anterior
portion of the body.]

[Illustration: Fig. 25.
A representation of the superficial layer of muscles on the posterior
portion of the body.]

Very rarely is motion produced by the action of a single muscle, but by
the harmonious action of several. There is infinite variety in the
arrangement of the muscles, each being adapted to its purpose, in
strength, tenacity, or elasticity. While some involuntarily respond to
the wants of organic life, others obey, with mechanical precision, the
edicts of the will. The peculiar characteristic of the muscles is their
contractility; for example, when the tip of the finger is placed in the