resenting a striped or striated appearance; and a microscopic
examination of them shows that even the most minute consist of parallel
filaments marked by longitudinal and transverse _striae_, or minute
channels. The fibers are nearly the same length as the muscles to which
they belong. Each muscular fiber is capable of contraction; it may act
singly, though usually it acts in unison with others. By a close
inspection, it has been found that fibers may be drawn apart
longitudinally, in which case they are termed _fibrillae_, or they may
be separated transversely, forming a series of discs. The _Sarcolemma_,
or investing sheath of the muscles, appears to be formed even before
there are any visible traces of the muscle itself. It is a transparent
and delicate membrane, but very elastic. The _Involuntary Muscles_ are
influenced by the sympathetic nervous system, and their action pertains
to the nutritive functions of the body. They differ from the voluntary
muscles in not being striated, having no tendons, and in the net-work
arrangements of their fibers. The _Intermediate Muscles_ are composed of
striated and unstriated fibers; they are, therefore, both voluntary and
involuntary in their functions. The muscles employed in respiration are
of this class, for we can breathe rapidly or slowly, and, for a short
time, even suspend their action; but soon, however, the organic muscles
assert their instinctive control, and respiration is resumed.

[Illustration: Fig. 22.
Unstriated muscular fiber; at _b_, in its natural
state; at _a_, showing the nuclei after the action of
acetic acid. ]

[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 o