PHYSIOLOGICAL ANATOMY.
THE MUSCLES.
[Illustration: Fig. 19.
Muscular fillers highly
magnified.]
The _Muscles_ are those organs of the body by which motion is produced,
and are commonly known as _flesh_. A muscle is composed of _fascieuli_,
or bundles of fibers, parallel to one another. They are soft, varying in
size, of a reddish color, and inclosed in a cellular, membranous sheath.
Each _fasciculus_ contains a number of small fibers, which, when
subjected to a microscopic examination, are found to consist of
_fibrillae_, or little fibers; each of these fibrillae in turn being
invested with a delicate sheath. The fibers terminate in a glistening,
white _tendon_, or hard cord, which is attached to the bone. So firmly
are they united, that the bone will break before the tendon can be
released. When the tendon is spread out, so as to resemble a membrane,
it is called _fascia_. Being of various extent and thickness, it is
distributed over the body, as a covering and protection for the more
delicate parts, and aids also in motion, by firmly uniting the muscular
fibers. The spaces between the muscles are frequently filled with fat,
which gives roundness and beauty to the limbs. The muscles are of
various forms; some are longitudinal, each extremity terminating in a
tendon, which gives them a _fusiform_ or spindle-shaped appearance;
others are either fan-shaped, flat, or cylindrical.
[Illustration: Fig. 20.
1. A spindle-shaped muscle, with tendinous
terminations. 2. Fan-shaped muscle.
3. Penniform muscle. 4. Bipenniform
muscle.]
[Illustration: Fig. 21.
Striped muscular fibre showing cleavage in
opposite directions. 1. Longitudinal cleavage.
2. Transverse cleavage. 3. Transverse section of
disc. 4. Disc nearly detached. 5. Detached disc,
showing the sarcous elements. 6. Fibrillæ. 7,8.
Separated fibrillae highly magnified.]
Every muscle has an _origin_ and an _insertion_. The term _origin_ is
applied to the more fixed or central attachment of a muscle, and the
term _insertion_ to the movable point to which the force of the muscle
is directed; but the origin is not absolutely fixed, except in a small
number of muscles, as those of the face, which are attached at one
extremity to the bone, and at the other to the movable integument, or
skin. In most instances, the muscles may act from either extremity. The
muscles are divided into the Voluntary, or muscles of animal life, and
the Involuntary, or muscles of organic life. There are, however, some
muscles which cannot properly be classified with either, termed
Intermediate. The _Voluntary Muscles_ are chiefly controlled by the
will, relaxing and contracting at its pleasure, as in the motion of the
eyes, mouth, and limbs. The fibers are of a dark red color, and possess
great strength. These fibers are parallel, seldom interlacing, but
presenting 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 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
ear, an incessant vibration, due to the contraction of the muscles of
the ear, can be heard. When the muscles contract, they become shorter;
but what is lost in length is gained in breadth and thickness, so that
their actual volume remains the same. Muscles alternately contract and
relax, and thus act upon the bones. The economy of muscular power thus
displayed is truly remarkable. In easy and graceful walking, the forward
motion of the limbs is not altogether due to the exercise of muscular
power, but partly to the force of gravity, and only a slight assistance
of the muscles is required to elevate the leg sufficiently to allow it
to oscillate.
Motion is a characteristic of living bodies. This is true, not only in
animals, but also in plants. The oyster, although not possessing the
power of locomotion, opens and closes its shell at pleasure. The coral
insect appears at the door of its cell, and retreats at will. All the
varied motions of animals are due to a peculiar property of the muscles,
termed _contractility_. Although plants are influenced by external
agents, as light, heat, electricity, etc., yet it is supposed that they
may move in response to inward impulses. The sensitive stamens of the
barberry, when touched at their base on the inner side, resent the
intrusion, by making a sudden jerk forward. Venus's fly-trap, a plant
found in North Carolina, is remarkable for the sensitiveness of its
leaves; which close suddenly and capture insects which chance to alight
upon them. The muscles of the articulates are situated within the solid
framework, unlike the vertebrates, whose muscles are external to the
bony skeleton. All animals have the power of motion, from the lowest
radiate to the highest vertebrate, from the most repulsive polyp to that
type of organized life made in the very image of God.
The muscles, then, subserve an endless variety of purposes. By their aid
the farmer employs his implements of husbandry, the mechanic deftly
wields his tools, the artist plies his brush, while the fervid orator
gives utterance to thoughts glowing with heavenly emotions. It is by
their agency that the sublimest spiritual conceptions can be brought to
the sphere of the senses, and the noblest, loftiest aims of to-day can
be made glorious realizations of the future.
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