Illustration: Fig. 19. Muscular fillers highly magnified.
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 fibrillÆ, or little fibers; each of these fibrillÆ 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.

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Illustration: Fig. 20.
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.
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 fibrillÆ 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 striÆ, or minute channels. The fibers are nearly the same length as the muscles to which they belong. Each muscular fiber is capable of [pg 32]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 fibrillÆ, 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;
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.
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 [pg 33][pg 34]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.
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.
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 [pg 35][pg 36]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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