The Muscular System : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 15 of 27)

As already stated, the skeleton, the nervous system, and the muscular system are concerned in the production of motion. The skeleton and the nervous system, however, serve other purposes in the body, while the muscular system is devoted exclusively to the production of motion. For this reason it is looked upon as the special motor system. The muscular tissue is the most abundant of all the tissues, forming about 41 percent of the weight of the body.

Properties of Muscles. - The ability of muscular tissue to produce motion depends primarily upon two properties - the property of irritability and the property of contractility. Irritability is that property of a substance which enables it to respond to a stimulus, or to act when acted upon. Contractility is the property which enables the muscle when stimulated to draw up, thereby becoming shorter and thicker (a condition called contraction), and when the stimulation ceases, to return to its former condition (of relaxation). The property of contractility enables the muscles to produce motion. Irritability is a condition necessary to their control in the body.

Kinds of Muscular Tissue. - Three kinds of muscular tissue are found in the body. These are known as the striated, or striped, muscular tissue; the non-striated, or plain, muscular tissue; and the muscular tissue of the heart. These are made up of different kinds of muscle cells and act in different ways to cause motion. The striated muscular tissue far exceeds the others in amount and forms all those muscles that can be felt from the surface of the body. The non-striated muscle is found in the walls of the food canal, blood vessels, air passages, and other tubes of the body; while the muscular tissue of the heart is confined entirely to that organ.

Striated Muscle Cells. - The cells of the striated muscles are slender, thread-like structures, having an average length of 1-1/2 inches and a diameter of about 1/400 of an inch. Because of their great length they are called fibers, or fiber cells. They are marked by a number of dark, transverse bands, or stripes, called striations, which seem to divide them into a number of sections, or disks. A thin sac-like covering, called the sarcolemma, surrounds the entire cell and just beneath this are a number of nuclei.

Within the sarcolemma are minute fibrils and a semi liquid substance, called the sarcoplasma. At each end the cell tapers to a point from which the sarcolemma appears to continue as a fine thread, and this, by attaching itself to the inclosing sheath, holds the cell in place. Most of the muscle cells receive, at some portion of their length, the termination of a nerve fiber. This penetrates the sarcolemma and spreads out upon a kind of disk, having several nuclei, known as the end plate.

The "Muscle-organ." - We must distinguish between the term "muscle" as applied to the muscular tissue and the term as applied to a working group of muscular tissue, which is an organ. In the muscle, or muscle-organ, is found a definite grouping of muscle fibers such as will enable a large number of them to act together in the production of the same movement. An examination of one of the striated muscles shows the individual fibers to lie parallel in small bundles, each bundle being surrounded by a thin layer of connective tissue. These small bundles are bound into larger ones by thicker sheaths and these in turn may be bound into bundles of still larger size. The sheaths surrounding the fiber bundles are connected with one another and also with the outer covering of the muscle, known as

The Perimysium. - The plan of the muscle-organ is revealed through a study of the perimysium. This is not limited to the surface of the muscle, as the name suggests, but properly includes the sheaths that surround the bundles of fibers. Furthermore, the surface perimysium and that within the muscle are both continuous with the strong, white cords, called tendons, that connect the muscles with the bones. By uniting with the bone at one end and blending with the perimysium and fiber bundles at the other, the tendon forms a very secure attachment for the muscle. The perimysium and the tendon are therefore the means through which the fiber cells in any muscle-organ are made to pull together upon the same part of the body.

Purpose of Striated Muscles. - The striated muscles, by their attachments to the bones, supply motion to all the mechanical devices, or machines, located in the skeleton. Through them the body is moved from place to place and all the external organs are supplied with such motion as they require. Because of the attachment of the striated muscles to the skeleton, and their action upon it, they are called skeletal muscles. As most of them are under the control of the will, they are also called voluntary muscles. They are of special value in adapting the body to its surroundings.

Structure of the Non-striated Muscles. - The cells of the non-striated muscles differ from those of the striated muscles in being decidedly spindle-shaped and in having but a single well-defined nucleus. Furthermore, they have no striations, and their connection with the nerve fibers is less marked. They are also much smaller than the striated cells, being less than one one-hundredth of an inch in length and one three-thousandth of an inch in diameter.

In the formation of the non-striated muscles, the cells are attached to one another by a kind of muscle cement to form thin sheets or slender bundles. These differ from the striated muscles in several particularys. They are of a pale, whitish color, and they have no tendons. Instead of being attached to the bones, they usually form a distinct layer in the walls of small cavities or of tubes. Since they are controlled by the part of the nervous system which acts independently of the will, they are said to be involuntary. They contract and relax slowly.

Work of the Non-striated Muscles. - The work of the non-striated muscles, both in purpose and in method, is radically different from that of the striated. They do not change the position of parts of the body, as do the striated muscles, but they alter the size and shape of the parts which they surround. Their purpose, as a rule, is to move, or control the movement of, materials within cavities and tubes, and they do this by means of the pressure which they exert. Examples of their action have already been studied in the propulsion of the food through the alimentary canal and in the regulation of the flow of blood through the arteries. While they do not contract so quickly, nor with such great force as the striated muscles, their work is more closely related to the vital processes.

D.C. Heath and Co. - Publishers
Original copyright 1909