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,[pg 246] 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 thus the means through which the fiber cells in any muscle-organ are made to pull together upon the same part of the body (Fig. 110).
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 (Fig. 111). 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 particulars. They are of a pale, whitish color, and they have no tendons. Instead of[pg 247] being attached to the bones, they usually form a distinct layer in the walls of small cavities or of tubes (Fig. 111). 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.
Fig. 111—Non-striated muscle cells. A. Cross section of small artery magnified, showing (1) the layer of non-striated cells. B. Three non-striated cells highly magnified.
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 (pages 159 and 49). 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.
Structure of the Heart Muscle.—The cells of the heart combine the structure and properties of the striated and the non-striated muscle cells, and form an intermediate type between the two. They are cross-striped like the striated cells, and are nearly as wide, but are rather short (Fig. 112). Each cell has a well-defined nucleus, but the sarcolemma is absent. They are placed end to end to form fibers, and[pg 248] many of the cells have branches by which they are united to the cells in neighboring fibers. In this way they interlace more or less with each other, but are also cemented together. They contract quickly and with great force, but are not under control of the will. Muscular tissue of this variety seems excellently adapted to the work of the heart.
Fig. 112—Muscle cells from the heart, highly magnified (after Schäfer).