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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. Because of their great length they are called fibers, or fiber cells. A thin sac-like covering, called the sarcolemma, surrounds the entire cell and just beneath this are a number of nuclei.
The microscope shows these muscles to consist not of fibers, but of long spindle-shaped cells, united to form sheets or bands. They have no sarcolemma, stripes, or cross markings like those of the voluntary muscles. Hence their name of non-striated, or unstriped, and smooth muscles. The involuntary muscles respond to irritation much less rapidly than do the voluntary.
Attached to the cell is the termination of a nerve fiber. Within the sarcolemma are minute fibrils and a semiliquid substance, called the sarcoplasm. 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.
The fibrillæ, then, are bound together in a bundle to form a fiber, which is enveloped in its own sheath, the sarcolemma. These fibers, in turn, are further bound together to form larger bundles called fasciculi, and these, too, are enclosed in a sheath of connective tissue. The muscle itself is made up of a number of these fasciculi bound together by a denser layer of connective tissue.
Then cover with water on a glass slide, and with a couple of fine needles tease out the small muscle threads. Protect with a cover glass and examine with a microscope, first with a low and then with a high power. The striations, sarcolemma, and sometimes the nuclei and nerve plates, may be distinguished in such a preparation.
*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. Each cell has a well-defined nucleus, but the sarcolemma is absent. They are placed end to end to form fibers, and many of the cells have branches by which they are united to the cells in neighboring fibers.
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.
Muscles are not usually connected directly with bones, but by means of white, glistening cords called tendons. If a small piece of muscle be examined under a microscope it is found to be made up of bundles of fibers. Each fiber is enclosed within a delicate, transparent sheath, known as the sarcolemma.
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