The muscle tissues of the higher animals may be divided into two classes: (i) non-striated or smooth, and (2) striated, in which again there are some slight variations.
The non-striated muscle tissue is that in which the elements are most like contractile protoplasmic cells, and have so far retained the typical form as to be easily recognizable as cells when separated one from the other. These cells are more or less elongated, flattened, homogeneous elements with a single, long, rod-shaped nucleus and no cell wall. They are tightly cemented together by a tough elastic substance, so that their tapering extremities fit closely together and form commonly a dense mass or sheet. Sometimes they branch more or less regularly, and then are arranged in networks.
These cells vary greatly in size as well as in the relation of their length to their width, in some places deserving the name fibres, or fibre cells, and in others being only elongated cells.
Fig. 177. Muscle cells, showing different condition 0f the protoplasm of the cell and nucleus.
The striated muscle tissue is that of which the skeletal muscles and the heart are composed. It therefore forms the larger proportion of the animal, known as flesh. The flesh can, by judicious dissection, easily be divided into single parts called muscles, each of which contains many other tissues, and is so attached as to carry on certain movements, and may, therefore, be regarded as an organ.
Such a muscle is enclosed in a sheath of connective tissue, for which sheet-like partitions or septa pass into the mass of the muscle and divide it into bundles of fibres, which they enclose. These septa also act as the bed in which the vessels and nerves lie.
The tissue of the heart differs from the striated muscle in being made up of truncated, oblong branching cells with a central nucleus and no sarcolemma (see page 262).
The bundles of fibres of skeletal muscle vary much in size, giving a coarse or fine grain in different muscles; they are composed of a greater or less number of fibres, which, lying side by side, run parallel one to the other. The single fibres of striated muscle vary in length, sometimes reaching 4-5 cm. (2 inches), but being on an average much shorter, they only extend the entire length of a muscle in the case of very short muscles. In long muscles their tapering points are made to correspond with those of other fibres to which they are firmly attached. The soft fibres are pressed by juxtaposition into prismatic forms, so that in a fresh condition they appear polygonal in transverse section. When freed from all pressure or traction they become cylindrical, and the transverse striation of the contractile substance appears regular, and is easily recognized. Each fibre consists of a delicate case of thin, elastic, homogeneous membrane, forming a sheath called sarcolemma, within which the essential contractile substance is enclosed. The soft contractile substance completely fills and distends the elastic sarcolemma, so that when the latter is broken its contents bulge out or escape. After death, particularly if preserved in weak acid (HC1), the striation becomes more marked, and the dead and now rigid contractile substance can easily be broken up into transverse plates or discs.
Fig. 178. Short striated cells of the heart muscles, separated one showing the truncated (a), or divided (c), ends and branches (6).
Besides the transverse striation, a longitudinal marking can be seen in the muscle fibre, which indicates the subdivision of the contractile substance into thin threads called primitive fibrillae. Each primitive fibril shows a transverse marking, corresponding with the transverse striation, which divides the fibrils into short blocks called sarcous, or muscle elements. These markings, and the transverse striations of the muscle fibre in general, depend on different parts of the contractile substance having different powers of refraction, and giving the appearance of dark and light hands.
In the muscle fibre are found long granular masses like protoplasm; these are the nuclei of the contractile substance. They must not be confounded with the nuclei of the sarcolemma, which are much more numerous along the edge of the fibre, or with the other short nuclei seen in such numbers between the fibres, which indicate the position of the capillary vessels.
Fig. 179. Two fibres of striated muscle, in which the contractile substance (ni) has been ruptured and separated from the sarcolemma (a) and (s); (B) shows a thin strip of torn contractile substance in which the transverse markings are clearer; (n) nuclei, (p) space under sarcolemma, (Ranvier).
It is stated that each striated muscle fibre has a nerve fibre passing directly into it, but the exact details of the mode of union in mammalia are not yet satisfactorily made out.