Diagram 18.—Muscle.
The two varieties are: the smooth, or involuntary, and the striped, or voluntary, muscle. Smooth muscle consists of spindle-shaped cells with one elongated nucleus. ([See Diagram 18, Fig. 1.]) It only contracts very slowly, and is not under control of the will; but it is very abundant in the body, since it effects practically all the movements of the alimentary canal and bloodvessels. Voluntary or striped muscle, so called from its appearance under the low power of a microscope, consists of long fibres, each containing many nuclei. ([See Diagram 18, Fig. 2.]) Its protoplasm is rich in hæmoglobin, and in it, under powerful microscopes, can be made out two kinds of fibrils: Rutherford’s fibrils, the complicated structure of which gives muscle its striped appearance; and Marshall’s fibrils, which are much finer and more difficult to see. The muscle of the heart, though not under control of the will, is striped; but it differs from ordinary striped muscle in being made up of small branched cells with only one nucleus.
The way in which the three elements of striped muscle contribute to a contraction is practically unknown, and the subject of much dispute. In fact, one could hardly wish for a better soil for theories, and some which grow in it are very wonderful indeed. We have reason for supposing that there are two contractile substances—one which gives a sharp twitch, the other a slow, hard pull; and on the whole there seems good reason to believe that Rutherford’s fibrils give the sudden movements, while Marshall’s give the more forcible ones; and that the ordinary protoplasm of the cell is restricted to the duty of nourishing the fibrils.
Diagram 19.—Striped Muscle Fibre, more highly magnified than in [Diagram 18].
The muscle cells are modified from among those of the bud forming the middle layers of the embryo. ([See Diagram 5.]) Other cells of this bud form connective tissue, by, so to speak, spinning long fibres of the substance called collagen, which turns to gelatin when boiled. ([See Diagram 20.]) This connective tissue permeates the whole body, affording a firm foundation for the many layers of cells which form the skin and the single layer of digestive cells; supporting the other organs throughout, and keeping the different parts of the body in their places, in doing which, however, it is assisted by other fibres which are not collagenous, but elastic. It also forms tracts which become lymph and blood vessels.
In parts of the animal which require special support it forms solid rods, the collagen combining with calcium salts to form a clear, hard substance—cartilage. At one period in the development of an animal or animals we find the only solid support is cartilage, but cartilage is not sufficiently rigid for a very large beast, especially on land, so is only used for outlying parts, the main framework being bone.
Diagram 20.—A Connective-tissue Cell giving rise to Long Collagenous Fibres.
Bone is formed very much as if Nature were rectifying a mistake. When a rod of cartilage is unequal to its work it is eaten hollow, and fresh connective-tissue cells immigrate and fill up the cavity, eventually laying down a fine network of cells in its place, the meshes of which are filled with inorganic calcium salts, chiefly phosphate of lime. Nature then benefits by experience, and the last bones to be formed are not preceded by any makeshift cartilage, but built up straight away in ordinary connective tissue.