Muscle when most highly developed has an extraordinarily definite structure. It is minutely subdivided into units which appear, looked at separately, simple in design. We are tempted to believe that the explanation of the way in which each of these units works is not far to seek. It is disappointing to be obliged to admit that, notwithstanding all the thought which has been devoted to the problem, we are as far as ever from a definitive solution. We understand the principles on which steam-engines, combustion-engines, electric motors are planned. We compare muscle with each of these mechanical contrivances in turn, expecting to discover the principle of its construction. Many ingenious hypotheses have been formulated; but the fact that some of these are mutually destructive shows clearly enough that as yet no approach to certainty has been made. Probably the fundamental error lies in attempting to compare muscle with a mechanical contrivance. The apparent simplicity and regularity of structure of “striped muscle” misleads us. We ought to have commenced our investigations at the other end of the scale of mobile tissue—to have begun with semifluid and apparently homogeneous animal matter, working upwards to the tissue which, being limited to the one function of movement, and movement in one direction only, has, as it were, crystallized along the lines of force.
All protoplasm is mobile. Its particles move one on another. Hence follows either circulation of the living matter within the cell or change in shape of the cell. The two phenomena are identical in nature. Circulation is best studied in a large-celled, transparent part of a plant. A filamentous water-weed is suitable for the purpose. If this be examined with a microscope while still alive, its cells are seen to contain a watery juice enclosed in spaces of denser cell-substance. Bridges of cell-substance span the spaces. The particles of which these bridges consist are in a state of constant streaming motion, which has, it is needless to say, no effect upon the shape of the cell ([cf. p. 9]).
The unicellular animal amœba, leucocytes, and certain spores of plants, are devoid of cell-wall ([cf. p. 28]). Their soft protoplasm is not limited by a rigid case. When it streams, the form of the cell is changed. True, we must not think of the body-substance of an amœba as homogeneous. It exhibits an internal structure. Yet its architecture is not, so far as we can see, sufficiently fixed to restrict the directions in which it can stream. Any change of shape is possible. We cannot find in Nature an isolated clump of living protoplasm; nor do we suppose that, if we found it, it would prove to be homogeneous. It appears to be necessary that protoplasm and metaplasm—the terms have no chemical significance; “primary” and “secondary,” or “chief” and “subsidiary” would be equally distinctive—should be intermixed. Streaming is apparently due to alterations in the surface relations of the two substances.
In multicellular animals certain elongated cells are arranged in groups, with their long axes all pointing in the same direction. They can change in shape, diminishing in length, with equivalent increase in breadth. Since all the cells of a group undergo this change of form at the same time, the result is an alteration in the shape of the animal of which they are a part. Applying the experience which we have gained in studying the movements of unicellular organisms, we conclude that these elongated cells are composed of two substances—protoplasm and metaplasm. The restriction of their capacity for altering their shape to one direction indicates that their protoplasms and metaplasm are not indifferently mixed. The two substances set in lines in the direction of the long axis of the cell. Hence, when streaming occurs—when the force which keeps the molecules of protoplasm and of metaplasm in their respective rows is relaxed—the lines thicken. The cell broadens, with an equivalent diminution of length.
Muscle-fibres exhibit all degrees of specialization. The simplest, “plain muscle-fibres,” are found in the wall of the alimentary canal, of bloodvessels, of ducts, in the tissue of the spleen, in the skin, and elsewhere. Each fibre is a fusiform cell. Save for its central nucleus and a little granular protoplasm in which the nucleus is embedded, the cell may show no architectural features. But in most varieties of plain muscle, and especially in that of the alimentary canal, the substance of the fibres is striated longitudinally. This is visible evidence of the orientation of the molecules of protoplasm and metaplasm in the direction of the long axis of the fibre. It shows that the streaming of particles occurs along these lines. It is, as it were, a diagram of the lines of force.
Heart-muscle has been described already ([p. 224]). Its striation, which is both transverse and longitudinal, is so delicate as almost to defy microscopical analysis. The transverse striæ are the darker and more distinct. But close examination shows that the transverse striæ do not indicate the direction in which the particles of cell-substance are oriented. They are oriented longitudinally. The cell is a bundle of rods of substance A, embedded in substance B. The transverse markings are very thin lines which cross the bundles at right angles.
The third variety of muscle is the kind by which locomotion is effected. It is present in large masses—all the red tissue to which the term “meat” is commonly applied. It accounts for about 35 per cent. of the body-weight. This kind of muscle is not composed of single cells, but of compound cells, or cell-complexes, termed “fibres.” A fibre may attain a length of upwards of 2 inches, with a breadth of about ¹/₅₀₀ inch. In most cases the fibres are attached by one end to a bone, by the other to a tendon; and since they are shorter than the muscle as a whole, the tendon commences as a membrane which covers the surface of the muscle, sloping to it from the bone to which by their other ends the fibres are attached. A fibre is developed from a single cell. The cell elongates, its nucleus divides, and the daughter-nuclei divide until several hundred have been formed; but cell division does not follow. The result is a cylindrical mass enclosed within a delicate membranous sheath, the sarcolemma. In the early stages of its development its nuclei are in the axis of the fibre, but subsequently they are displaced outwards. In the most highly specialized muscle, known as the “white” variety, they lie just beneath the sarcolemma ([cf. Fig. 16, B]).
The feature of this type of muscle is its transverse striation, almost mathematically regular. Commonly striated muscle is spoken of as “voluntary,” because, for the most part, it is under the control of the Will; but the term, in so far as it implies a connection between structure and mode of actuation, is misleading. Transverse striation is evidence of capacity for rapid action. The muscles which the Will directs exhibit promptitude; but striated muscle, which is not under the direction of the Will, is found in certain situations—e.g., the upper part of the œsophagus. Conversely, many animals can voluntarily call into action muscle which is not striped. A turkey erects its feathers by setting in motion little groups of “plain” fibres, which pull on elastic tendons attached to the tips of the buried ends of their shafts. Plain muscle contracts less promptly and relaxes more slowly than the striped variety. Cardiac muscle is quicker in acting than plain, but does not hold the contraction so long.
All striped muscle is not equally rapid. Two varieties are distinguishable: “white fibres,” which respond suddenly to a single stimulus and quickly relax; “red fibres,” which respond in a more leisurely way, but remain contracted longer. In some muscles these two types of fibre are intermixed. Others are wholly red or wholly white. Everyone is familiar with the contrast which the white flesh of a turkey or of the domestic fowl presents to the red flesh of game-birds and birds of prey. In the breast of a blackcock a sheet of white muscle overlies a mass of red. When the bird is cooked the difference in colour is strongly marked. Of the two muscles which, in a rabbit, correspond to our muscles of the calf, the superficial, gastrocnemius, is white; the deeper, soleus, red. The former acts over both knee and ankle joints; the latter over the ankle only. The muscle which, acting over a longer range, has to contract more quickly is white; the shorter, more slowly acting muscle is red. Experiment shows that red and white muscles are distinguished by a difference in the promptitude with which they respond to an electric current. It shows, too, that the white muscle is exhausted sooner than the red. It cannot give so many successive responses to stimulation without a rest. We shall find, when we are considering the minute structure of striped muscle, a difference between its two varieties which we can correlate with their different modes of action. All human muscles belong to the red kind.
The most efficient muscle-fibres in the animal kingdom are found in insects. This will not surprise anyone who thinks of an insect’s power of movement. If a man could jump as many times his own height as a flea can, he would clear the dome of St. Paul’s. An ant can drag an object sixty times as heavy as itself, with no wheels beneath it to diminish friction. Under the same conditions a horse cannot drag much more than its own weight. A dragon-fly, it is asserted—although we have not met a man who guarantees that he has made the observation—will support its heavy body in the air by the rapid vibration of its wings for four-and-twenty hours without alighting. The chirp of a cricket is produced by the rubbing together of its hind-legs. A mosquito sounds its war-cry much in the same way. The pitch of the note proves that the insect’s muscles are contracting and relaxing at least 300 times a second. None of these figures must be applied without qualifications in estimating the relative strength of insect and human muscle. Weight for weight, the muscle of a flea is not so much stronger than ours as the figures might lead one to infer. To ascertain the numerical relation, it is necessary to compare the total cross-section of the two chief segments of a flea’s leg with the cross-section of the extensor muscles of a man’s thigh and calf, and a man’s weight with the weight of a flea. Nevertheless, after all deductions have been made, a considerable balance of superiority lies with the insect as regards the strength of its muscles, their rapidity of contraction, and power of repeating contraction without fatigue. An insect’s muscle is the most suitable that can be obtained for microscopic examination. Its pattern is larger and more distinct than that of other animals. That the pattern should be larger is not quite what might have been expected. It would not have surprised us had we found the pattern finer in the more effective type.