I.
In the preceding essay we regarded protoplasm as a chemical factor in the universe.
We have seen how it is always changing, always taking in food, always giving off waste materials. We have seen, too, that it grows and that it does work, and that in a large mass the cells which compose it share the labour instead of each component cell performing all the vital functions. We have now to consider the work which protoplasm does—in a word, the mechanical effect of the chemical actions just described.
The simplest movement of protoplasm is to be seen by the aid of the microscope in certain vegetable cells, where granules seem always streaming about in different directions. A step higher, and we find this streaming movement converted into movements of the whole cell. In the simplest unicellular animals the fluid protoplasm is contained in a membrane, or denser bounding layer, to which are attached fine filaments springing from a minute body known as the centrosome. These centrosomes—for there are sometimes several in a cell—seem to control the mechanical department, just as the nucleus does the chemical. Along the fibrils at intervals are minute globules, and by watching the distance between them it is seen that the fibrils undergo changes in length, pulling in the membrane when they shorten, and letting the cell flow out in any direction when they relax. By adjusting these two movements to balance one another, the cell can move in any direction, surround and engulf particles of food, and assume a strange variety of shapes. ([See Diagram 1.])
Diagram 15.—Cell Division.
In some cells, probably in all, the centrosome presides over division. Cells, however, do not always divide in the same way. Some simply lengthen, the nucleus also lengthening inside, become constricted in the middle like a dumb-bell, and separate. ([See Diagram 15.])
Diagram 16.—Cell Division.
Others manage differently. In them the nucleus simply bursts, and turns its essential elements, a number—always a constant number—of coarse threads, adrift. Meanwhile, two centrosomes have moved to opposite ends of the cell, and there anchored themselves by fibrils; other fibrils springing from them become attached to the nuclear threads, and when all is ready pull them apart, equally divided, to their respective ends, where they re-form into two fresh nuclei. ([See Diagram 16.])
Unicellular animals, which are constant in shape and swim instead of flowing when they want to get anywhere, have at first sight nothing in common with those which do the latter. From their surfaces spring fringes of free protoplasmic threads, called cilia, from their fancied resemblance to eyelashes, which serve as motor organs, and beat the water like oars. ([See Diagram 2.]) Waves of movement, as they lash one after another, all in the same direction, seem to pass over the cell, and it is propelled through the water; while others, which are situated in the neighbourhood of the cell’s mouth, stir the water into eddies, and drive food particles into it.
Diagram 17.—Cilia of an Epithelial Cell.
These cilia are important, as they are adapted for many purposes in large animals. The cells which line the cavity at the back of the nose, the tubes of the lungs, and other parts of the body, have a few cilia on their free surface, and it is in them that the structure of these organs can best be made out. At the foot of each cilium is a minute globule, from which a fine fibril passes into the cell, and the fibrils, collectively forming a leash, are attached to its opposite end. ([See Diagram 17.]) It seems highly probable that the globule is a centrosome giving rise to two fibrils, one attached as described, the other passing up one side of the cilium, and fast to its apex. The result of this arrangement is that when the fibrils contract the cilium is bent over with a jerk to the side up which the fibril runs, and when they relax it slowly straightens itself. There is, therefore, no fundamental difference between this and the other mode of progression; both are dependent upon the centrosome.
Finally we have muscle cells. These are only found in a fairly complicated animal, since they are a product of the division of labour principle, and their sole business is movement. There are two varieties of muscle, but the principle is the same in both—a long thin cell, with fibrils traversing its length whose contraction causes the cell to shorten and thicken, thus reducing the distance between its two ends. At present the development of muscle and the way in which it ‘contracts,’ to use the word accepted in this case for describing a redistribution of bulk, are little understood, and there are accordingly many opinions; but I think careful study will eventually show that some modification of the centrosome, with its contractile fibrils, is responsible for the movement.
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.
This brings us back again to muscle, for the object of nearly all the voluntary muscle is to cause movement among the bones. For this purpose the muscle cells or fibres are arranged parallel to one another, and bound up together by connective tissue, the whole bundle being known as ‘a muscle.’ The two ends of a muscle are attached to two bones by connective tissue, which sometimes forms a short cord, or tendon. Then, when the muscle contracts, the two places of its attachment are pulled towards one another, and something has to move. But before saying more about the way in which the bones are jointed and muscles attached—in fact, what movements are possible in the human body—it would be as well here to describe the chief properties of muscle and the way in which they are studied.