The principle that knowledge consists in a perception of relations will now be applied to the structure and functions or uses of the different parts of the body.
Fig. 1. Muscle-fibres from the heart, much magnified, showing cross-stripings, nuclei, or the darkly stained central bodies very important to the life of the cell, also the divisions and points of union. (Schäfer's Histology.)
Fig. 2. Appearance of a small portion of muscle under a moderate magnification. Between the muscle-cells proper a form of binding tissue may be seen.
The demonstration that all animals, even all living things, have certain properties or functions in common is one of the great results of modern science. Man no longer can be rightly viewed apart from other animals. In many respects he is in no wise superior to them. The most desirable course to pursue is to learn wherein animals resemble and wherein they differ, without dwelling at great length on the question of relative superiority or inferiority. It may be unhesitatingly asserted that all animals live, move, and have their being, in every essential respect, in the same way. Whether one considers those creatures of microscopic size living in stagnant ponds, or man himself, it is found that certain qualities characterize them all. That minute mass of jelly-like substance known as protoplasm, constituting the one-celled animal amœba, may be described as ingestive, digestive, secretory, excretory, assimilative, respiratory, irritable, contractile, and reproductive: that is to say, the amœba must take in food; must digest it, or change its form; must produce some fluid within itself which acts on food; must cast out from itself what is no longer of any use; must convert the digested material into its own substance—perhaps the most wonderful property of living things; must take up into its own substance oxygen, and expel carbonic acid gas (carbon dioxide); and possess the power to respond to a stimulus, or cause of change, the property of changing form, and, finally, the ability to bring into being others like itself.
Fig. 3. Muscle-cells isolated from the muscular coats of the intestine. Similar cells are found in some part of most of the internal organs, including the bronchial tubes. These cells are less ready in responding to a stimulus, contract more slowly, and tend to remain longer contracted when they pass into this condition than striped muscle cells. (Schäfer.)
Before justifying these statements in detail it will be desirable to say something of the anatomy or structure of a mammal, and we may select man himself, though it is to be remembered that one might apply exactly the same treatment to a dog, pig, mouse, or any other member of this group of animals. The amœba and creatures like it live immersed in water; man, at the bottom of an ocean of air. Both move in their own medium, the amœba creeping with extreme slowness, man moving with a speed incalculably greater. In each case the movements are determined by some cause from without which is termed by physiologists a stimulus. The slightest movement of the thin cover-glass placed over the drop of water in which an amœba is immersed, on a microscopic slide, suffices to act as a stimulus, and serves much the same purpose as an electric shock to the muscles of a man. In man an elaborate apparatus exists for the process known as respiration, but in this and in all other cases the mechanism is composed of what is known technically as cells, the latter being the units of structure, the individual bricks of the building, so to speak; and just as any edifice is made up of individual pieces some of which differ from one another while others do not to any appreciable extent, so is it with the body. The individual cells of a muscle are alike in structure and function, but they differ widely from those of a gland or secreting organ, as the liver. But it is to be ever remembered that the statements with which we set out hold: that is, that however cells may differ, they have in all animals certain properties in common. Of the muscle-cell, the liver-cell, and the one-celled animal we may affirm the same properties, but the difference is that while all are secretory the liver-cell is eminently so, and produces bile, which other cells do not; that while it is but feebly contractile, or susceptible of change of form, the muscle-cell is characterized by this property above all others.
The lower we descend in the animal scale the more simple are the mechanisms by which results are attained. The one-celled animal may be said to breathe with its whole body, while the man employs a large number of muscles, not to speak, at present, of other arrangements. But when a muscle is examined under the microscope, it is found to consist of cells, each one of which is physiologically in all essentials like an amœba, so that we may say that a muscle or other tissue or organ is really a sort of colony of cells of similar structure and function, all working in harmony like a happy family. We actually do find colonies of unicellular animals much like amœba, so that the muscle-cells and all other cells of the body may be compared to amœba and other one-celled animals.