GENERAL AND PHYSIOLOGICAL CONSIDERATIONS. 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. 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 amoeba, may be described as _ingestive_, _digestive_, _secretory_, _excretory_, _assimilative_, _respiratory_, _irritable_, _contractile_, and _reproductive_: that is to say, the amoeba 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. [Illustration: 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. (Schaefer's _Histology_.)] [Illustration: 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.] [Illustration: 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. (Schaefer.)]