1. A definite arrangement of the cells to form the tissues. A tissue is a group of like cells.
2. A definite arrangement of the tissues in the organ. Each organ contains the tissues needed for its work.
3. In several instances there is a definite arrangement of organs to form systems.
4. The body as a whole is made up of organs and systems, together with the structures necessary for their support and protection.
There now remains a further question for consideration. What is the one supreme end, or purpose, toward which all the activities of the body organization are directed? This purpose will naturally have some relation to the maintenance, or preservation, of the cell group which we call the body.
The Maintenance of Life.—The preservation of any cell group in its natural condition, whether it be plant or animal, is accomplished through keeping it alive. If life ceases, the group quickly disintegrates and its elements become scattered, a fact which is verified through everyday observation. Though the nature of life is unknown, it may be looked upon as the organizer and preserver of the protoplasm. But in preserving the protoplasm it also preserves the entire cell group, or body. Life is thus the most essential condition of the body. With life all portions of the body are concerned, and toward its maintenance all the activities of the body organization are directed.
The Nutrient Fluid in its Relations to the Cells.—The maintenance of life within the cells requires, as we have seen, that they be supplied with water, food, and oxygen, and that they be relieved of such wastes as they form.[pg 021] This double purpose is accomplished through the agency of an internal nutrient fluid, a portion of which has already been referred to as the lymph. Not only does this fluid supply the means for keeping the cells alive, but, through the cells, it is also the means of preserving the life of the body as a whole.
The cells, however, rapidly exhaust the nutrient fluid. They take from it food and oxygen and they put into it their wastes. To prevent its becoming unfit for supplying their needs, food and oxygen must be continually added to this fluid, and waste materials must be continually removed. This is not an easy task. As a matter of fact, the preparation, distribution, and purification of the nutrient fluid requires the direct or indirect aid of practically all parts of the body. It supplies for this reason a broad basis for the division of labor on the part of the cells.
Relation of the Body to its Environment.—While life is directly dependent upon the internal nutrient fluid, it is indirectly dependent upon the physical surroundings of the body. Herein lies the need of the external organs—the feet and legs for moving about, the hands for handling things, the eyes for directing movements, etc. That the great needs of the body are supplied from its surroundings are facts of common experience. Food, shelter, air, clothing, water, and the means of protection are external to the body and form a part of its environment. In making the things about him contribute to his needs, man encounters a problem which taxes all his powers. Only by toil and hardship, "by the sweat of his brow," has he been able to wrest from his surroundings the means of his sustenance.
The Main Physiological Problems.—The study of the body is thus seen to resolve itself naturally into the consideration of two main problems: