[pg 022]1. That of maintaining in the body a nutrient fluid for the cells.
2. That of bringing the body into such relations with its surroundings as will enable it to secure materials for the nutrient fluid and satisfy its other needs.
The first problem is internal and includes the so-called vital processes, known as digestion, circulation, respiration, and excretion. The second problem is external, as it were, and includes the work of the external organs—the organs of motion and of locomotion and the organs of special sense. These problems are closely related, since they are the two divisions of the one problem of maintaining life. Neither can be considered independently of the other. In the chapter following is taken up the first of these problems.
Summary.—The individual parts, or units, that form the body organization are known as cells. These consist of minute but definitely arranged portions of protoplasm and are held together by the intercellular material. They build up the body and carry on its different activities. The tissues are groups of like cells. By certain general activities the cells maintain their existence in the tissues and by the exercise of certain special activities they adapt the tissues to their purposes in the body. The body, as a cell organization, has its activities directed under normal conditions toward a single purpose—that of maintaining life. In the accomplishment of this purpose a nutrient fluid is provided for the cells and proper relations between the body and its surroundings are established.
Exercises.—1. If a tissue be compared to a brick wall, to what do the separate bricks correspond? To what the mortar between the bricks?
2. Draw an outline of a typical cell, locating and naming the main divisions.
3. How do the cells enable the body to grow? Describe the process of cell-division.
[pg 023]4. How does the general work of cells differ from their special work? Define absorption, excretion, and assimilation as applied to the cells.
5. Compare the conditions surrounding a one-celled animal, living in water, to the conditions surrounding the cells in the body.
6. What is meant by the term "environment"? How does man's environment differ from that of a fish?