Physiology and Hygiene for Secondary Schools | Page 9

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:
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.
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?
7. What is the necessity for a nutrient fluid in the body?
8. Why is the maintenance of life necessarily the chief aim of all the activities of the body?
9. State the two main problems in the study of the body.

PRACTICAL WORK
*Observations.*--1. Make some scrapings from the inside of the cheek with a dull knife and mix these with a little water on a glass slide. Place a cover-glass on the same and examine with a compound microscope. The large pale cells that can be seen in this way are a variety of epithelial cells.
2. Mount in water on a glass slide some thin slices of cartilage and examine first with a low and then with a high power of microscope. (Suitable slices may be cut, with a sharp razor, from the cartilage found at the end of the rib of a young animal.) Note the small groups of cells surrounded by, and imbedded in, the intercellular material.
3. Mount and examine with the microscope thin slices of