_Absorption_ is the process of taking water, food, and oxygen into the cells.

_a.s.similation_ is a complex process which results in the addition of the absorbed materials to the protoplasm. Through a.s.similation the protoplasm is built up or renewed.

_Excretion_ is the throwing off of such waste materials as have been formed in the cells. These are pa.s.sed into the lymph and thence to the surface of the body.

Absorption, a.s.similation, excretion, and also reproduction are performed by all cla.s.ses of cells. They are, on this account, referred to as the _general work of cells_.

*The Special Work of Cells.*-In addition to the general work which all cells do in common, each cla.s.s of cells in the body is able to do some particular kind of work-a work which the others cannot do or which they can do only to a limited extent. This is spoken of as the _special work of cells_. Examples of the special work of cells are found in the production of motion by muscle cells and in the secretion of liquids by gland cells.

It may be noted that while the general work of cells benefits them individually, their special work benefits the body as a whole. Another example of the special work of cells is found in the

[Fig. 7]

Fig. 7-Cartilage cells, surrounded by the intercellular material which they have deposited.

*Production of the Intercellular Material.*-Though most of the cells of the body deposit to a slight extent this material, the greater part of it is produced by a single cla.s.s of cells found in bone, cartilage, and connective tissue. Cartilage, bone, and connective tissue differ greatly from the other tissues in the amount of intercellular material which they contain, the difference being due to these cells. In the connective tissue they deposit the fibrous material so important in holding the different parts of the body together. In the cartilage they produce the gristly substance which forms by far its larger portion (Fig. 7). In the bones they deposit a material similar to that in the cartilage, except that with it is mixed a mineral substance which gives the bones their hardness and stiffness.(4) The intercellular material, in addition to connecting the cells, supplies to certain tissues important properties, such as the elasticity of cartilage and the stiffness of the bones.

*Nature of the Body Organization.*-The division of labor carried on by the different organs, as shown in the preceding chapter, is in reality carried on by the cells that form the organs. To see that this is true we have only to observe the relation of cells to tissues and of tissues to organs.

The cells form the tissues and the tissues form the organs. This arrangement enables the special work of different kinds of cells to be combined in the work of the organ as a whole. This is seen in the hand which, in grasping, uses motion supplied by the muscle cells, a controlling influence supplied by the nerve cells, a framework supplied by the bone cells, and so on. The cells supply the basis for the body organization and, properly speaking, the body is _an organization of cells_(5) (Recall the definition of an organization, page 10.) In this organization there are to be observed:

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. 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:

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 a.s.similation 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 sc.r.a.pings from the inside of the cheek with a dull knife and mix these with a little water on a gla.s.s slide. Place a cover-gla.s.s 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 gla.s.s 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 elder pith, potato, and the stems of growing plants. Make drawings of the cells thus observed.

4. Examine with the microscope a small piece of the freshly sloughed off epidermis of a frog's skin. Examine it first in its natural condition, and then after soaking for an hour or two in a solution of carmine. Make drawings.

5. Mount on a gla.s.s slide some of the sc.u.m found on stagnant water and examine it with a compound microscope. Note the variety and relative size of the different things moving about. The forms most frequently seen by such an examination are one-celled plants. Many of these have the power of motion.

6. Examine tissues of the body, such as nervous, muscular, and glandular tissues, which have been suitably prepared and mounted for microscopic study, using low and high powers of the microscope. Make drawings of the cells in the different tissues thus observed.

CHAPTER IV - THE BLOOD

Two liquids of similar nature are found in the body, known as the blood and the lymph. These are closely related in function and together they form the nutrient fluid referred to in the preceding chapter. The blood is the more familiar of the two liquids, and the one which can best be considered at this time.

*The Blood: where Found.*-The blood occupies and moves through a system of closed tubes, known as the blood vessels. By means of these vessels the blood is made to circulate through all parts of the body, but from them it does not escape under normal conditions. Though provisions exist whereby liquid materials may both enter and leave the blood stream, it is only when the blood vessels are cut or broken that the blood, as blood, is able to escape from its inclosures.