Physiology and Hygiene for Secondary Schools | Page 7

4. Food particles and waste. 5. Cell-wall. 6. Masses of active material found in certain cells, called plastids.
Surrounding the nucleus is the main body of the cell, sometimes referred to as the "protoplasm." Since the protoplasm forms all parts of the cell, this substance is more properly called the cytoplasm, or cell plasm. Surrounding and inclosing the cytoplasm, in many cells, is a thin outer layer, or membrane, which affords more or less protection to the contents of the cell. This is usually referred to as the cell-wall. A fourth part of the cell is also described, being called the attraction sphere. This is a small body lying near the nucleus and co?perating with that body in the formation of new cells. Food particles, wastes, and other substances may also be present in the cytoplasm. The parts of a typical cell are shown in Fig. 4.
*Importance of the Cells.*--The cells must be regarded as the living, working parts of the body. They are the active agents in all of the tissues, enabling them to serve their various purposes. Working through the tissues, they build up the body and carry on its different activities. They are recognized on this account as the units of structure and of function, and are the "individuals" in the body organization. Among the most important and interesting of the activities of the cells are those by which they build up the body, or cause it to grow.
*How the Cells enable the Body to Grow.*--Every cell is able to take new material into itself and to add this to the protoplasm. This tends to increase the amount of the protoplasm, thereby causing the cells to increase in size. A general increase in the size of the cells has the effect of increasing the size of the entire body, and this is one way by which they cause it to grow. There is, however, a fixed limit, varying with different cells, to the size which they attain, and this is quite low. (The largest cells are scarcely visible to the naked eye.) Any marked increase in the size of the body must, therefore, be brought about by other means. Such a means is found in the formation of new cells, or cell reproduction. The new cells are always formed by and from the old cells, the essential process being known as cell-division.
[Fig. 5]
Fig. 5--Steps in cell-division (after Wilson). Note that the process begins with the division of the attraction sphere, then involves the nucleus, and finally separates the main body.
*Cell-Division.*--By dividing, a single cell will, on attaining its growth, separate into two or more new cells. The process is quite complex and is imperfectly understood. It is known, however, that the act of separation is preceded by a series of changes in which the attraction sphere and the nucleus actively participate, and that, as a result of these changes, the contents of the old cell are rearranged to form the new cells. Some of the different stages in the process, as they have been studied under the microscope, are indicated in Fig. 5.
Gradually, through the formation of new cells and by the growth of these cells after they have been formed, the body attains its full size. When growth is complete, cell reproduction is supposed to cease except where the tissues are injured, as in the breaking of a bone, or where cells, like those at the surface of the skin, are subject to wear. Then new material continues to be added to the protoplasm throughout life, but in amount only sufficient to replace that lost from the protoplasm as waste.
[Fig. 6]
Fig. 6--A tumbler partly filled with marbles covered with water, suggesting the relations of the cells to the lymph.
*Cell Surroundings.*--All cells are said to be aquatic. This means simply that they require water for carrying on their various activities. The cells, in order to live, must take in and give out materials, and water is necessary to both processes. It is also an essential part of the protoplasm. Deprived of water, cells become inactive and usually die. Aquatic surroundings are provided for the cells of the body through a liquid known as the lymph, which is distributed throughout the intercellular material (Fig. 6). This consists of water containing oxygen and food substances in solution. Besides supplying these to the cells, the lymph also receives their wastes. Through the lymph the necessary conditions for cell life are provided in the body.
*The General Work of Cells.*--In handling the materials derived from the lymph, the cells carry on three well-defined processes, known as absorption, assimilation, and excretion.
Absorption is the process of taking water, food, and oxygen into the cells.
Assimilation is a complex process which results in the addition of the absorbed materials to the protoplasm. Through assimilation the protoplasm is