Physiology and Hygiene for Secondary Schools | Page 8

built up or renewed.
Excretion is the throwing off of such waste materials as have been formed in the cells. These are passed into the lymph and thence to the surface of the body.
Absorption, assimilation, excretion, and also reproduction are performed by all classes 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 class 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 class 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