Physiology and Hygiene for Secondary Schools

by Francis M. Walters

The General Work of Cells.—In handling the materials[pg 018] 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 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—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.[pg 019] 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[pg 020] of an organization, page 10.) In this organization there are to be observed: