There are some organisms lower down in the scale, whose whole activity is confined within the narrow limits of a single cell. Thus, the amœba begins its life as a cell split off from its parent. This divides in its turn, and each half is a complete amœba. When we come a little higher than the amœba, we find organisms which consist of several cells, and a specialization of function begins to appear. As we ascend in the animal scale, specialization of structure and of function is found continually advancing, and the various kinds of cells are grouped together into colonies or organs.
12. Cells and the Human Organism. If the body be studied in its development, it is found to originate from a single mass of nucleated protoplasm, a single cell with a nucleus and nucleolus. From this original cell, by growth and development, the body, with all its various tissues, is built up. Many fully formed organs, like the liver, consist chiefly of cells. Again, the cells are modified to form fibers, such as tendon, muscle, and nerve. Later on, we shall see the white blood corpuscles exhibit all the characters of the amœba (Fig. 2). Even such dense structures as bone, cartilage, and the teeth are formed from cells.
Fig. 2.--Amœboid Movement of a Human White Blood Corpuscle. (Showing various phases of movement.)
In short, cells may be regarded as the histological units of animal structures; by the combination, association, and modification of these the body is built up. Of the real nature of the changes going on within the living protoplasm, the process of building up lifeless material into living structures, and the process of breaking down by which waste is produced, we know absolutely nothing. Could we learn that, perhaps we should know the secret of life.
13. Kinds of Cells. Cells vary greatly in size, some of the smallest being only ⅓500 an inch or less in diameter. They also vary greatly in form, as may be seen in Figs. 3 and 5. The typical cell is usually globular in form, other shapes being the result of pressure or of similar modifying influences. The globular, as well as the large, flat cells, are well shown in a drop of saliva. Then there are the columnar cells, found in various parts of the intestines, in which they are closely arranged side by side. These cells sometimes have on the free surface delicate prolongations called cilia. Under the microscope they resemble a wave, as when the wind blows over a field of grain (Fig. 5). There are besides cells known as spindle, stellate, squamous or pavement, and various other names suggested by their shapes. Cells are also described as to their contents. Thus fat and pigment cells are alluded to in succeeding sections. Again, they may be described as to their functions or location or the tissue in which they are found, as epithelial cells, blood cells (corpuscles, Figs. 2 and 66), nerve cells (Fig. 4), and connective-tissue cells.
14. Vital Properties of Cells. Each cell has a life of its own. It manifests its vital properties in that it is born, grows, multiplies, decays, and at last dies.[3] During its life it assimilates food, works, rests, and is capable of spontaneous motion and frequently of locomotion. The cell can secrete and excrete substance, and, in brief, presents nearly all the phenomena of a human being.
Cells are produced only from cells by a process of self-division, consisting of a cleavage of the whole cell into parts, each of which becomes a separate and independent organism. Cells rapidly increase in size up to a certain definite point which they maintain during adult life. A most interesting quality of cell life is motion, a beautiful form of which is found in ciliated epithelium. Cells may move actively and passively. In the blood the cells are swept along by the current, but the white corpuscles, seem able to make their way actively through the tissues, as if guided by some sort of instinct.
Fig. 3.--Various Forms of Cells.
A, columnar cells found lining various parts of the intestines (called columnar epithelium);
B, cells of a fusiform or spindle shape found in the loose tissue under the skin and in other parts (called connective-tissue cells);
C, cell having many processes or projections--such are found in connective tissue, D, primitive cells composed of protoplasm with nucleus, and having no cell wall. All are represented about 400 times their real size.
Some cells live a brief life of 12 to 24 hours, as is probably the case with many of the cells lining the alimentary canal; others may live for years, as do the cells of cartilage and bone. In fact each cell goes through the same cycle of changes as the whole organism, though doubtless in a much shorter time. The work of cells is of the most varied kind, and embraces the formation of every tissue and product,--solid, liquid, or gaseous. Thus we shall learn that the cells of the liver form bile, those of the salivary glands and of the glands of the stomach and pancreas form juices which aid in the digestion of food.
15. The Process of Life. All living structures are subject to constant decay. Life is a condition of incessant changes, dependent upon two opposite processes, repair and decay. Thus our bodies are not composed of exactly the same particles from day to day, or even from one moment to another, although to all appearance we remain the same individuals. The change is so gradual, and the renewal of that which is lost may be so exact, that no difference can be noticed except at long intervals of time.[4] (See under "Bacteria," Chapter XIV.)
The entire series of chemical changes that take place in the living body, beginning with assimilation and ending with excretion, is included in one word, metabolism. The process of building up living material, or the change by which complex substances (including the living matter itself) are built up from simpler materials, is called anabolism. The breaking down of material into simple products, or the changes in which complex materials (including the living substance) are broken down into comparatively simple products, is known as katabolism. This reduction of complex substances to simple, results in the production of animal force and energy. Thus a complex substance, like a piece of beef-steak, is built up of a large number of molecules which required the expenditure of force or energy to store up. Now when this material is reduced by the process of digestion to simpler bodies with fewer molecules, such as carbon dioxid, urea, and water, the force stored up in the meat as potential energy becomes manifest and is used as active life-force known as kinetic energy.
16. Epithelium. Cells are associated and combined in many ways to form a simple tissue. Such a simple tissue is called an epithelium or surface-limiting tissue, and the cells are known as epithelial cells. These are united by a very small amount of a cement substance which belongs to the proteid class of material. The epithelial cells, from their shape, are known as squamous, columnar, glandular, or ciliated. Again, the cells may be arranged in only a single layer, or they may be several layers deep. In the former case the epithelium is said to be simple; in the latter, stratified. No blood-vessels pass into these tissues; the cells derive their nourishment by the imbibition of the plasma of the blood exuded into the subjacent tissue.
Fig. 4.--Nerve Cells from the Gray Matter of the Cerebellum. (Magnified 260 diameters.)
17. Varieties of Epithelium. The squamous or pavement epithelium consists of very thin, flattened scales, usually with a small nucleus in the center. When the nucleus has disappeared, they become mere horny plates, easily detached. Such cells will be described as forming the outer layer of the skin, the lining of the mouth and the lower part of the nostrils.
The columnar epithelium consists of pear-shaped or elongated cells, frequently