13. Kinds of Cells. Cells vary greatly in size, some of the smallest being only 1/3500 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. Illustration: 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.