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.
]

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.)