Even such hard and dense structures as bone, gristle, alai tooth are formed originally from and by the agency of cells.
The cell is therefore the histological unit of the body. By association, combination, and modification of cells the body is built up. It may also be safely asserted that if the true character of the changes going on in living protoplasm, the building up and the breaking down process, were accurately understood, the secret of life would be laid bare.
Cells vary in size, sonic being only of an inch or less in diameter, whilst the largest, the ovum, is only from to of an inch in breadth. They also vary greatly in form, as may be seen from the accompanying figures.
The typical cell is more or less globular in form, and such are found in glands. A drop of saliva from the mouth will usually exhibit, besides large flat cells, globular cells from the salivary glands.
There are also columnar cells, in which the cells are closely ranged side by side (Fig. 6).
Another form is called ciliated columnar, in which the columnar cells have delicate pro longations—cilia—from the free surface (Fig. 6). These cilia are, when the cell is living, in constant movement, and, when observed under the microscope, present the appearance of the passage of a wave, as is seen when the wind blows over a corn-field.
There are cells of spindle shape, cells with numerous branching processes, branched or stellate cells, cells flattened into a pavement form, squamous or pavement cells (Fig. 5), polyhedral cells, and so on.
Besides being thus classified according to shape, cells may be designated according to contents. Thus fat cells and pigment cells are spoken of, the former being filled with- oil, so that it appears as a mere sac of oil, the latter being loaded with dark-coloured granules.
Moreover, cells may be described according to their function, or situation, or tissue in which they are found, as epithelial cells, blood cells, gland cells, nerve cells, connective-tissue cells.
The functions of cells have been already partly indicated. They manifest vitality in various ways: (1) absorption of matter; (2) transformation of the same either into proto plasm or some material formed by the cell, such as fat or pigment (colouring matter); (3) separation of waste matters of no further use to the cell (excretion); (4) growth or increase in size and development of parts by taking up new matter; (5) development of new cells or descendants which succeed the old ones; and (6) special properties, such as that of changing their form or contracting, or that of nervous activity, as shown by the cells of the nervous system. Each cell has a life of its own. Some live for from 12 to 24 hours, as is likely the case with many of the cells lining the alimentary canal; others may live for many years, as in cartilage or gristle and bone. In glands they are constantly engaged in separating various matters front the blood, and altering or elabo rating these to form new substances, to be made use of in time body or expelled from it. Thus the cells of the liver form the bile, those of the kidney separate certain substances from the blood, which are cast away in the urine, while those of the salivary glands and of the glands of the stomach and pancreas form the juices by whose agency food is digested.