Cells are associated and combined in various ways to form simple tissues. Such a simple tissue is called an epithelium, and the cells are called epithelial cells. The cells are united together by a very small amount of a cement substance. The cells forming an epithelium may be globular, squanions, columnar, or cili ated, and so squamous epithelium, columnar epithelium, ciliated columnar epithelium, and so on, are spoken of. Moreover, the cells form ing an epithelium may be in a single layer only, or may be several layers deep. In time former case the epithelium is said to be simple, in the latter case stratified. It is, then, easy to understand what is meant by simple squamous epithelium, and what by simple columnar epithelium, and what by simple columnar ciliated epithelium. In each case what is meant is a tissue formed of a single layer of cells, but in the first the cells are squamous, in the second colum nar, and in the third ciliated coin mar. Then there is stratified squamous, stratified columnar, and stratified columnar ciliated epithelium. In the case of stratified epithelia it is the character of the uppermost layer of cells that gives the designa tion. So that, of the three last phrases, the first means a tissue formed of several layers of cells of which the uppermost is squanions, the second signifies a similar structure, the uppermost layer being columnar, and in time third ease the top layer is columnar and ciliated. Now such epithelia are found on the whole surface of the skin, lining the mouth, throat, and whole length of the alimentary canal, and all canals communicating with it, lining the air passages and recesses of the lungs, the nostrils, canal of the ear, surface of eyelids and eye balls, lining the tubes and recesses of glands, lining all time closed cavities and tubes of the body, &c., and epithelial structures form the essential parts of the terminal organs of the senses.
Functions of Epithelium. — Such struc
tures may be divided, as regards their function, into two main divisions. One set of them are obviously chiefly protective in character. The layers of epithelium which together form the epidermis, or superficial layer of the skin, have little beyond such an office to discharge. So is it with the cells coveting the mucous mem brane of the mouth, and those lining the inner surface of the eyelids and front of the eyeball, of which Fig. 7 is a representation. A similar duty belongs to the epithelium lining the air passages and air-cells of the lungs. Epithelia which discharge so inactive a function are com monly formed of squamous or short columnar cells, and if the situation they protect be much exposed, they are generally stratified. The second great division of epithelia consists of those whose cells are formed of highly active protoplasm, and are busily engaged in some sort of secretion. Such are time cells of glands —the cells of the salivary glands, which secrete the saliva, of time gastric glands, which secrete gastric juice, of the intestinal glands, the cells of the liver, time cells lining time tubules of the kidney, the sweat glands, and so on. Such active epithelial structures are usually formed of a single layer of cells, which are more or less globular in form or long columnar.
Of ciliated epithelium it is necessary to say a further word. The cilia are delicate pro longations of the protoplasm of time cell. They execute a rapid whip-like lashing movement as often as ten or more times a second, and the movement may be quickened or slowed by favourable or unfavourable circumstances. All the cilia move in the same direction. In time case of the respiratory passages this serves to sweep mucus up the passages; and in other canals a similar valuable office is filled by the ciliated epithelium.