18.  Functions of Epithelial Tissues.  The epithelial structures may be divided, as to their functions, into two main divisions.  One is chiefly protective in character.  Thus the layers of epithelium which form the superficial layer of the skin have little beyond such an office to discharge.  The same is to a certain extent true of the epithelial cells covering the mucous membrane of the mouth, and those lining the air passages and air cells of the lungs.

[Illustration:  Fig. 5.—­Various Kinds of Epithelial Cells

  A, columnar cells of intestine;
  B, polyhedral cells of the conjunctiva;
  C, ciliated conical cells of the trachea;
  D, ciliated cell of frog’s mouth;
  E, inverted conical cell of trachea;
  F, squamous cell of the cavity of mouth, seen from its broad surface;
  G, squamous cell, seen edgeways.
]

The second great division of the epithelial tissues consists of those whose cells are formed of highly active protoplasm, and are busily engaged in some sort of secretion.  Such are the cells of glands,—­the cells of the salivary glands, which secrete the saliva, of the gastric glands, which secrete the gastric juice, of the intestinal glands, and the cells of the liver and sweat glands.

19.  Connective Tissue.  This is the material, made up of fibers and cells, which serves to unite and bind together the different organs and tissues.  It forms a sort of flexible framework of the body, and so pervades every portion that if all the other tissues were removed, we should still have a complete representation of the bodily shape in every part.  In general, the connective tissues proper act as packing, binding, and supporting structures.  This name includes certain tissues which to all outward appearance vary greatly, but which are properly grouped together for the following reasons:  first, they all act as supporting structures; second, under certain conditions one may be substituted for another; third, in some places they merge into each other.

All these tissues consist of a ground-substance, or matrix, cells, and fibers.  The ground-substance is in small amount in connective tissues proper, and is obscured by a mass of fibers.  It is best seen in hyaline cartilage, where it has a glossy appearance.  In bone it is infiltrated with salts which give bone its hardness, and make it seem so unlike other tissues.  The cells are called connective-tissue corpuscles, cartilage cells, and bone corpuscles, according to the tissues in which they occur.  The fibers are the white fibrous and the yellow elastic tissues.

The following varieties are usually described: 

    I. Connective Tissues Proper: 

1.  White Fibrous Tissue. 2.  Yellow Elastic Tissue. 3.  Areolar or Cellular Tissue. 4.  Adipose or Fatty Tissue. 5.  Adenoid or Retiform Tissue.

    II.  Cartilage (Gristle): 

      1.  Hyaline.
      2.  White Fibro-cartilage.
      3.  Yellow Fibro-cartilage.