CHAPTER IV. 



HISTOLOGY OF THE EPITHELIAL AND CONNECTIVE TISSUES. 



i. EPITHELIAL TISSUE. 



The epithelial tissue consists of one or more layers of cells resting on a 

 homogeneous membrane, the other side of which is abundantly supplied 

 with blood-vessels and nerves. The form of the epithelial cell varies in 

 different situations, and may be flattened, cuboid, spheroid or columnar. 

 (See Figs. 4, 5, and 6.) The form of the cell in all instances is related to 

 some specific function. When arranged in layers or strata, the cells are 

 cemented together by an intercellular substance. 



The epithelial tissue forms a continuous covering for the surfaces of the 

 body. The external investment (the skin) and the internal investment (the 

 mucous membrane, which lines the entire alimentary canal as well as as- 

 sociated body cavities) are both formed, in all situations, by the homogeneous 

 basement membrane, covered with one or more layers of cells. The glands 



FIG. 3. EPITHELIAL CELLS OF RABBIT, ISOLATED. X 560. i. Squamous cells (mucous 

 membrane of mouth). 2. Columnar cells (corneal epithelium). 3. Columnar cells, with cuticular 

 border, s (intestinal epithelium). 4. Ciliated cells; h, cilia (bronchial epithelium). (Stohr.) 



of the skin, the lungs and the glands in connection with the alimentary canal 

 and the uro-genital apparatus are formed of the same elemental structures. 

 All materials, therefore, whether nutritive, secretory, or excretory, must pass 

 through epithelial cells before they can enter into the formation of the blood 

 or be eliminated from it. The nutrition of the epithelial tissue is maintained 

 by the nutritive material derived from the blood diffusing itself into and 

 through the basement membrane. Chemically, the epithelial cells of the 

 epidermis hair, nails, etc. are composed of a sclero-protein (keratin), a 

 small quantity of water, and inorganic salts. In other situations, especially 

 on the mucous membranes, the cells consist largely of mucin, in association 

 with other proteins. The consistency of epithelium varies in accordance with 

 external influences, such as the presence or absence of moisture, pressure, 

 friction, etc. This is well seen in the skin of the palms of the hands and the 

 soles of the feet situations where it acquires its greatest density. In the 

 alimentary canal, in the lungs, and in other cavities, where the reverse condi- 

 tions prevail, the epithelium is extremely soft. Epithelial tissues also possess 



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