108 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Epithelial Tissues 



Epithelium covers the outer surfaces of the body, Hnes its cavities such as 

 lungs, alimentary canal, the coelom or body cavity, and the blood and lymph 

 vessels (Fig. 7.1). It forms glands and the essential parts of the sense organs 

 — sensory cells in the eye and nose. 



Epithelium is an essential guardian of the integrity of the body. It has a 

 general and vital part in metabolism since all substances which take part in 

 metabolic activity must go through epithelial cells. All digested food is ab- 

 sorbed through epithelium, mostly in the small intestine. The amount of water 

 contained in an animal is controlled through epithelium, in the skin, alimen- 

 tary canal, and kidneys. In the liver and kidneys it takes part in excreting 

 waste substances. It secretes such varied products as oyster shells and pearls, 

 the chitinous cover of insects, the digestive fluids of all multicellular animals, 

 and the hormones of glands such as the thyroid and the pancreas. It is directly 

 the protection against all manner of mechanical and chemical injuries. It was 



Fig. 7.1. Epithelial tissues through which all substances that take part in the 

 metabolism of multicellular animals must pass. A, simple flattened or squamus 

 epithelium from the surface of the mesentery of a guinea pig; B, lining of a small 

 vein of mesentery. Intercellular cement is darkened by the preparation. xl200. 

 (Courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, 

 McGraw-HiU Book Co., Inc., 1953.) 



