EPITHELIUM. 



85 



THE EPITHELIUM. 



All the surfaces of the body, the external surface of the skin, the internal 

 surface of the digestive and respiratory tracts, the closed serous cavities, and 

 the ducts of all glands, are covered by one or more layers of simple cells, called 

 Epithelium or Epithelial Cells, which serve various purposes, both as a protec- 

 tive layer, and as an agent in secretion. Thus, in the skin, the main purpose 

 served by the epithelium (here called the epidermis) is that of protection. As 

 the surface is worn away by the agency of friction or change of temperature, 

 new cells are supplied, and thus the surface of the true skin, and the vessels 

 and nerves which it contains, are defended from damage. In the gastro-intestinal 

 mucous membrane and in the glands, the epithelial cells appear to be the prin- 

 cipal agents in separating the secretion from the blood or from the alimentary 

 fluids. In other situations (as the nose, fauces, and respiratory passages) the 

 chief office of the epithelial cells appears to be to maintain an equable tempera- 

 ture by the moisture with which they keep the surface always slightly lubri- 

 cated. In the serous cavities they also keep the opposed layers- moist, and thus 

 facilitate their movements on each other. Finally, in all internal parts they 

 insure a perfectly smooth surface. 



The epithelium is usually spoken of as tessellated or pavement, columnar, 

 spheroidal or glandular, and ciliated. 



Fig. 43. 



Epithelial cells in the oral cavity of man. a. Large, b. Middle-sized, c. The same with two nuclei, 

 (Magnified 350 times.) 



The pavement epithelium is composed of flat nucleated scales of various shapes, 

 usually polygonal, and varying in size. These scales often contain granules, as 



Fig. 44. 



Fig. 45. 



Epithelium of the intestinal villi of the rabbit, a. Base- 

 ment-membrane. (Magnified 300 times.) 



Spheroidal epithelium from the human 

 bladder. (Magnified 350 times.) 



in Fig. 43. This kind of epithelium is found on the surface of the skin (the 

 epidermis), on all the serous surfaces (unless the ventricles of the brain be an 



