THE STRUCTURE OF THE ELEMENTARY TISSUES. 39 



the interstices being filled by the transparent intercellular cement-sub- 

 stance. When this increases in quantity in inflammation the cells are 

 pushed further apart, and the connecting fibrils or " prickles " elongated 

 and therefore more clearly visible. 



The columnar cells of the deepest layer are distinctly nucleated; they 

 multiply rapidly by division; and as new cells are formed beneath, they 

 press the older cells forward to be in turn pressed forward themselves 

 toward the surface, gradually altering in shape and chemical composition 

 until they are cast off from the surface. 



Stratified epithelium is found in the following situations: (1.) Form- 

 ing the epidermis, covering the whole of the external surface of the body; 

 (2.) Covering the mucous membrane of the nose, tongue, mouth, pharynx, 

 and oesophagus; (3.) As the conjunctival epithelium, covering the cor- 

 nea; (4.) Lining the vagina and the vaginal part of the cervix uteri. 



Fig. 35. -Jagged cells from the middle layers of pavement epithelium, from a vertical section of the 

 gum of a new-born infant. (Klein.) 



Functions of Epithelium. According to function, epithelial cells 

 may be classified as: (1.) Protective, e.g., in the skin, mouth, blood- 

 vessels, etc. (2.) Protective and moving ciliated epithelium. (3.) 

 Secreting glandular epithelium; or, Secreting formed elements epi- 

 thelium of testicle secreting spermatozoa. (4.) Protective and secreting, 

 e.g., epithelium of intestine. (5) Sensorial, e.g., olfactory cells, rods and 

 cones of retina, organ of Corti. 



Epithelium forms a continuous smooth investment over the whole 

 body, being thickened into a hard, horny tissue at the points most ex- 

 posed to pressure, and developing various appendages, such as hairs and 

 nails, whose structure and functions will be considered in a future chapter. 

 Epithelium lines also the sensorial surfaces of the eye, ear, nose, and 

 mouth, and thus serves as the medium through which all impressions 

 from the external world touch, smell, taste, sight, hearing reach the 

 delicate nerve endings, whence they are conveyed to the brain. 



The ciliated epithelium which lines the air-passages serves not only 

 as a protective investment, but also by the movements of its cilia pro- 

 motes currents of the air in the bronchi and bronchia, and is enabled to 

 propel fluids and minute particles of solid matter so as to aid tln>ir ex- 



