28 HANDBOOK OF PHYSIOLOGY. 



the cells is very irregular, in consequence of processes passing from cell 



to cell across these intervals. 



Such cells (Fig. 27) are termed " ridge 

 and furrow," "cogged" or "prickle" 

 cells. These "prickles" are prolonga- 

 tions of the intra-cellular network which 

 run across from cell to cell, thus joining 

 them together (Martyn), the interstices 

 being filled by the transparent intercel- 

 lular cement substance. When this in- 

 creases in quantity in inflammation, the 



cells are P ushed further a P art > and the 



ofanew ' connecting fibrils or " prickles" elon- 

 gated, 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 forwards to be in turn pressed forwards themselves 

 towards the surface, gradually altering in shape and chemical composi- 

 tion until they are cast off from the surface. 



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

 Forming the epidermis, covering the whole of the external surface of 

 the body; (2.) Covering the mucous membrane of the tongue, mouth, 

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

 the cornea; (4.) Lining the vaginal part of the cervix uteri. 



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 chap- 

 ter. 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 

 promotes 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 their 

 expulsion from the body. In the case of the Fallopian tube, this agency 

 assists the progress of the ovum towards the .cavity of the uterus. Of 

 the purposes served by cilia in the ventricles of the brain nothing is 

 known. 



