8 General Part. 



be more precise, it is many cells deep, for the cells are generally not 

 arranged in layers. Whilst the deeper-lying cells are usually 

 undifPerentiated, the most superficial, or several of the outer layers, 

 exhibit many modifications. Sometimes they are flattened, and the 

 epithelium is termed stratified squamous epithelium ; or the outermost 

 layer consists of cylindrical cells, when it is known as stratified 

 columnar epithelium. 



The free surfaces of the cells, whether of simple or of stratified 

 epithelium, may be provided with cilia, delicate hair-like processes, 

 which are in constant motion. These are called ciliate cells, 

 or if only one large hair, a flagellum, is present, flagellate cells. 

 Sometimes isolated groups of such cells are found in simple, or in the 

 superficial layer of stratified epithelium ; or, again, they may compose 

 the whole, or almost the whole, layer (of course, the outer layer 

 only in stratified epithelium) . This is commonly called ciliate 

 epithelium. 



Fig. 6. A Ciliated cells. B Columnar cells, 

 with a outioular plate (c). — Orig. 



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Fig. 7. Simple epithelium, with a cuticle (c). 

 Orig. 



Not infrequently the epithelial cells secrete, at their free edge, a 

 firmer substance, a cuticular plate (often called the cuticular 

 border) ; the cuticular deposits of neighbouring cells usually become 

 closely connected, so as to form a continuous covering or cuticle, 

 which occasionally attains a considerable thickness and hardness. 



Fig. 8. A Columnar 

 epithelium, with goblet 

 cells, from one of these 

 mucus is escaping. B 

 Other gland cells. — Orig. 



The primary function of the epithelium is to form a protecting 

 covering for the other tissues, but it often performs a second, viz. 

 that of secreting, materials, usually fluids, which are either of great 

 importance to the body, or else need to be discharged from it (urea) . 

 Isolated gland cells, usually of a peculiar form, are often met with in 

 epithelia, such are the goblet cells of many animals. They have a 



