SIMPLE ISOLATED CELLS EPIDERMIS. 



141 



true skin, designed for its protection. It is now known, however, to 

 consist of a series of layers of cells, which are continually wearing off 

 at the external surface, and are being renewed at the surface of the 

 true skin ; so that the newest and deepest layers gradually become the 

 oldest and most superficial, and are at last thrown off by slow desqua- 

 mation. Occasionally this desquamation of the cuticle is much more 

 rapid ; as after Scarlatina and other inflammatory affections of the 

 Skin. 



225. In their progress from the internal to the external surface of 

 the Epidermis, the cells undergo a series of well-marked changes. 

 When we examine the innermost layer, we find it soft and granular ; 

 consisting of nuclei, in various stages of development into cells, held 

 together by a tenacious semi-fluid substance. This was formerly consi- 

 dered as a distinct tissue, and was supposed to be the peculiar seat of the 

 colour of the skin ; it received the designation of rete mucosum. Passing 

 outwards, we find the cells more completely formed ; at first nearly sphe- 

 rical in shape ; but becoming polygonal where they are flattened against 

 one another. As we proceed further towards the surface, we perceive 

 that the cells are gradually more and more flattened, until they become 

 mere horny scales, their cavity being obliterated ; their origin is indi- 

 cated, however, by the nucleus in the centre of each. This flattening 

 appears to result from the gradual desiccation or drying up of the con- 

 tents of the cells, which result from their exposure to the air. Thus 

 each cell of the Epidermis is developed from the nucleus on the surface 

 of the basement-membrane which nucleus is probably furnished by the 

 membrane itself ( 208), and is gradually brought to the surface by the 

 development of new cells beneath, and the removal of the superficial 

 layers ; whilst at the same time it is progressively changed in form, 

 until it is converted into a flattened scale. The accompanying repre- 



Fig. 20. 



Fig. 21. 



- :r 



"^ 



Oblique section of Epidermis, showing the 

 progressive development of its component 

 cells; a, nuclei, resting upon the surface 

 of the cutis vera /; these nuclei are seen to 

 be gradually developed into cells, at ~b, c, and 

 d; and the cells are flattened into lamellae, 

 forming the exterior portion of the epidermis 

 ate. 



Horny Epidermis, from conjunctiva covering 

 the cornea; a, single scales; b. single lamina of 

 epithelium; below is seen a double layer of the 

 same. 



sentation of an oblique section of the Epidermis, exhibits the principal 

 gradations of its component structures. 



