STRUCTURE OF THE EPIDERMIS. 523 



which it serves to envelope and defend. That surface of the epider- 

 mis which is exposed to the influence of the atmosphere and exterior 

 sources of injury is hard and horny in texture, while that which lies 

 in contact with the papillary layer is soft and cellular. Hence the 

 epidermis, like the dermis, is divisible into two layers, external and 

 internal, the latter being termed the rete mucosum. Moreover, the 

 epidermis is laminated in structure, and the laminae present a progres- 

 sively increasing tenuity and density as they advance from the inner 

 to the outer surface. This difference of density is dependent on the 

 mode of growth of the epidermis, for as the external surface is con- 

 stantly subjected to destruction from attrition and chemical action, so 

 the membrane is continually reproduced on its internal surface, new 

 layers being successively formed on the dermis to take the place of the 

 old. 



The mode of growth of the epidermis may be thus briefly explained; 

 a stratum of plastic lymph (liquor sanguinis) is poured out upon the 

 surface of the dermis. This fluid, by virtue of the vital force inherent 

 in itself, and communicated to it by contact with a living tissue, is con- 

 verted into granules, which are termed cell-germs or cyto-blasts. By 

 endosmosis, these cyto-blasts imbibe serum from the lymph and adja- 

 cent tissues, and the outermost layer or pellicle of the cyto-blast be- 

 comes gradually distended by the imbibed fluid. The cyto-blast has 

 now become a cell, and the solid portion of the cyto-blast, which always 

 remains adherent to some one point of the internal surface of the cell- 

 membrane, is the nucleus of the cell. Moreover, within the nucleus 

 one or several nuclei are formed 



which are termed nucleoli. By a Fig. 160.* 



continuance of the process of imbi- 

 bition, the cell becomes more or less 

 spherical ; so that, at this period, 

 every part of the surface of the pa- 

 pillary layer of the dermis is coated 

 by a thin and membranous stratum, 

 consisting of spherical cells lying 3 @ 



closely pressed together, and corre- 

 sponding with every irregularity 7 

 which the papillae present. But, as \ e 

 this production of cells is a function / . 

 constantly in operation, a new lay- 

 er is formed before the first is com- 

 pleted, and the latter is separated 

 by subsequent formations farther and farther from the surface of the 



* A diagram illustrative of the development of the epidermis, and of epithelia 

 in general. 1. A granule or cyto-blast. 2. The cell seen rising on the cyto- 

 blast ; the latter is now a nucleus and a nucleolus may be detected in its interior. 

 3. The spheroidal cell. 4. The oval cell. 5. The elliptical cell 6 The flat- 

 tened cell ; which, by contact of its walls, is speedily converted into a scale in 

 which the nucleus is lost. 7. A nucleated scale as seen upon its flat surface. 

 8. A cluster of such scales. 



