470 STRUCTURE OF THE EPIDERMA. 



papillaB of the derma are those which produce the nail; in the dermal 

 follicle of the nail they are long and filiform, while beneath its concave 

 surface they form longitudinal and parallel plications which extend for 

 nearly the entire length of that organ. In structure each papilla is com- 

 posed of a more or less convoluted capillary and a more or less convoluted 

 nervous loop. 



The EPIDERMA or cuticle (scarf-skin) is a product of the derma, which 

 it serves to envelope and defend. That surface of the epiderma 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 epideima, like the derma, 

 is divisible into two layers, external and internal, the latter being termed 

 the rete mucosum. Moreover, the epiderma is laminated in structure, and 

 the laminae present a progressively 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 epiderma, for as the external 

 surface is constantly 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 derma to take the place of 

 the old. 



The theory of growth of the epiderma, deduced from the observations 

 of Sclrsvann, is as follows : A stratum of plastic lymph (liquor sanguinis) 

 is poured out upon the surface of the derma. This fluid, by virtue of the 

 vital force inherent in itself, and communicated to it by contact with a 

 living tissue, is converted into granules, which are termed cell-germs, or 

 cyto-blasts. By endosmosis, these cyto-blasts imbibe serum from the. 

 plastic lymph and adjacent tissues, and the outermost layer or pellicle of 

 the cyto-blast becomes 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. 

 Fig. 209.* Moreover, within the nucleus one or several nuclei 



are formed which are termed nucleoli. By a con- 

 tinuance of the process of imbibition, the cell be- 

 comes more or less spherical ; so that, after a time, 

 every part of the surface of the papillary layer of 

 derma is coated by a thin and membranous 

 stratum, consisting of spherical cells lying closely 

 if ^^^ pressed together, and corresponding with every 



* 7/7#*/ irregularity which the papillae present. But, as 

 xV^/ this production of cells is a function constantly in 

 operation, a new layer is formed before the first is 



completed, and the latter is separated by subsequent formations farther 

 and farther from the surface of the papillary layer. As a consequence 

 of loss of contact with the derma, the vital force is progressively dimi- 



* A diagram illustrative of the development of the epiderma, and of epithelia in 

 general, according to the theory of Schwann. 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 de- 

 tected in its interior. 3. The spheroidal cell. 4. The oval cell. 5. The elliptical cell. 

 6 The flattened cell ; which, by contact of its walls, is speedily converted into a scale 

 *n which the nucleus is lost. 7. A nucleated scale as seen upon its flat surface. 8. A 

 cluster of such scale* 



