OF THE SKIN. M7 



remaining wholly unchanged), which gradually raises the bottom 

 of the wound, and brings it to a level with the surrounding 

 epidermis, the latter, in consequence of the pressure that it suffers 

 from the growing portion, becoming everted and exfoliating. 

 The reason of this phenomenon is to be sought, simply in this, 

 that the non-vascular epidermis draws the materials which it 

 requires for its nutrition and regeneration from the superficial 

 vessels of the corium. It is more difficult to ascertain from what 

 portion of the Malpighian layer the regeneration proceeds. If a 

 layer of cyto-blastema and of free nuclei existed upon the surface 

 of the corium, as many authors suppose, we might acquiesce 

 in the view, that the epidermis grows by free cell-development 

 in those innermost layers which rest immediately upon it ; 

 but such a cyto-blastema, as we have seen, does not exist, the 

 stratum Malpighii being invariably formed by perfect cells ; 

 and thence nothing remains, but to suppose an endogenous 

 cell-development around portions of contents in the deepest 

 round cells, or a multiplication by division, for which latter 

 view the occasional occurrence of two nuclei in some of the 

 softer epidermic cells, seems to speak. It can be more easily 

 made out, how, in the course of the growth of the epidermis, 

 the youngest epidermic cells become changed into horny plates. 

 The small and round vesicles of the deeper layers of the stratum 

 Malpighii become larger and flatter the more they approach the 

 surface, until at last they are completely converted into flattened 

 plates. In the meanwhile their nuclei at first grow a little, 

 and then, as a general rule, disappear wholly in the horny 

 layer; whilst the cell-contents, which are granular in the 

 mucous layer, clearly distinct from the cell-membrane, and 

 probably semi-fluid, become more solid and homogeneous in 

 the horny layer, and finally coalesce with the cell-membranes. 

 At the same time the latter are chemically altered, becoming 

 less and less soluble in caustic alkalies.] 



§47. 



Development of the Epidermis. — The first layers of the epi- 

 dermis are developed, in the Mammalia, by the metamorphosis of 

 the most superficial of the original formative cells which com- 

 pose the young embryo. When the rudiments of the stratum 

 Malpighii and horny layer are once indicated, the former con- 



