DEVELOPMENT OF THE EPIDERMIS. 



413 



The growth of epidermis continues throughout life. The cells of the Malpighian 

 layer are constantly undergoing multiplication, and the new cells thus produced 

 push outwards those which are previously formed. The more superficial cells of the 

 Malpighian layer are thus continually passing on to reinforce the horny layer, the 

 cells as they proceed outwards becoming flattened and transformed into horny 

 matter. This change seems to occur quite abruptly when the stratum lucidum is 

 reached ; beyond this the cells again swell out somewhat, until on reaching the 

 most superficial layers they are entirely transformed into structureless horny scales 

 which are constantly undergoing desquamation. The cells of the stratum lucidum 

 sometimes contain drops of a semi-fluid substance which stains similarly to the 

 eleidin of the stratum granulosum (fig. 467, Z>). It has accordingly been inferred 

 that the horny substance of these cells and therefore of the whole horny layer of the 

 epidermis is due to a chemical transformation of the eleidin which is formed in the 

 most superficial layer of the rete mucosum. But a genetic relation between eleidin 

 and keratin is rendered improbable if the statement which has been made by some 

 authors is correct, that in some parts of the epidermis where a large amount of keratin 

 is produced, eleidin granules are not formed within the cells of the rete mucosum. 



There is reason to suppose that the regeneration of epidermis, when a portion 

 has been removed by a blister or wound of any kind, takes place, like its growth, 

 only from cells of the Malpighian layer. If the whole of the epidermis has been 

 destroyed or removed over an extensive surface, the process of regeneration is very 

 slow, since the new covering has to grow in from the epidermis at the margins of 

 the wound. But if the deeper cells have not been wholly removed the regeneration 

 may start from the places where any of them still remain, and the formation of the 

 new covering is proportionately quicker. In the operation of skin-grafting so-called, 

 the surgeon endeavours to transplant from a healthy portion of skin small pieces of 

 the epidermis, including its deeper layers, to the denuded surface : if the operation 



Fig. 471. VERTICAL SECTION OP THE . 

 SKIN AND SUBCUTANEOUS TISSUE, 

 FROM THE END OF THE THUMB, 

 ACROSS THE RIDGES AND FURROWS. 

 MAGNIFIED 20 DIAMETERS. 



(Kblliker.) 



a, horny, and b, Malpighian layer of 

 the epidermis ; c, corium ; d, panni- 

 culus adiposus ; e, papillse on the 

 ridges ; /, fat-clusters ; g, sweat-glands ; 

 h, sweat-ducts ; i, their openings on 

 the surface. 



succeed, each such graft acts as 

 a centre from which the new 

 formation of epidermis may 

 spread, and in this way the raw 

 surface may be much more 

 speedily covered. 



The True Skin, Cutis 

 Vera, Derma, or Corium, is 

 a sentient and vascular fibrous 

 texture. It is covered and de- 

 fended, as already explained, by 

 the non-vascular cuticle, and is 



attached to the parts beneath by a layer of areolar tissue, named "subcuta- 

 neous," which, excepting in a few parts, contains fat, and has therefore been 

 called also the " panniculus adiposus " (fig. 471, d): The connection is in many 



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