STRUCTURE OP THE SKIN I EPIDERMIS. 49 



outwards, until we arrive at layers composed entirely of dry 

 flat scales, which show but little indication of ever having 

 been cells. There is no doubt, however, that all these forms 

 are but different stages of the existence of one and the 

 same set of epidermic cells ; these taking their origin in the 

 formative fluid exuded on the surface of the basement-mem- 

 brane, and being progressively carried towards the surface by 

 the . successive development of new layers beneath them, 

 whilst the layers above them are thrown off, or are worn 

 away ; and at the same time undergoing a change of form, in 

 the first instance from mutual pressure, and afterwards from 

 the loss of their contained fluid. At the same time they are 

 rendered more firm in texture, by the formation of a horny 

 secretion in their interior ; so that the outer layers of epi- 

 dermis form a consistent membrane, which is raised from the 

 surface of the Cutis when fluid infiltrates between them (as 

 when the hand has been long soaked in water), or is poured 

 out by the vessels of the latter (as when a blister is applied) ; 

 whilst the soft internal layers remain in contact with the 

 basement-membrane. The number of layers varies greatly in 

 different parts, being usually found to be greatest where 

 there is most pressure or friction, as if the irritation deter- 

 mined an increased supply of blood to the spot, and thus 

 favoured an augmented development of epidermic cells. 

 Thus, on the soles of the feet, particularly at the heel and 

 the ball of the great toe, the Epidermis is extremely thick ; 

 and the palms of the hands of the labouring man are 

 distinguished by the horny hardness of their thick cuticle. 

 It was formerly supposed that a special layer of a soft 

 spongy tissue, termed the rete mucosum, intervenes between 

 the Cutis and the Epidermis ; and that this was the special 

 seat of the colour of the skin in the dark races. It is 

 now well ascertained, however, that this supposed rete con- 

 sists of nothing else than the newly-forming soft layers 

 of the true epidermis; and that the colouring matter is 

 diffused through the epidermic cells, so as to tinge the 

 entire thickness of the cuticle, although its presence is 

 particularly obvious in the deeper layers. The Nails may 

 be considered as nothing more than an altered form of 

 Epidermis ; when examined near their origin, they are found 

 to consist of cells which gradually dry into scales that remain 



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