CHAP. XVIII.] THE SKIK 213 



form one special function. It is an important excretory organ, 

 but it is also an absorbing organ; it is likewise the principal 

 seat of the sense of touch, and serves, too, as a protective cover- 

 ing for the deeper tissues lying beneath it. 



The skin, like a mucous membrane, consists of two distinct 

 layers; an epithelial covering, and a connective tissue basis. 

 The epithelium is a stratified epithelium and is called the epi- 

 dermis, or scarf -skin; the connective tissue layer is called the 

 derma, cutis vera (true skin), or corium. The epidermis is com- 

 posed of layers of cells, the deeper of which are soft and pro- 

 toplasmic, while the superficial layers are hard and horny. 

 Between the two layers is a fairly distinct line of granular- 

 looking cells, the granules in which have been thought to form 

 the horny matter in the superficial cells. In the coloured races 

 the single layer of elongated cells next the corium contains 

 pigment granules. 



The growth of the epidermis takes place by the multiplication 

 of the cells in the deeper or Malpighian layer. As these cells 

 multiply by cell-division, they push upwards towards the surface 

 those previously formed. In their upward progress they 

 undergo a chemical transformation, and the soft protoplasmic 

 cells become converted into the flat, horny scales which are 

 constantly being rubbed off the surface of the skin. 



The thickness of the epidermis varies in different parts of the 

 body, measuring in some places not more than ^^th of an 

 inch (0.106 mm.), and in others as much as ^th of an inch 

 (1.06 mm.). It is thickest in the palms of the hands and on 

 the soles of the feet where the skin is most exposed to friction 

 and pressure, but it forms a protective covering over every 

 part of the true skin, upon which it is closely moulded. 



No blood-vessels pass into the epidermis ; it, however, receives 

 fine nerve-fibrils between the cells of the Malpighian layer. 



The cutis vera or true skin is a highly sensitive and vascular 

 layer of connective tissue. It is, like the mucous membranes, 

 attached to the parts beneath it by a layer of areolar tissue, 

 here named " subcutaneous," which layer, with very few excep- 

 tions, contains fat. The connection in some parts is loose and 

 movable, as on the front of the neck ; in others, close and firm, 

 as on the palmar surface of the hand and on the sole of the 

 foot. 



