420 



THE SKIX. 



superficial cells are flattened and scaly (fig. 109). The 



deeper part of the epider- 

 Firj. 109.* mis, which is softer and 



more opaque than the su- 

 perficial, is called the rete 

 mucosum. Many of the 

 epidermal cells contain 

 pigment, and the varying 

 quantity of this is the 

 source of the different 

 shades of tint in the skin, 

 both of individuals and 

 races. The colouring mat- 

 ter is contained chiefly in 

 the deeper cells composing 

 the rete mucosum, and be- 

 comes less evident in them 

 as they are gradually 



pushed up by those under them, and become, like their 

 predecessors, flattened and scale-like (fig. 109). It is by 

 this process of production from beneath, to make up for 

 the waste at the surface, that the growth of the cuticle is 

 effected. 



The thickness of the epidermis on different portions of 

 the skin is directly proportioned to the friction, pressure, 

 and other sources of injury to which it is exposed; and the 

 more it is subjected to such injury, within certain limits, 

 the more does it grow, and the thicker and more horny 

 does it become ; for it serves as well to protect the sen- 

 sitive and vascular cutis from injury from without, as to 

 limit the evaporation of fluid from the blood-vessels. The 



* Fig. 109. Skin of the negro, in a vertical section, magnified 250 

 diameters, a, a, cutaneous papilla? ; &, undermost and dark coloured 

 layer of oblong vertical epidermis-cells ; c, mucous or Malpigliiaii 

 layer ; d, horny layer (from Sharper). 



