THE SKIN AND ITS APPENDAGES 



125 



the underlying derma. The cells which are immediately in contact with 

 the true skin are more or less rounded, but become gradually altered in 

 figure as they approach the surface, passing through the oval, the elongated 

 oval, the elliptical, and finally being resolved into flattened scales which 

 overlap each other much in the same way as do the tiles of a house (fig. 258). 

 These changes in form and substance are occasioned under the combined 

 influence of the upward pressure from below of the new crops of cells, 

 which are constantly being developed, and of the downward pressure of 

 the atmosphere on the surface, aided by the gradual evaporation of the 

 fluid contents of the cells. The outermost layer of the cuticle is commonly 



Fig. 259. 1, Simple, and 2, Compound Papillse of the Skin. A, Nerves. B, Capillary Loops. 



c, Tactile Corpuscles. 



called the horny layer, while the deeper portion, which consists of the 

 younger and more recently-developed nucleated cells,, some of them con- 

 taining coloured matter (pigment cells), forms what is termed the rete 

 mucosum. These two layers pass gradually one into the other, so that it 

 would be difficult to say at what precise point the separation can be made. 

 The cuticle forms a sufficiently dense covering to protect the sensitive 

 and vascular parts beneath, and as it is constantly being thrown off from 

 the surface in the form of scales, which are familiarly known as scurf, it 

 follows that it must be as constantly renewed from below. It is not, 

 however, to be supposed that the cuticle forms a perfectly continuous coat 

 spread over the structure of the true skin. On the contrary, it is pierced 

 by innumerable openings to afford an exit to the watery excretion of the 

 sweat glands, the fat-like material of the sebaceous follicles, and the hairs 

 which emerge from the openings of the hair follicles. 



