CHAPTER V. 



TEGUMENTARY ORGANS SKIN STRUCTURES. 



SECTION I. 



Vertebrates. 



It will be remembered that the whole surface, both 

 external and internal (by infolding of the external), is 

 covered with a pavement of living nucleated cells, and 

 that all exchange by absorption or elimination is by the 

 agency of these cells. When these are very active the 

 surface is soft and covered with a slimy mucus by the 

 continual decay and solution of dead cells, and is 

 called epithelium. When less active the old cells accumu- 

 late in many layers and dry up, mummify, and flatten, 

 and finally pass away as scales. This is called the 

 epidermis. 



The epiderm, therefore, is said to consist of two 

 layers — viz., a layer of living nucleated cells in direct 

 contact with the dermis and called the mucous or Mal- 

 pighian layer, and a layer of accumulated cells in all 

 stages of dying and flattening and mummification, and 

 called the cuticular layer. The color of the skin, whether 

 blond or brunette, or brown or black, is determined by 

 the amount of pigment in the mucous layer. The 

 cuticular layer, in the act of drying up, may harden in 

 various degrees. 



For example, if the epithelial cells only mummify 

 they form the ordinary cuticle. If they simply harden, 

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