EPIDERMAL STRUCTURES. 



8 9 



fied layers of cells are known as the Malpighian layer, the cells 

 of which are polygonal in outline and rich in protoplasm In 

 the mammals, be- 

 tween horny and 

 Malpighian lay- 

 ers, is a thin stra- 

 tum lucidum, 

 consisting of ex- 



FiG. 94. Skin of mammalian embryo, showing the 

 epitrichium, e, after Minot. b, basal layer ; m, Malpighian 

 layer. 



tremely flattened 



cells closely compacted together. 



As will readily be understood, the cells of the basal layer 



are continually dividing, thus producing new cells, which come 



to lie between the basal 

 layer and those layers 

 previously formed, and 

 in this way tending to 

 increase the thickness 

 of the epidermis. As 

 these cells grow older 

 they gradually pass into 

 the conditions found in 

 the different layers, 

 lucidum, corneum, 

 and at last are cast off 

 from the outer surface, 

 either a few cells at a 

 time, or in larger sheets, 



FIG. 95. Diagrammatic section through mam- as in the amphibians 



malian skin. C, stratum corneum; Z>, derma; and reptiles. The Strata 



G, sweat gland; //, hair; L, stratum lucidum; corneum an d lucidum 

 A/, Malpighian layer ; A 7 , nerve ; T, tactile 



corpuscles. are clearly protective 



in nature, and only in 



the turtles is there an absence of this shedding of the external 

 layers of the skin. 



Dermal Glands. The epidermis gives rise to numerous 

 glands. In the fishes the unicellular mucous glands have already 

 been referred to. In some fishes (teleosts) in addition the 

 skin also contains poison glands, sometimes upon the back, 



