8 



The Skin and its Derivatives 



The skin of the vertebrate consists of two layers, an outer 

 epidermis of ectodermal origin and an inner dermis of meso- 

 dermal origin. Each of the layers, in contrast to those of the 

 invertebrates, is formed of several kinds of cells and several 

 forms of the same kind of cell (Figures 8-1, 8-2). Both parts 

 may be involved in the skin derivatives, which include teeth, 

 scales, bony plates, keratinized scales, nails, feathers, hair, 

 several types of glands (including the mammary glands), 

 and many sensory structures. The latter, with one exception 

 (prototrichs), will be described as parts of the nervous system. 



SKIN 



The epidermis of the higher forms is stratified into several 

 layers of cells. Essentially one type of cell is envolved in the 

 epidermis, and it undergoes several changes in form as it 

 moves from the base of the epidermis to the surface. This 

 outer layer of the skin is generally without a blood supply 

 and without nerves, although sensory cells, are found here 

 in the lower forms. In a few instances, nerve endings may 

 penetrate the epidermis. Capillaries lie close to the basal 

 layer of the epidermis, the Malpighian layer or stratum 

 germinativum, from which the cells of the epidermis are 



hair 



sweat gland pore 



interpapillory peg 



stratum cornium 

 stratum lucidum 

 stratum granulosum 

 .stratum spinosum 

 stratum germinativum 



dermal papilla 



sebaceous gland 



sweat gland 



Figure 8-1. A steriodiagram of human skin. The outer layer of epidermis is several times thicker 

 than it should be in order to show its layering. The hair follicle is proportionally reduced in order to 

 keep the follicle within the limits of the diogrom; the diameter of the hair is actually somewhat greater 

 than the thickness of the epidermis (see Figure 8-3). 



217 



