DISEASES OF THE SKIN. 459 



STRUCTURE OF THE SKIN. 



The skin consists primarily of two parts: (1) The superficial non- 

 vascular (without blood vessels) layer, the cuticle, or epidermis; and 

 (2) the deep vascular (with blood vessels) layer, the corium, dermis, 

 or true sMn. (See PI. XXXVIII, fig. 1.) 



The cuticle is made up of cells placed side by side and more or less 

 modified in shape by their mutual compression and by surface evapo- 

 ration and drying. The superficial stratum consists of the cells dried 

 in the form of scales, which fall off continually and form dandruff. 

 The deep stratum (the mucous layer) is formed of somewhat rounded 

 cells with large central nuclei, and in colored skin containing numer- 

 ous pigment granules. These cells have prolongations, or branches, 

 by which they communicate with one another and with the superficial 

 layer of cells in the true sMn beneath. Through these prolongations 

 they receive nutrient liquids for their growth and increase, and pass 

 on liquids absorbed by the skin into the vessels of the true skin 

 beneath. The living matter in the cells exercises an equally selective 

 power on what they shall take up for their own nourishment and on 

 what they shall admit into the circulation from without. Thus, cer- 

 tain agents, like iodin and belladonna, are readily admitted, whereas 

 others, like arsenic, are excluded by the sound, unbroken epidermis. 

 Between the deep and superficial layers of the epidermis there is a 

 thin, translucent layer (septum lucidum) consisting of a double 

 stratum of cells, and forming a medium of transition from the deep 

 spheroidal to the superficial scaly cuticle. 



The true skin, or dermis, has a framework of interlacing bundles 

 of white and yellow fibers, large and coarse in the deeper layers, and 

 fine in the superficial, where they approach the cuticle. Between the 

 fibrous bundles are left interspaces which, like the bundles, become 

 finer as they approach the surface, and inclose cells, vessels, nerves, 

 glands, gland ducts, hairs, and in the deeper layers fat. 



The superficial layer of the dermis is formed into a series of 

 minute, conical elevations, or papillae, projecting into the deep por- 

 tion of the cuticle, from which they are separated by a very fine 

 transparent membrane. This papiUary layer is very richly supplied 

 with capillary blood vessels and nerves, and is at once the seat of 

 acute sensation and the point from which the nutrient liquid is 

 supplied to the cells of the cuticle above. It is also at this point that 

 the active changes of inflammation are especially concentrated ; it is 

 the immediately superposed cell layers (mucous) that become mor- 

 bidly increased in the earlier stages of inflammation; it is on the 

 surface of the papillary layer that the liquid is thrown out which 

 raises the cuticle in the form of a blister, and it is at this point 

 mainly that pus forms in the ordinary pustule. 



