DISEASES OF THE SKIN. 459 
STRUCTURE OF THE SKIN. 
The skin consists primarily of two parts: (1) The superficiat non- 
vascular (without blood vessels) layer, the cuticle, or epidermis; and 
(2) the deep vascular (with blood vessels) layer, the corium, dermis, 
or true skin. (See Pl. XX XVIII, 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 skin 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 papille, projecting into the deep por- 
tion of the cuticle, from which they are separated by a very fine 
transparent membrane. This papillary 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. 
