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144 ANATOMY.

mencing at its foundation or lower portion, it is subdivided as follows :
stratum Malpighii or rete mucosum, stratum granulosum, stratum
lucidum, and stratum corneum. The continued growth of the epidermis
as a whole, with its appendages, the hair, nails, and the enamel of
the teeth, depends upon this function of development of the stratum
Malpighii.
Slratum 31alpighii.—Just above the papillary layer of the corium is
a layer of oval cells, each containing nuclei. These cells are constantly
undergoing proliferation, and are connected by numerous fine filaments
(imbricated cells). As the cells multiply, those which have been pre-
viously formed are pushed upward toward the surface, die, and are
cast off.
Stratum Granulosum.—Above the stratum Malpighii the cells change
their form—become more flattened and possess large and distinct nuclei.
The prot(^3lasmic contents of the cells also exhibit numerous granular
masses, and from this appearance the layer thus formed receives its
name.
Stratum Lucidum.—Above the stratum granulosum the cells flatten
out still more, become narrower and homogeneous, and sections of them
freely transmit light; hence the name.
Stratum Corneum.—Above the stratum lucidum another change in
the cells takes place. At first they appear to swell up, but soon assume
a more flattened appearance, tlie most superficial of them becoming
structurally horny scales are constantly undergoing desquamation.
The True Skin, Derm, Corium, or Cutis Vera is a tough, flexible,
elastic, highly vascular, and nervous tissue, containing lymphatic ves-
sels. It is developed from the mesoblastic or middle layer of the
blastoderm.
Structure.—The true derm is principally composed of a reticulum
of white fibrous connective tissue largely interwoven with elastic fibres.
There are also found in it many independent lymphoid cells and a
complicated network formed by an intimate association of the processes
of the connective-tissue corpuscles. Unstriated or involuntary muscular
fibres are found in the vicinity of the nipples and their alveoli, etc., and
striated or voluntary muscular fibres are to be met with in the region of
the face, head, neck, and portions of the hand.

The Papilla.
The true skin is divided into two portions, upper and lower, known
as the jxipillary and reticular (or vascular) layers. These receive their
name from their anatomical formations. The papillse of these layers
arc considered to be the organs of touch, as they are found more highly
developed at those ]X)ints wliere the sense of touch is most delicate.
They act by extending the surface for the production of cuticular
tissue, and hence are fi)und large and numerous under the nails. The
pulps of the teeth and the papillie of the hairs are developed from
them. They are conical or finger-like in shape, and are either
simple or compound, sometimes dividing near their apices into two
or more projections. They vary both in shape and size in diflerent