Malpighii. In the white race, dark pigment is usually pres-
ent in greatest quantity in the areolse of the nipples and in
the scrotum and labise.
General structure. The skin is composed of the follow^
ing parts : epidermis, corium, subcutaneous connective tissue,
-.-.*
e
FIG. 114. Diagrammatic perpendicular section through the normal skin : a, epidermis ; b, rete Mal-
pighii; c, papillary layer; d, corium; e, panniculus adiposus; /, spirally bent end of excretory sweat-
dnct ; (7, straight portion of excretory duct of sweat-gland ; A, coil of sweat-duct ; ', hair-shaft ; *, root
of hair ; /, sebaceous gland. After Neumann.
blood-vessels, nerves, lymphatics, sweat and sebaceous glands,
hairs, and nails.
A perpendicular section through the skin shows (Fig. 114)
three well-marked layers ; the most superficial is called the
epidermis proper, a, b ; the middle layer is the corium or cutis,
d ; and the deepest layer the subcutaneous connective tissue, e.
The limit of the epidermis at its place of union with the corium
is sharply defined, but the corium and subcutaneous connec-
THE SKIN". 271
live tissue gradually merge into eacli other, the boundary be-
tween them being only an artificial one.
Commencing with the epidermis, we will describe in detail
the minute structure of the different tissues and organs of the
skin, omitting only the lymphatics.
Description of the different layers. The epidermis is
generally subdivided into several layers, with specially distinc-
tive names for each layer ; but though such a division has
some practical value, histologically it
is incorrect, as the cells of the lowest
layer are transformed, at some period
of their existence, in their movement
toward the free surface, into the cells
of the other layers. Examination with
high powers also shows that the chan-
ges in the molecular constitution or
chemical condition of the cells of the
epidermis changes which produce
differences in their appearance are
quite gradual. Consequently, sharply defined layers are not
found. For practical reasons, however, it is well to adopt the
usual classification. In Fig. 115 these layers are shown.
Another division is into Malpighian and corneous layers
only, the former comprising the rete and the granular layer,
and the latter the stratum lucidum and corneous layer. The
Malpighian layer, as compared with the corneous layer, pre-
sents a more or less dark, granular appearance, while the latter
is homogeneous, and its cells have a lamellar arrangement.
The rete MalpigMi consists of nucleated corpuscles, rich in
protoplasm, granular in appearance, and disposed more or less
in parallel strata, the elements of the different layers differing
somewhat from each other as regards their size and shape. The
lowest layer consists of columnar-shaped cells arranged pali-
sade-like, with their long axes more or less perpendicular to the
surface of the corium. Where the papillae are well developed,
this perpendicular arrangement is not so marked. The base of
some of these bodies terminates in a pointed extremity, which
passes a short distance into the underlying corium. Each of
them has an oval nucleus. The cell-body consists of a small
quantity of slightly granular, shining protoplasm. The cor-
puscles of this layer are not united to each other by bands, as
272 MANUAL OF HISTOLOGY.
in the other layers. The next two or three strata consist of
more or less polygonal-shaped bodies, each with a spherical
nucleus. The cells of these layers are large, their contours
sharply defined, and they contain more or less pigment. It is
this substance deposited in the corpuscles that gives the charac-
teristic color to the different races of mankind. Their cell-bod-
ies are larger in proportion to the nucleus than in the first layer.
In the succeeding laj^ers the cells increase in size and are more
granular in appearance, the cells and nuclei become flatter as
they approach the granular layer, and, finally, lie with their long
axes parallel to the surface. The granular structure which in
the lowest layer is most marked around the nucleus, gradu-
ally extends toward the margin of the cells, as the surface is
approached, so that finally a clear area is seen around the
nucleus, whilst the remainder of the cell-body is markedly
granular. At the same time the cell-body becomes firmer and
the nucleus smaller.
All the cells of the rete Malpighii, except those of the first
row, are united to each other by filaments (Martin, Bizzozero,
Heitzmann), the so-called prickles of Max
Schultze (Fig. 116). These uniting filaments
or bands vary much as regards their size
and length in different parts of the bod}'.
They are most distinct wherever the Mal-
pighian layer is well developed, but are
thicker and longer in the lower rows of
ceiis cells than in the upper. At the stratum
of the rete. x 1600. lucidum they cease to exist. Between
neighboring corpuscles the length of these bands is in direct
proportion to the distance between the borders of the cell-
bodies. Hence, where three or four cells meet at one place,
as in the centre of Fig. 116, the minute filaments are much
longer than those uniting the bodies of closely adjoining cells.
Examining these prickle-cells with the microscope, alternate
dark and light bands are seen between the cell-borders. With
a low power, these light bands appear to consist of spaces be-
tween the connecting filaments, the dark lines being the con-
necting filaments, but with a high power the latter can be
recognized as spaces between the former. The light bands
can be traced from the surface of one cell to the surface of
another, whilst the dark lines are the spaces between these
THE SKIN. 273
bands. These connecting cords sometimes divide and anas-
tomose with each other, forming a sort of network between the
cells. In this case, the dark spaces do not always extend
from one cell-body to another, since they may correspond to
the space between anastomosing filaments. These bands are
therefore not the prickles of adjoining cells, which interlock
with each other, but are true connecting filaments between
cells of a common origin, and which iiave not yet become sepa-
rated from each other. The connecting bands or fibres gradu-
ally diminish in length and thickness from below upward, and
finally cease to exist when the granular layer is reached.
The spaces between the bands are filled with an inter-
cellular albuminous substance, and they may be regarded as
minute channels for the conveyance of nutriment to the cells
of the epidermis. The above view of the " prickles" corre-
sponds very closely with that held by Dr. Martin, and differs
from that of later observers, who maintain that the dark lines
are connecting bands, and the light lines the spaces between
them.
Owing to the close union of the Malpighian elements it is
very difficult to isolate them. Perhaps the best way to accom-
plish this result is by long immersion in iodized serum. Fig.
117 represents a cell isolated in this manner. Here
the bands have been torn apart and the cell-surface is
studded with thorn-like projections. Hardening in
chromic acid, with subsequent boiling in a moderately
strong solution of potash, causes a separation of the
mucous layer from the corium and a falling apart of the rete
cells (Biesiadecki). The structure of the corpuscles, however,
can be best studied when their normal relations with each
other are preserved. Yariations in the number of cellular lay-
ers are of normal occurrence in the rete, although this portion
of the skin shows the least variation as regards its thickness.
The arrangement of the elements in these different strata is the
same in all parts of the body, and appears to be independent
of the thickness of this layer.
As regards the direction of the long axes of the cells there
is a gradual passing from the perpendicularly seated cells of
the first layer to the horizontally lying cells of the uppermost
>w. The lower surface of the rete adapts itself to the upper
mrface of the corium, and between the papillae projects down-
274
MANUAL OF HISTOLOGY.
PIG. 118. Horizontal section of skin through a
papilla. The migrating cells are observed as dark
bands between the epithelial cells and amongst the
connective tissue of the papilla. Pagenstecher.
ward and forms the interpapillary rete Malpigliii. Wandering
lymphoid cells are frequently present in the rete. They are
especially numerous in some pathological conditions. They
(Fig. 118) are elongated spindle-shaped bodies lying between
the rete cells, and sending out minute processes. They color
deeply in carmine, have a
small nucleus, and are most
numerous in the lower part
of the rete mucosum.
The granular layer (Fig.
115, b) consists of one or two
strata of flattened, granular-
looking bodies, which, in
perpendicular section appear
spindle - shaped, with their
long diameter parallel to the
free surface of the epidermis.
In this stratum the cells are
no longer connected with each
other by bands, as in the pre-
ceding layer. The nuclei of
these corpuscles are very distinct, and flattened in the same
direction as the cell-body. The latter has a very coarsely gran-
ular appearance, which is most marked near the nucleus, and
gradually diminishes in degree as the periphery of the cell is
approached. The structure of these bodies is best shown with
hsematoxylon.
The stratum lucidum, also called the stratum of Oehl, is
composed of at least three layers (Fig. 115, c). It presents a
clear, homogeneous, or striated appearance. Within the flat-
tened cells composing it, a staff-shaped nucleus is found. The
cells of this layer are formed from those of the granular stra-
tum. In their movement to the free surface the latter become
less granular and the inter-granular substance grows more trans-
parent and shining (Unna). This change from a granular to a
homogeneous translucent appearance commences around the
nucleus, whence it gradually extends to the periphery of the
cell. The nucleus, also, usually becomes invisible.
In vertical section the corneous layer appears (Fig. 115, d)
to be composed of wavy fibres and horny, transparent cells
of various sizes and shapes. This variation in bulk and form
THE SKIN. 275
depends in great measure upon the thickness of the layer.
The nearer we approach to the stratum lucidum, the more dis-
tinct are the cells. If the layer is very thin the cells appear
as elongated, flat, or curved bodies, giving to this part of the
epidermis a fibrous appearance. When the corneous stratum
is thick these cells present various forms and sizes. The cor-
puscles of the lower layers color slightly in carmine, are poly-
gonal or spindle-shaped, and frequently contain a shrivelled
nucleus. As the surface is approached they grow flatter and
drier, are more bent upon themselves, and color less and less in
carmine. The nucleus also becomes invisible. The most su-
perficial layers are composed of elongated, flat, dried-up cells,
the so-called epidermic scales. These bodies are best studied
after they have been subjected to the action of liquor potassse,
which ca-uses them to swell up.
The corpuscles of the stratum corneum are arranged in lay-
ers as in the other parts of the epidermis, but the elements
forming a layer are more closely united with each other than
with those of the adjoining layers. Hence this stratum can be
separated into lamellae, as occurs in some pathological states
of the skin. It accompanies, for example, the formation of
some vesicles, where the exuded liquid, prevented from pass-
ing toward the surface, accumulates between the layers, and
thus separates them from each other.
The corneous layer participates in the elevations and de-
pressions of the underlying layers. This causes the undulat-
ing or wavy appearance of the lamellae, as observed in sections
where the papillae are well developed. It varies greatly in
thickness in different parts of the body, and reaches its great-
est development on the palms of the hands and soles of the feet.
Its thickness does not depend upon the rete Malpighii, as it
sometimes forms a thin layer where the rete is thick, and vice
versa.
The subcutaneous connective-tissue layer of the skin con-
sists principally of connective-tissue bundles, which, coming
from the underlying fasciae of the muscles or from the peri-
osteum, pass in an oblique direction to the corium. These
fasciculi are generally cylindrical in form, and variable in size ;
by their anastomoses or divisions they form larger or smaller
networks, with correspondingly large or small interfascicular
spaces. Generally large bundles anastomose with each other
276 MANUAL OF HISTOLOGY.
in this layer, and hence a loose connective tissue is formed.
Within this layer adipose tissue is found in greater or less
quantity. The 'fat-cells are collected into masses or lobules, the
number of cells which form a lobule varying greatly in num-
ber. Each of these latter may be regarded as a fat-gland, as it
is provided with an afferent artery, a capillary plexus between
the corpuscles, and one or more efferent veins. Several lobules
are sometimes united together in the form of an acinous-like
gland, and are likewise seen to be surrounded by a general
sheath of connective tissue. The individual fat-cells are round,
flattened, polyhedral, or oval-shaped, the form depending upon
the degree and direction of the pressure exerted upon them.
Owing to the amount of fat-tissue so often found in this layer,
it has been called the panniculus adiposus. Such fat-lobules
are absent in the penis, scrotum, labise minorse, eyelids, and
pinna. The corresponding spaces in these regions are tra-
versed by fine connective- tissue bands or single fibrils. From
this adipose tissue fat-columns pass upward in a somewhat
oblique direction to the bases of the hair-follicles, especially
to those of the fine hairs. Their long axes form a slight angle
with the axes of the follicles, and they are nearly parallel to
the erector pili muscles (Warren). In cases of starvation, in
the so-called wasting diseases, and in all acute diseases at-
tended with excessive loss of tissue, the fat-cells disappear to
a greater or less extent. The skin, in such instances, becomes
correspondingly flaccid and wrinkled. Adipose tissue gives to
the skin its tension and fulness, and to the body its appear-
ance of roundness or plumpness. Obesity consists in an exces-
sive production of fat-cells.
The interfascicular spaces differ in size in proportion to the
amount of lymph present, and to the closeness of the anasto-
moses between the bundles. In oedema the lymph-spaces are
increased in size proportionately to the increased amount of
liquid present. The interfascicular spaces all communicate
with each other, as is shown by the rapidity with which a
hypodermically injected liquid can be dispersed by manipu-
lation.
The connective-tissue cells of this layer and of the corium
consist of branched cells (Ravogli) which surround the white
fibrous bundles and send in processes between the fibres. Ac-
cording to some observers, these cells are epithelioid in charac-
THE SKIN. 277
ter. The elastic-tissue fibres are developed from the processes
of the branched cells.
Besides connective-tissue fibres and cells, lympJioid corpus-
cles are present in this layer. They exist in greatest number
near the blood-vessels and glands. In this situation they are
of a roundish form, but in the parts distant from the blood-
vessels they are more or less spindle-shaped, and are to be
regarded as wandering cells.
The convoluted part of the sweat-glands and the lower part
of the hair-follicles of deep-seated hairs lie in this layer.
Blood-vessels, lymphatics, and nerves are present. The
blood-vessels are large, and after giving off small branches to
the hair- follicles, sweat-glands, and fat-lobules, pass upward
to the corium.
Pacinian corpuscles are found in connection with some of
the nerves. For a description of these bodies the reader is re-
ferred to the article on the nerves.
The principal part of the corium consists of white fibrous and
elastic tissue, the latter increasing in amount with advancing
age. Here the white fibrous tissue forms a much denser, firmer
structure than in the previous layer. It consists of deep
oblique, and superficial horizontal bundles. The latter com-
prise fine bundles of connective tissue which run parallel with
the surface of the skin, and by their division and anastomoses
form a very fine network with small interfascicular spaces.
From this layer bundles pass upward into the papillae, and
these form a second denser network. The deeper layer is
formed by a continuation upward of the subcutaneous con-
nective-tissue bundles. These pass upward in an oblique direc-
tion, and as they reach the corium divide into fasciculi. Here
they continue to divide and anastomose with each other and
with fibres from the horizontally running bundles. The anas-
tomoses are very close ; hence, the corium is formed of a dense
network of connective tissue, except in those parts which are
traversed by blood-vessels, lymphatic vessels, nerves, hair-folli-
cles, and sebaceous and sweat glands. Immediately around
the hair-follicles, sweat-ducts, and sebaceous glands the con-
nective tissue is dense, and the fibres run parallel with the di;
rection of the organs. Owing to the greater size of the connec-
tive-tissue bundles in the lower part of the corium, and the
consequent looseness of the network formed by their anasto-
278 MANUAL OF HISTOLOGY.
moses, this part of the corium lias been called the pars reticu-
laris corn, in contradistinction from the finer network formed
in the upper part, to which the name pars papillarls has been
applied. But neither between these two parts nor between the
subcutaneous layer and the corium is there any sharp dividing
line, the transition being a gradual one.
As already mentioned, the size of the interfascicular spaces
depends upon the closeness of the anastomosis between the
bundles and fibres. The direction of the bundles corresponds
with that taken by the blood-vessels.
The connective-tissue corpuscles of the corium resemble
those found in the subcutaneous layer, and also bear the same
relation to its connective-tissue bundles. From the upper
portion of the corium fibres pass upward to make the papillae.
The form of the papillae is very variable in different parts of
the body. Where they are most developed, as on the inner
surface of the terminal phalanges of the fingers and toes, they
are conical in shape. In some other regions they form only
slight elevations on the corium, giving a wave-like appearance
to its upper surface. They consist of a close network of white,
fibrous connective tissue combined, especially in the central
part of the papilla, with a large number of elastic fibres. Those
papillae which contain tactile corpuscles are called nerve-pa-
pillae.
The corium is separated from the stratum mucosum by a
thin, transparent basement-membrane, containing oval nuclei.
Its under surface is not sharply defined, and from it prolonga-
tions pass upward between the cylindrical cells of the rete,
giving this surface a notched appearance similar to that ob-
served on the inner margin of the internal sheath of the hair-
follicle.
Elastic fibres are present in large numbers in the corium,
especially in its upper part, where they form a network around
and between the white fibrous tissue-bundles. In the lower
part of the corium they form a large network, which becomes
finer as the surface is approached. The number of elastic
fibres increases with advancing years. With this increase of
elastic fibres there is a corresponding decrease of the white
fibrous connective-tissue cells (Ravogli). Numerous wander-
ing cells are met with in the corium, especially in the vicinity
of the blood-vessels and glands. Hair-follicles, sebaceous
THE SKIN. 279
glands, sweat-ducts, nerves, lymphatic vessels, and non-striated
muscles are also present in this layer. For a fuller descrip-
tion of the intimate structure of the connective-tissue bundles
and cells, see the subject of connective tissues.
Blood-vessels. Only the corium and subcutaneous tissue
are provided with blood-vessels. The arterial blood-vessels
supplying the skin form two parallel horizontal layers, a su-
perficial and a deep one. The deep layer lies in the subcuta-
neous tissue, and consists of large vessels running parallel to
the general surface. From this horizontally lying deep layer,
branches are distributed to the sweat-glands and fat-follicles of
this region. The principal branches, however, pass perpendicu-
larly or obliquely upward through the corium to its upper part,
and form immediately beneath the papillae (after free branch-
ing and anastomosis) a superficial horizontal layer, the stratum
subpapillare. From the vessels ascending
through the corium branches are given off to
the hair-follicles, sebaceous glands, and gen-
eral tissue of the corium. From the stratum
subpapillare small branches pass upward into
the papillse, where they become capillary ves-
sels, which proceed to the summit of the
papilla. (See Fig. 119.) Before reaching this point, however,
they frequently divide into two or more branches. Frequently,
those papillse in which tactile corpuscles are seated have no
blood-vessels.
The veins are arranged on the same plan as the arteries :
they form a superficial and a deep layer, and have their origin
in the papillae. From the superficial layer larger vessels pass
downward, receiving blood from the veins of the hair-follicles,
sebaceous glands, and the general tissue of the corium, thus
forming a deep subcutaneous layer or venous network.
Nerves. Medullated and non-medullated nerve-fibres are
present in the skin. They are found in combination in the nerve-
trunks of the subcutaneous tissue, the medullated fibres being
most numerous in those regions of the skin where the Pacinian
and tactile corpuscles are most abundant. In the subcuta-
neous connective-tissue region, and in the lower part of the
corium, some nerve-fibres leave the nerve-trunks and pass to
the glands, blood-vessels, and Pacinian corpuscles found in this
region. In the corium some of the fibres lose their medullary
280 MANUAL OF HISTOLOGY.
sheath, and afterward continue their course as non-medullated
fibres. The nerve-bundles pass upward in a more or less oblique
direction from the subcutaneous connective tissue through the
corium to the subpapillary network of blood-vessels, around
which they form a plexus. From this subpapillary plexus
medullated fibres run upward and pass into the tactile cor-
puscles.
The non-medullated nerve-fibres form a reticulum around
the blood-vessels of the pars reticularis corii and the capilla-
ries of the papillae. They consist of thick or fine, smooth,
varicose fibres with numerous nuclei. These fibres proceed
from the network around the subpapillary blood-vessels up-
ward toward the rete Malpighii, and either pass directly into
the rete or run for a short distance parallel to its under sur-
i'ace, and then finally enter that layer. Within the epider-
mis the fibres run between the cells and terminate in a manner
not yet definitely known. Their mode of division and termina-
tion within the epidermis is probably similar to that occurring
in the cornea. Within the papillae the nerve-fibres frequently
divide before entering the rete.
The manner of distribution and termination of the non-
medullated nerve-fibres can only be studied successfully in tis-
sue stained with gold chloride. The tissue must be fresh, and
a weak solution of the gold chloride used. When sufficiently
stained the tissue is placed in -distilled water slightly acidu-
lated with acetic acid and exposed to the light.
The Pacinian corpuscles are found in greatest abundance in
the skin of the fingers, toes, palm of the hand, sole of the foot,
but also occasionally in other regions of the skin. Their struc-
ture is described in the article on the nervous system.
Tactile corpuscles. As already mentioned, some of the
medullated nerve-fibres forming the plexus surrounding the
subpapillary blood-vessels, pass upward and enter the so-called
tactile corpuscles. These corpuscles are generally seated in the
papillae, but occasionally they are found in the subpapillary
region, i.e., the upper part of the corium. The majority of
the papillae containing such corpuscles have no blood-vessels.