THE SKIN
Podiatry deals largely with ailments involving the skin or its
appendages and it is deemed advisable to describe briefly the anatomy
and physiology of that organ, so as to refresh the memories of those
who study this work.
It is not the intention of the authors to enter deeply into this
subject and the reader is referred to the works in this series
which deal exclusively with anatomy and physiology, for a more
intimate knowledge of the skin. It is an accepted fact that no one
can intelligently comprehend pathology without knowing the normal
structure and functions of the tissues of the body to be considered,
and it is for that reason that the pages to follow have been written.
[Illustration: 1. Stratum Corneum; 2. Stratum Lucidum; 3. Stratum
Granulosum; 4. Stratum Mucosum; 5. Papillary Layer; 6. Reticular Layer;
7. Subcutaneous Tissue; 8. Hairs; 9. Fat Cells; 10. Sudoriferous
Glands; 11. Sebaceous Glands; 12. Pacinian Body or Touch Corpuscle; 13.
Connective Tissue Cells; 14. Nerve Endings 15. Vascular Papillæ and
Capillary Loops]
ANATOMY OF THE SKIN
The skin as a whole is composed of two distinct layers resting upon a
third structure, the subcutaneous tissue. The outer portion is called
the epidermis, cuticle or scarf skin, and is without blood and nerve
supply, while the inner portion is called the corium, derma or cutis
vera, and contains the capillary loops and nerve endings.
THE EPIDERMIS
=The Epidermis= is divided into four layers, named from without inward,
the stratum corneum, the stratum lucidum, the stratum granulosum and
the stratum mucosum or rete Malpighii.
=The Stratum Corneum=, or horny layer of the skin, is composed of many
layers of horny, non-nucleated scales which are being continuously
displaced by exposure to weather, water, etc., and are being as
continuously renewed by the deeper layers. This layer of the skin is
involved in the simpler foot lesions such as heloma and callositas.
=The Stratum Lucidum=, or clear layer of the skin, is composed of a few
rows of transparent cells, without distinct boundary, and, except on
the palms and soles, is considered a part of the stratum corneum. It is
composed of from two to four layers of cells, which are like the cells
of the horny layer except that they are brighter and more homogeneous.
This layer is not often clearly defined and is of no importance.
=The Stratum Granulosum=, or granular layer of the skin is composed
of several rows of polygonal shaped cells which are well marked on
the soles of the feet. The nuclei of the cells are not well defined
and the cell itself refracts light. The granules found in this layer
are varied in shape and contain a fluid called eleidin and a peculiar
solid substance called keratohyalin. This substance is derived from
the cytoplasm of the cells and represents the first process in the
cornification of the cells in the outer layers of the epidermis.
=The Stratum Mucosum=, or mucous layer of the skin, usually called the
rete, or rete Malpighii, is the deepest and most important layer of the
epidermis. The basal layers of cells are separated from the corium by a
basement membrane or membrana propria, and these layers, which are made
up of columnar cells, contain the pigment of the skin. The next few
layers show elongated, oval or rounded shaped cells, the form varying
with the locality, the tendency being to a rounded shape, owing to the
more even pressure on the cells from above and below. The cells are
irregularly formed and are made up of a soft substance with large oval
or rounded nuclei. In the intercellular spaces is found a fluid which
is nutrient in character. In the stratum mucosum are found the prickle
cells. These cells have hairlike processes on them which serve to hold
the cells together.
THE DERMA
=The Derma, or Corium=, is divided into two layers, the outer, called
the papillary layer, or pars papillaris, and the inner, called the
reticular layer, or pars reticularis. It is composed of bundles of
fibrous tissue, yellow elastic tissue and connective tissue cells, the
reticular layer being more compact than the papillary layer. The derma
contains blood vessels, nerves, lymphatics, touch corpuscles, hairs,
sweat glands and sebaceous glands.
=The Papillary Layer= of the skin is composed of small conical
elevations called papillæ, which blend with the prolongations of the
rete above. The best developed papillæ are found on the under or flexor
surfaces of the fingers and toes and attain their greatest length at
this point. They are placed in double rows that underlie the cutaneous
ridges on the fingers and toes. These cutaneous ridges remain unchanged
throughout life and are so characteristic of each individual, that they
are used as a means of detecting and identifying criminals and others.
Papillæ of two kinds are noticed, the one being very well supplied with
blood vessels, and are called vascular, the others being only scantily
supplied with blood, containing medullated nerves, and are called
sensory papillæ.
=The Reticular Layer= of the derma is composed of loosely arranged
bundles of connective tissue which merge with the papillary layer
without a distinct line of demarcation. In these bundles of connective
tissue are found the sweat glands, the sebaceous glands, the hair
follicles and the deeper lymphatics. This layer of the derma is made up
of fasciculi of connective tissue which blend into each other obliquely
and give it a plexiform appearance. As the bundles ascend towards
the surface they divide into smaller and finer bundles, and when the
papillary layer is reached, they have a close, felt-like appearance.
=The Subcutaneous Areolar Tissue=, or tela subcutanea, connects the
skin with the deeper structures and should be considered a part of the
true skin. It is made up of loosely arranged bundles of connective
tissue which cross each other repeatedly and form well defined spaces.
These spaces contain fat, and where there are large quantities of this
fat, as on the soles of the feet, the tissue is designated as adipose.
The subcutaneous areolar tissue also contains the deeper hair follicles
and the deeper sweat glands.
=Blood Supply.= The layers of the epidermis are without vascular
supply, but the derma and the subcutaneous tissue are well supplied
with blood vessels. There are two plexuses, one superficial in the
upper layer of the derma, and the other deep, in the subcutaneous
tissue. The vessels of the upper layer arise from the deeper plexus and
give off branches in all directions supplying the hair follicles, sweat
and sebaceous glands. The papillary layer is richly supplied with
delicate capillaries, which terminate in the papillæ, and are called
capillary loops.
=Lymphatics.= The lymphatics follow the vessels in a general way, there
being two plexuses, viz.: deep and superficial. Lymph spaces are found
in the rete Malpighii, which connect with the channels of those in the
derma. The papillæ and the glands also have lymph channels.
=Nerve Supply.= The skin contains both medullated and non-medullated
nerve fibres; these fibres are especially abundant in the soles of the
feet and at the ends of the toes. They enter the skin with the more
important ascending blood vessels. The non-medullated nerves terminate
in the rete as fine filaments, and the medullated nerves end in the
corium and subcutaneous tissue in special terminals called corpuscles.
Examples of these are Pacinian corpuscles, tactile corpuscles and the
end bulbs of Krause.
In addition to the sensory nerves, the skin also contains vasomotor
nerves. These nerves are found on the smooth muscles of the skin and
on all glands having such muscles, and have a direct action on these
glands.
=Muscles.= Both striated and non-striated or smooth muscles are found
in the skin. Those of the latter variety are most common, while the
former are sparingly found. The smooth muscle fibres are found in
connection with the hair follicles, the sebaceous and the sudoriferous
glands, and they act upon these organisms.
=Sudoriferous Glands.= The sudoriferous glands, or sweat glands, are
found in the reticular layer of the corium and in the subcutaneous
tissue. They are simple tubular glands which are coiled into globular
shape. The tubule of the gland empties into a gland duct which passes
through the corium and the epidermis and opens on the surface of the
skin in a funnel-shaped sweat pore. The sweat glands are very numerous,
particularly on the soles of the feet. It is estimated that there are
2,000,000 sweat glands in the adult human body.
=Sebaceous Glands.= The sebaceous glands, or oil glands, are found in
the reticular layer of the derma, usually associated with or in close
proximity to a hair follicle. They may occur independent of the hairs
however, as is the case in the lips. They vary in size from a simple
pouch to a many pouched or multilobular gland. These pouches empty into
a common duct, which in turn empties between the hair and the inner
sheath. The ducts secrete sebum, which consists of fatty degenerated
cells, in which is found epithelial waste matter. The sebum keeps the
skin and the hair soft and oily.
NAILS
=The Nails= are a specialized form of epidermis, and are considered
by many to correspond to the stratum lucidum of that structure. They
are horny, elastic, transparent, quadrilateral plates, and are found
at the distal ends of the fingers and toes, on their dorsal surfaces.
The nails are convex on the outer surface and concave within. The
nail itself is called the body and rests upon the nail bed. It has a
free edge distally and two lateral and a proximal or short edge which
latter lie in a groove called the nail or ungual fold. The ungual wall
overlies the lateral and proximal portions. The nail is embedded into
the derma at its proximal end by a root. This part of the nail is found
beneath the ungual wall and is composed of cells which have not yet
become horny.
The thin layer of skin, which extends forward from the nail groove at
the beginning of the body of the nail, is called the eponychium or nail
skin. The lunula is the little whitish, crescentic spot, a portion of
the nail bed, which is found in front of the nail fold, and extends to
the lateral edges of the nail.
The matrix of the nail is situated beneath the root of the nail,
and is so-called because it is from this structure that the nail is
produced. The matrix is thick, and raised in a series of longitudinal
ridges, which are readily seen through the transparent nail tissue. It
corresponds to the mucous layer of the epidermis, and is essentially
of the same structure. The matrix is highly vascular, which accounts
for the pink color seen through the nail, except at the lunula.
PHYSIOLOGY OF THE SKIN
The functions of the skin may be subdivided as follows:
Touch Organ
Protective Covering
Excretory and Secretory Organ
Temperature Regulator
Organ of Respiration
The skin acts as a touch organ or as an organ of tactile sensibility;
this power is supplied by special bodies found in the papillæ. The
degrees of consistency, of size, of form and of other qualities are
recognized by this function. Other sensations are conveyed by these
special nerve endings, such as heat and cold, burning, itching,
tingling, etc. The sense of touch is well developed, particularly in
the skin at the ends of the fingers, and this sense may be farther
increased, as is the case with blind persons.
The skin acts as a protective organ to the body within, by excluding
harmful agents such as bacteria, chemicals, heat, cold, etc. It is
elastic and thick and is without sensation and thus protects the
delicate structures beneath it from injury from various causes.
The functions of excretion and of secretion are performed by the
glands. The sudoriferous, or sweat glands, excrete the perspiration,
and in this way also act as elimination organs, accessory to the
kidneys. The body is continuously sweating. When there is no indication
of this function, when the skin seems dry, the name “insensible
perspiration” is applied; when the function is apparent, by the
formation of drops of moisture on the surface, it is called “sensible
perspiration.”
The sebaceous glands are organs of secretion. They give off an oily
substance called sebum, which lubricates the hairs, and gives an oily,
soft appearance to the skin. This tends to keep the outer layers
elastic and pliable; where this function is absent, the skin becomes
dry and is likely to form cracks or fissures.
The skin acts as a regulator of the body heat, by controlling the
radiation of the heat as brought to the surface from within, and
by regulating evaporation. The normal tension of the skin on the
various parts of the body has an influence in the regulation of body
temperature.
The function of respiration is, to some extent, duplicated by the skin,
the process being analogous to the respiration that takes place in the
lungs. The amount of oxygen absorbed is small, but water and carbon
dioxide are freely given off.