STRUCTURE OF FUR
Fur is made up of two main components, the hair and the skin, and each of
these has a very complex structure.
In the living animal the skin serves as a protective covering, and also
constitutes an organ of secretion and of feeling; consequently it is
of a highly complicated nature. The skin of all fur-bearing animals is
essentially the same in structure, although varying considerably as to
thickness and texture. It consists of two principal layers, which are
entirely different in structure and purpose, and correspondingly different
in both physical and chemical respects: the epidermis, epithelium or
cuticle, which is the outer layer, and the dermis or corium, which is the
true skin. (Fig. 1A).
The epidermis is very thin as compared with the corium. Its outer layer
consists of a tissue of cells, somewhat analogous to the horny matter
of nails and hair. The inner surface, called the ‘rete malpighi,’ rests
on the true skin, and is a soft, mucous layer of cells. These cells are
spherical when first formed, but as they approach the surface become
flattened, and dry up, forming the horny outer layer of the epidermis,
which is constantly throwing off the dead scales, and which is constantly
being renewed from below. It is from this inner layer of the epidermis
that the hair, the sweat-glands, and the fat-glands are developed.
The corium, or true skin, consists essentially of white, interlacing
fibres of the kind known as connective tissue. These fibres are themselves
made up of extremely fine smaller fibres, or fibrils, cemented together by
a substance of a somewhat different nature from the fibres, the coriin.
Towards the center of the skin, the texture of the interweaving fibres
is looser, becoming much more compact at the surface just beneath the
inner layer of the epidermis. This part of the corium is so exceedingly
close that the fibrils are scarcely recognizable. It is in this part that
the fat-glands are situated, while the hair-roots and sweat-glands pass
through it into the looser texture of the corium. The surface next to the
flesh is also closer in structure than the middle portions of the skin,
and has somewhat of a membranous character due to the fibres running
almost parallel to the surface of the skin. The skin is joined to the body
proper by a network of connective tissue, frequently full of fat-cells.
This layer, together with portions of the flesh which may adhere to it,
is removed by the process called ‘fleshing,’ and this side of the skin
is known as the flesh side. The corium also contains a small proportion
of yellow fibres, known as ‘elastic fibres,’ which differ physically and
chemically from the rest of the skin substance.
During the course of the development of the embryo animal, a small group
of cells forms like a bulb on the inner side of the epidermis, above a
knot of very fine blood-vessels in the corium. This group of cells grows
downward into the true skin, and the hair-root which is formed within it,
surrounds the capillary blood-vessels, drawing nourishment from them, and
thus forming the papilla. (Fig. 1A). Smaller projections also form on the
bulb, and the fat-glands are gradually developed. The sweat-glands are
formed in a manner similar to the development of hair.
The individual hair fibre is quite as complicated in structure as the
skin, and is made up of four distinct parts. (Fig. 1B).[2]
[2] Descriptions and figures taken from “Mammal Fur Under the
Microscope,” by Dr. L. A. Hausman, in _Natural History_,
Sept.–Oct., 1920.
The medulla, or pith, is the innermost portion of the hair, and is
composed of many shrunken cells, often connected by a network which may
fill the medullary column partially or wholly.
Surrounding the medulla is the cortex, which is made up of spindle-shaped
cells fused into a horny, almost homogeneous, transparent mass, and
forming a large proportion of the hair shaft.
In the majority of the fur-bearing animals, there is distributed within
and among the cells of the cortex a pigment in the form of granules or
minute particles, arranged in the different hairs in fairly definite and
characteristic patterns. It is to these pigment granules that the color
of the hair is due primarily. In some cases the coloring matter of the
shaft is uniformly diffused and not granular.
[Illustration: FIG. 1
_A._ STRUCTURE OF SKIN.
_B._ STRUCTURE OF HAIR.]
The outermost coat of the hair, or cuticle, is composed of thin,
colorless, transparent scales of varying forms and sizes, and arranged
in series like the shingles of a roof. It is on these scales that the
lustre or gloss of the hair depends. Since lustre is due to the unbroken
reflection of light from the surface of the hair, the smoother the
surface, the glossier it will appear. When the scales of the cuticle are
irregular and uneven, the surface of the hair will not be uniform and
smooth, and the light reflected from it will be broken and scattered, and
consequently the hair will not possess a high degree of lustre. As a rule,
the stiff, straight hairs have the most regular and uniform arrangement
of the scales of the cuticle, and hence are the smoothest and glossiest.
Fur hairs are in general either circular or elliptical in cross-section,
those which are circular being straight or only slightly curved, while
those which are elliptical in cross-section are curly like the hair of
the various kinds of lambs.
Most fur-bearing animals have two different kinds of hair on their bodies.
Nearest to the skin is a coat of short, thick, soft and fine hair, usually
of a woolly nature, and called the under-hair, under-wool, or fur-hair.
Overlying the fur-hair is a protective layer of hair, longer and coarser
than the under-hair, and usually straight, hard, smooth and glossy. This
is called the top-hair, over-hair, guard-hair or protective hair. In some
furs, the top-hair constitutes one of the chief elements of their beauty,
while in others, the top-hairs are removed, so as better to display the
attractive features of the under-hair. The roots of the top-hair are
generally deeper in the skin than those of the fur-hair, and in some
instances where the top-hair is removed, as in the seal, the roots are
destroyed by the action of chemicals applied to the skin side, the roots
of the fur-hair being wholly unaffected by this treatment.
The fur-hair and the top-hair in the same animal have different
medullary and cuticular structures, and these characteristics may be
used to distinguish the two kinds of hair. Figs. 2A and B illustrate
these differences. In each case, the two large hairs on the left of the
illustration are the guard-hairs, showing respectively the cuticular
scales and the medulla. On the right are the two fur-hairs showing the
scales and the medulla.
Although composed of many different kinds of tissues, and varying so
greatly in physical structure, both the skin and the hair belong to
the same class of chemical compounds, namely the proteins. These are
highly complex substances, forming the basis of all animal and vegetable
tissues. There are many different kinds of proteins, varying somewhat in
their constitutions, but all show, on analysis the following approximate
composition of chemical elements:
Carbon 50–55%
Hydrogen 6.5–7.3%
Nitrogen 15–17.6%
Oxygen 19–24%
Sulphur 0.3–5%
The principal kinds of proteins found in the various fur structures
are albumins, keratin, collagen, and mucines. Albumins, of which the
white of egg is the most familiar variety, occurs to some extent in the
corium as serum in the blood-vessels, and also as the liquid filling
the connective tissues, known as the lymph. They are soluble in cold
water, but when heated to about 70° C., they coagulate and are then
insoluble. Concentrated mineral acids and strong alcohol will also effect
coagulation.
[Illustration: FIG. 2
_A._ HAIR OF EUROPEAN BEAVER.
_a._ TOP-HAIR. _b._ UNDER-HAIR.
_B._ HAIR OF SKUNK.
_a._ TOP-HAIR. _b._ UNDER-HAIR.]
Keratin is the chief substance of which all horny parts of the animal
body are composed, such as the hair, nails or hoofs. It is the principal
constituent of the hair, the epidermis, and the walls of the cells of
the inner layer of the epidermis, or the ‘rete malpighi.’ Keratin is
particularly rich in sulphur, and is quite insoluble in cold water.
Caustic alkalies attack keratin-containing parts.
The collagens are the principal proteins of the skin, forming largely the
substance of the connective tissue fibres, and consequently the framework
of the skin. They are insoluble in cold water, dilute acids and salt
solutions, and are only very slowly attacked by dilute alkalies. Dilute
acids and alkalies cause collagen to swell; concentrated acids, vegetable
tanning materials, basic chrome or iron salts cause it to shrink. By
boiling with water, dilute acids or dilute alkalies, collagen is split up
into gelatin or glutin.
The mucines of the skin, intercellular material or coriin, are soluble
in dilute acids, in dilute solutions of alkalies and of alkaline earths
such as lime, and in 10% salt solution, but insoluble in water, and in
salt solutions of greater or less concentration than 10%. On drying the
skin, the mucines cement the connective tissue fibres, causing the skin to
become stiff, horny and translucent. The mucines are also constituents of
the cells of the ‘rete malpighi.’ The solubility of the mucines in dilute
solutions of alkalies and of alkaline earths causes the epidermis to be
loosened from the corium, when the skins are treated with such solutions
for some time.
When raw skins are boiled with water, the greater part goes into solution,
the residue consisting chiefly of the keratins of the hair and epidermis
cells. On cooling, the solution solidifies to a jelly of gelatine. It
combines with both acids and alkalies. A property of the skin which is
of importance in the tanning operation of fur-dressing, and a quality
which also characterizes gelatine, is the capacity to absorb liquids
and swell up, without changing chemically. Raw pelts swell up easily
in pure cold water, but much more easily in solutions of dilute acids
or dilute alkalies, only a little of the skin material being dissolved.
In stronger solutions, the skins swell up less, while more of the skin
substance dissolves, and by prolonged action of strong acids or alkalies,
an almost complete solution of the skin is obtained, without, however,
any of the material decomposing. With very strong alkalies or acids,
the skin substance is broken up into simpler compounds, such as various
amines and ammonia. The swelling action of acids or of alkalies increases
with the increase in concentration of the acid or alkali, but only up
to a certain point, after which further increase in the strength of the
acid or alkaline solution causes a reduction in the swelling, and even
produces shrinkage. In the presence of neutral salts, like common table
salt, sodium chloride, the swelling action of acids, is reduced, but the
action of alkalies remains practically unaffected.
When treated with the various chemicals, fur hair acts in a manner quite
similar to wool. If it be remembered that certain classes of furs are
derived from animals of the sheep family, such furs as Persian lamb,
krimmer, etc., it becomes apparent why chemicals should affect furs in
nearly the same way as wool. The great majority of furs differ from those
of the sheep family, in possessing much greater resistance to the action
of chemicals. The range is a wide one however, and no exact criterion can
be adopted. As a general rule, the reactions are most marked with fur-hair
of a woolly nature, so this may be taken as a standard of reference.
Acids have relatively little action on the hair, when applied in dilute
solutions. The scales of the cuticle or epithelium are somewhat opened,
the fibre becoming slightly roughened thereby. Even at high temperatures,
the hair is quite resistant to the action of dilute acids. Concentrated
acids destroy the hair with the liberation or formation of ammonia,
hydrogen sulphide, and various amino acids. When treated with dilute
acids, the hair, especially if it is of a very woolly nature, retains
considerable quantities of acid, this phenomenon being probably due to
the fixation of the acid by the basic groups in the hair. Nitric acid
produces a yellow coloration when applied in dilute solution for a short
time. Sulphurous acid, the acid formed by the burning of sulphur, has a
bleaching action on the hair.
Alkalies attack the hair, even in dilute solutions, and by longer action
complete decomposition sets in, with formation of ammonia and amino-acids.
Ammonium carbonate, soap, and borax are practically harmless in their
effect on the hair. Sodium and potassium carbonates roughen the hair on
prolonged action, even in dilute solutions. Calcium hydroxide on continued
action removes sulphur from the hair, causing it to become brittle.
Salts of alkalies and alkaline earths do not affect the hair at all.
Salts of the heavy metals on the other hand, are absorbed in appreciable
quantities. From a dilute solution of alum, aluminum hydroxide is absorbed
by the hair, the potassium sulphate remaining in solution. Similarly with
copper, iron, and chromium salts, the metal oxides are fixed by the fibre.