THE MAMMALIA. 
MAxV AND BEASTS. 
89 
GENERAL REVIEW OF THE MAMMALIA CONTINUED. 
Tht Structure of Skin , Hair, Iforns^ Nails, Scales, and other inteiyumejits. 
Thk entire surface of all organized bodies is terminated by an envelope of a peculiar 
nature, varying in thickness according to the species of animal or plant, or the dif- 
ferent parts which it covers. In animals, this iiitegnment commonly receives the 
name of skin, and seems to bo esseiilially the samo in all the \ertGhrated animals, 
the external difleronces being merely owing to the development of certain additional 
parts. One of its surfaces is always intimately united to the body of the animal or 
plant ; while the other, remaining unattached, bears immediate and various relations 
to the surrounding bodies. 
In the Mammalia generally, the skin is composed of four substances, more or less 
distinct, and varying in their properties. The roost external is termed the epidermis 
or cuticle ; the second from the surface is the mucous tissue or veto viiccnsum ; the 
third is the /ja^i7/rtry or rtervous substance; and the fourfh, or innermost, forms 
the true skin, chnri.on, cutis, or dermis. These successive layers may be of greater 
or less thickness, and some of them may not always be present in the several species 
of animals belonging to this class. 
The epidermis or cuticle is the most universal of all the layers, being found on iho 
bark of trees, the stalks of herbs, the petals of flowers, the pellicle of the fruit, — as 
well as upon the entire surface of all animals. It appears to be an intermediate sub- 
stance between horn and true skin, being nothing more than a tliin membrane, form- 
ed by the hardening and drying up of the most superficial layer of the mucous tissue 
immediately beneath it, and of the albuminous fluids with which the latter is impreg- 
nated. It does not possess life in common with the other animal tissue^ being 
merely composed of a greater or loss number of inanimate layers placed one over 
the other. This cuticle is not confined to the surface of the body, but extends 
into its several apertures, protecting them, as well as all the nerves of the body, 
from a prejudicial contact with the media of air and water, to which they arc con- 
tinually exposed. The consistency of the cuticle varies with the nature of the cir- 
vumjacont fluid; thn", it is observed to be dry and almost horny in animals living 
permanently in the air, while it is viscous and mucous in the aquatic species. The 
cuticle appears to be folded in a variety of ways, among tlioso Mammalia, which re- 
main continually exposed to tlie drying influence of Iho air. Sometimes these folds 
take the form of circles, wrinkles, or spiral curves, corresponding to the elevations 
and depressions of the skin, or that part of it called the. imieims tissme. The thick- 
ness of the cuticle becomes considerable whenever a part of the body i.s exposed to a 
continuous friction ; for example, upon the solo of the foot, the palm of the hand, and 
other parts used for holding or grasping, such as the prehensile tails of some Ameri- 
can apes. The holes through which the hairs protrude may be perceived in the fur- 
rows of the cuticle. These appc.ir to be conical elongations, forced outwards by the 
hairs, to which they serve as rudimentary shearhs. 
The epidermis is very thin in Man, excepting on those parts which cover the palm 
of the Irnnd and the solo of the foot. Yet it may be considerably hardened, and even 
changed into a substance nearly approaching to horn in consistency, either by fric- 
tion, long exposure to a dry air, or to certain chemical agents, while the .sense of 
touch becomes deadened in consequence, and almost wholly obliterated. We see 
frequent instances of this in the hands of hard labourers, of blacksmiths, dyers, or in 
those nativ<is of Africa who walk barefooted upon burning sands. On the back of 
the human hand the furrows of the cuticle exhibit angular figures of various forms ; 
on the palm they assume* the appearance of pavalle! and elongated lines; while under 
the extremities of the toes they take the form of arcs of circles, curves of dilferent 
kinds, and especially some very remarkable close and syramettical spirals. 
Among the other Mammalia, the cuticle, being always thinner in proportion as the 
hairs which protect it arc more compact, is found to exhibit nearly the same appear- 
ttnee as in Man. The epidermis, covering the wings of the Bat, is very thin, and 
possesses furrows of many angles, very similar to those aeon on the back of the human 
hand. This intc^'ument is thin in »he Porcupine, and not very distinct from the 
Either strata of the skin, which, in these animals, is always gelatinous. In the 
tails of the Beaver, Bats, Ondatra, and others, the epidermis is remarkably dry and 
*<^aly, as well as upon the surface of those scales which cover the body of the Manis 
*ind the Armadillo. Wlicre the skin is very thick and deeply furrowed, as in the 
Blephant, lUiinoeeros, and Hippopotamus, tlio epidermis is likewise thick, being co- 
vered with small plates, which sink into the several furrows, and may bo separated 
scales. The soles of their feet exhibit a rem.ukahle structure in respect to the 
^ttticlc, being divided externally by deep depressions, nearly circular, with six or eight 
*urfaces more or less regular, each of wliich contains an infinite number of small poly- 
gons of great irregularity. The entire surface of the skin thus acquires the appear- 
ance of shagreen. When separated from the foot, the epidermis exhibits elevated 
^'nes upon its external surface, corresponding to the furrows of the greater polygons, 
^ well as smaller ones, corresponding to the lesser polygons. This arrangement 
Sives it the appearance of net-work t« relievo, of a pretty regular design, and resom- 
lace with large points. The Cetacea arc covered with a very smooth cpidermi.s 
'vithout any remarkable fold, and arc always moistened with a raucous oily secretion, 
'^hich prevents the surfaces of these animals from becoming macerated by the action 
of the water. 
^he rete mucosum, or mucous tissue, is situate immediately between the epidcr- 
and the villous .surface of the skin. It is not membraneous, but forms a mucous 
iayor, the colour of which varies in different species and races of animals, and some- 
in different sexes and individuals, or even in the parts of the same animal. This 
apparent colour of the surface depends upon that of the mucous tissue ; for the epi- 
^Winis when removed is almost transparent, and the cutis or true skin is also destitute 
Colour. ' 
'fhe villous or papillary surface of the skin is placed between the cutis and mucous 
t»asue, and imracdiauly beneath the latter. It docs not possess the membraneous 
structure of the epidermU, but is a surface produced By the aggregation and approxi- 
*''ation of a number cf miiiule juipilloj or small tubercles of various shapes, and lormcd 
apparenlly by the external extremities of the cutaneous nerves. The figures of these 
nerves are exceedingly various, but their structure is nearly the same. They are 
easily exhibited on being macerated in water for some days. Each tubercle may 
then be observed to consist of a bundle of minute fibres, united at their base, like the 
hairs of a pencil. The fibres of the centre are sometimes longer than those of the 
eircurafcrencc, and then the papilla assumes the form of a cono ; often they are of the 
same length, in which c.i3e it appe.ars flat. As the sense of touch resides more par- 
ticularly in these papilljr, they are accordingly found in the greatest number, and 
movt conspicuously, on the tongue, the lips, and at the extremities of the fingers. 
In Man, the p.ipilljc on the soles of the feet and the palms of the hands are particu- 
larly remarkable, as they arc placed close together in a compact manner, and distri 
buted in lines corresponding to the external grooves of the epidermis already noticed. 
The papilla; under the nails present a villous surface, tlic minute and compact fibres 
of which are all directed obliquely towards the extremity of the fingers. The 
minute fibres of the lips are diijposcd in the same manner, but are .still longer, closer, 
and more delicate. In the other Mammalia the same rule is constantly observed, 
and the papilla; are always more developed in proportion as the parts to which they 
belong are employed in touch. Thu.«, the nervous papilhi; are very visible on the 
snout of the Moles, tlie Shrew?, and the Hogs, where they form tufts consisting of 
very close fibres. They mov also be remarked on the proboscis of the Elephant, 
and very distinctly on the tail of the Cayenne Opossum, and it is probable that they 
exist in the same manner in all Mammalia with prehensile tails. Cuvier was unable 
to detect them on the skin of tho Dolphin and Porpoise. 
The cutis or true skin is situate most internally. Its structure has been deve- 
loped by anatomists in a very distinct manner, by certain modes of preparation, and 
especially through maceration in water. They liave demonstrated that it is com- 
posed of a tissue of gelatinous fibres, crossing each other in every direction, and so 
interwoven that the substance may bo compared to felt. Among these fibres may be 
observed a great number of line ramifications of nerves, as well as arterial, venous, 
and lymphatic vessels. The organization of the cutis is such, that the fibres com- 
posing it are capable of elongation ami extension in every direction, and we may 
easily perceive that these qualities were necessary to give the surfaces of animals tho 
power of evading tho mechanical action of other bodies. These properties of elonga- 
tion and extension, possessed so remarkably by the skins of the Mammalia, have on- 
ablcil manufacturers to apply luem to diffyrent purposes, whore ‘streuglh and flexi- 
bility are necessary, or where great friction has to be sustained; and the process of 
inducing these requisites constitutes the art of the currier. The fibres arc further 
approximated or separatcil, to form the leather and adapt it for different use?, and this 
again is the fuumiation of the arts of the tanner, skinner, parchment and morocco- 
maker. 'J’ho cutis in Man is from a line to a line and a half in thickne.ss in certain 
parts of the body. From maceration, as well as the process used by skinners, wo 
perceive that the fibres wliich enter into its composition are long, fine, and very 
solid, but united in a lax manner. In the Mammalia generally, the cutis is thickest 
on the dorsal, and thimiest on the ventral region. It is also much thinner on birds 
than on the Mammalia. 
The obvious intention of Nature, in providing Animals with a skin or epidermis, 
was to protect them from an injurious contact with surrounding bodies. For some 
purposes this covering is insufficient, in whicli case other appendages are added, 
differing in form and consistency, and suited to their several purposes. These integu- 
ments have received the names of Hair, Horns, Nails, Scales, &c. 
Hairs are filaments of a hnniy substance, more especially intended to cover the skin 
of the Mammalia. One extremity of each hair is implanted in the cutis, and some- 
times penetrates even as far as tlio muscular layer beneath. This extremity is en- 
larged into a bulb, more or loss thick, sometimes containing a small drop of blood, 
the whole being inclosed in a membraneous sheath. NVheu the hair is young this 
cell is large, and its size diminishes in proportion as the hair grows older. If punc- 
tured during its earlier stage, the blood flows, and it becomes soft and flaccid. 
Tliat entire portion of the Hair placed externally to the skin, is termed the Shaft. 
It forms a very elongated cone, the free extremity being the apex. The hairs grow 
from their base, and hence are finer in young Animals than in the old; for a similar 
reason, they appear to nugmeiit in number when cut, though in fact their extremities 
only are increased in diameter. When the nails rise out of the skin, they carry 
with them a small portion of the epidermis, which forms a kind of sheath aj’ound 
their base ; this becomes gradually detached, under the appearance of transparent and 
whitish scales. 
Some Animals have the hair in some parts of their bodies more or less developed 
at the time of their birth. In other parts no hair appears until a more advanced 
period of life. 
Linnffius remarks that “ Mammalia have hairs, Birds have feathers, and Fishes 
have scales.” These assertions, as w'e have already remarked, are true only in a 
genc^ral manner, for many Mammalia cither want hairs altogether, or arc furnished 
with a very small number. This fact did not escape the observation of Linnjeus 
himself, who alludes to it in atiuthcr part of his S^stenia Naturof, Some species, 
such as those of the genus Manis, arc in fact covered with true scales, and others, 
like the Cetacea, luive a naked skin. These exceptions are, however, more apparent 
than real in respect to the Manis, in which the scales are little else than compound 
h^rs. According to M. Blainville, the last remark is equally applicable to the Ce- 
tacea, where the hairs, becoming blended together, unite in forming a kind of crust 
or general envelope. This celebrated Naturalist is even of opinion that the term 
Pili/ercs, or hair-bearers, might form an advantageous substitute for that of Mam- 
miferes, an observation with which wc can by no means agree, as true hairs are 
also foimd on many Birds, so that the term Ilair-hearcrs would apply equally to 
them. 
In general, the Mammalia have two kinds of hair ; the one bristly, more or less 
sliS', and external; and the other woolly, very fine, soft to the touch, and commonly 
hidden beneath the stronger hairs. The domestic species of Sheep form a remarkable 
exception to this observation, on account of tho abundance and length of the woolly 
hairs, and at the same time of the almost total disappearance of the coarse haiis. 
