THE MAMMALIA— MAN AND BEASTS. 



89 



GENERAL REVIEW OF THE MAMMALIA CONTINUED. 



The Structure of Skin, Jlair, Horns, Naihf ScaleSy and other integuments^ 



The 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 integument commonly receives the 

 name of skm, and seems to be essentially the same in all the Vertebrated animals, 

 the external differences 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 most external is termed the epidermis 

 or cuticle ; the second from the surface is the mucous tissue or rete mucosum ; the 

 third is the papillary or nervous substance ; and the fourth, or innermost, forms 

 the true skin, chorion, c7Uis, 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 the 

 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- 

 Ertance between horn and true skin, being nothing more than a thin 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 tissues, being 

 merely composed of a greater or less 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 are con- 

 tinually exposed. The consistency of the cuticle varies with the nature of the cir- 

 cumjacent fluid ; thus, it is observed to be dry and almost horny in animals living 

 termanently 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 those Mammalia, which re- 

 main continually exposed to the drying influence of the air. Sometimes these folds 

 tal':e the form of circles, wrinkles, or spiral curves, corresponding to the elevations 

 and depressions of the skin, or that part of it called the mucous tissue. The thick- 

 ness of the cuticle becomes considerable whenever a part of the body is exposed to a 

 continuous friction ; for example, upon the sole 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 appear to be conical elongations, forced outwards by tlie 

 hairs, to which they serve as rudimentary sheaths. 



The epidermis is very thin in Man, excepting on those parts which cover the palm 

 of the hand and the sole 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 sec 

 frequent instances of this in the hands of hard labourers, of blacksmiths, dyers, or in 

 those natives 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 parallel and elongated lines ; while under 

 the extremities of the toes they take the form of arcs of circles, curves of different 

 kinds, and especially some very remarkable close and symmetrical spirals. 



Among the other Mammaha, the cuticle, being always thinner in proportion as the 

 hairs which protect it ai*e more compact, is found to exhibit nearly the same appear- 

 ance as in Man. The epidermis, covering the wings of the Bat, is very thin, and 

 possesses furrows of many angles, very similar to those seen on the back of the human 

 hand. This integument is thin in the Porcupine, and not very distinct from the 

 other strata of the skin, which, in these animals, is always gelatinous. In the 

 tails of the Beaver, Rats, Ondatra, and others, the epidermis is remarkably dry and 

 scaly, as well as upon the surface of those scales which cover the body of the Manis 

 and the Armadillo. Where the skin is very thick and deeply furrowed, as in the 

 Elephant, Rhinoceros, and Hippopotamus, the epidermis is likewise thick, being co- 

 vered with small plates, which sink into the several furrows, and may be separated 

 like scales. The soles of their feet exhibit a remarkable structure in respect to the 

 cuticle, being divided externally by deep depressions, nearly circular, with six or eight 

 surfaces more or less regular, each of which 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 

 lines upon its external surface, corresponding to the furrows of the greater polygons, 

 as well as smaller ones, corresponding to the lesser polygons. This arrangement 

 gives it the appearance of net-work in reUevOj of a pretty regular design, and resem- 

 bling lace with large points. The Cetacea are covered with a very smooth epidermis 

 without ?.ny remarkable fold, and are always moistened with a mucous oily secretion, 

 which prevents the surfaces of these animals from becoming macerated by the action 

 of the water. 



The rete mucosum, or mucous tissue, is situate immediately between the epider- 

 mis and the villous surface of the skin. It is not membraneous, but forms a mucous 

 layer, the colour of which varies in diflferent species and races of animals, and some- 

 times 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- 

 dermis when removed is almost transparent, and the cutis or true skin is also destitute 

 of colour. 



The villous or papillary surface of the skin is placed between the cutis and mucous 

 tissue, and immediately beneath the latter. It does not possess the membraneous 

 structure of the epidermis, but is a surface produced by the aggregation and approxi- 

 mation of a number of minute papilla; or small tubercles of various shapes, and formed 



apparently 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 

 circumference, and then the papilla assumes the form of a cono ; often they are of the 

 same length, in which case it appears flat. As the sense of touch resides more par- 

 ticularly in these papillae, they are accordingly found in the greatest number, and 

 most conspicuously, on the tongue, the lips, and at the extremities of the fingers. 

 In Man, the papilla; on the soles of the feet and the palms of the hands are particu- 

 larly remarkable, as they are placed close together in a compact manner, and distri 

 buted in lines corresponding to the external grooves of the epidermis already noticed. 

 The papillae under the nails present a villous surface, the minute and compact fibres 

 of which are all directed obliquely towards the extremity of the fingers. The 

 minute fibres of the lips are disposed in the same manner, but are still longer, closer, 

 and more delicate. In the other Mammalia the same rule is constantly observed, 

 and the papillae are always more developed in proportion as the parts to which they 

 belong are employed in touch. Thus, the nervous papillae are very visible on the 

 snout of the Moles, the Shrews, and the Hogs, where they form tufts consisting of 

 very close fibres. They may 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 the Dolphin and Porpoise. 



The cutis or true skin is situate most internally. Its structure has bc;en deve- 

 loped by anatomists in a very distinct manner, by certain modes of preparation, and 

 especially through maceration in water. They have 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 be compared to felt. Among these fibres may be 

 observed a great number of fine 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 and extension in every direction, and we may 

 easily perceive that these qualities were necessary to give the surfaces of animals the 

 power of evading the mechanical action of other bodies. These properties of elonga- 

 tion and extension, possessed so remarkably by the skins of the IMammalia, have en- 

 abled. manufacturers to apply them to different purposes, where strength and flexi- 

 bility arc necessary, or where great friction has to be sustained; and the process of 

 ■ inducing these requisites constitutes the art of the currier. The fibres are further 

 approximated or separated, to form the leather and adapt it for different uses, and this 

 again is the foundation of the arts of the tanner, skinner, parchment and moroeco- 

 maker. The cutis in Man is from a line to a line and a half in thickness in certain 

 parts of the body. From maceration, as well as the process used by skinners, wo 

 perceive that the fibres which enter into its comi)osition are long, fine, and very 

 solid, but united in a lax manner. In the Mammalia generally, the cutis is thickest 

 on the dorsal, and thinnest 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 which 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 horny 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 the muscular layer beneath. This extremity is en- 

 larged into a bulb, more or less thick, sometimes containing a small drop of blood, 

 the whole being inclosed in a membraneous sheath. When 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. 



That 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 o-row 

 from their base, and hence are finer in young Animals than in the old; for a similar 

 reason, they appear to augment 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 around 

 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 hfe. 



Linneeus remarks that " Mammalia have hairs. Birds have feathers, and Fishes 

 have scales.' These assertions, as we have already remarked, are true only in a 

 general manner, for many Mammalia either want hairs altogether, or are furnished 

 with a very small number. This fact did not escape the observation of Linnaeus 

 himself, who alludes to it in another part of his Systema Naturce. Some species, 

 such as those of the genus Manis, are in fact covered with true scales, and others, 

 like the Cetacea, have 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 

 hairs. According to I\I. Blalnville, the last remark is equally applicable to the Ce- 

 tacea, where the hairs, becoming blended together, unite in farming a kind of crust 

 or general envelope. This celebrated Naturalist is even of opinion that the teim 

 Pilijeres, or hair-bearers, might form an advantageous substitute for that of Ma?})- 

 inifcres, an observation with which we can by no means agree, as true hairs are 

 also found on many Birds, so that the term Hair-bearers would apply equally to 

 them. 



In general, the Mammalia have two kinds of hair; the one bristly, more or le=is 

 stiff, 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 

 cxceptiiin to this observation, on account of the abundance and length of the woolly 

 hairs, and at the same time of the almost total disappearance of the coarse hairs. 



