CHAPTER XXXV.

TEGUMENTARY SYSTEM AND APPENDAGES OF MAMMALIA.

§ 357. Derm. The main constituent of the skin of Mammals consists of an interlacement of fibres of the white or sclerous kind, fig. 486, f, continuous with those of the subjacent areolar tissue, i, but more or less abruptly defining a firm sheet of strong and tough fasciculate framework investing the body: the looser central or initial texture, i, includes, in its larger meshes, fat, sweat-glands, h, bulbs of hair, of bristles, or of spines, with sebaceous follicles, according to the species: it is traversed by the nerves of the sensitive or tactile papillæ, d, by sweat-ducts and by arteries, veins, and absorbents: it is covered by the epiderm, c, a. With the sclerous fibres of the derm are blended a varying proportion of the yellow elastic fibres, and of unstriped muscular tissue, especially in relation to the roots of the hairs or spines.

The texture of the derm is firmest at its periphery, where its surface is best defined: its thickness varies in relation to the bulk of the species and to other circumstances; it is such, e.g., in certain Perissodactyles and the Hippopotamus, as to have suggested the name of Pachyderm' for an artificial group of Ungulates in the Cuvierian system. In the full-grown Giraffe the corium. hardly exceeds half an inch in thickness at any part: in the Indian Rhinoceros, of about the same weight, the average thickness of the derm is between two and three inches: it is thinner on the less exposed surfaces and at the bending of the joints. In the large specimen which I dissected the integument on the middle line of the abdomen presented a general thickness of threefourths of an inch on the inner side of the extremities, it was about one-fourth of an inch in thickness. It was connected to the abdominal parietes by a loose cellular tissue, and by a closer one to most of the other parts of the body; but the parts to which the stiff and ponderous hide most firmly adhered were the spinous processes of the posterior lumbar and sacral vertebræ, and the anterior extremities of the iliac bones, at which places the corium was blended with the periosteum, and was thin. The derm adhered over the jugal bones to a kind of movable

fibro-cartilage; but its attachment along the median line of the fore part of the head was so firm as to require, especially beneath the horn, the use of a chisel in order to separate it from the skull. Besides its attachment to subcutaneous cellular substance, fasciæ, elastic tissue, fibro-cartilages, and periosteum, the derm is connected with parts which are destined for its motions and adjustment upon the body. The panniculus carnosus' of the Rhinoceros is developed in certain parts to an extraordinary thickness; the permanent folds in the hide of the Indian species serving to afford, like the processes of bone, a firmer insertion to the aponeuroses of the cutaneous muscles than a plane surface of integument could have done. A sheet of these muscles situated on each side of the thoracic or scapular region sends its fascia into the interstice of the fold in front of the anterior extremities, the skin being bent upon itself, as it were, to grasp this fascia. Similar portions of panniculus carnosus send their aponeuroses into the posterior folds of the skin.

The derm, in Cetacea, is a somewhat gradual condensation of the close fibrous reticulation in the areola of which the oil is contained; the thickness of such subcutaneous tissue, called 'blubber,' being enormous in the large Whales: it is wanting at the fins, and here the derm is closely connected with the sclerous tissue. covering the bony framework of the pectorals, and contributing mainly to form the dorsal and caudal fins: in the latter the subdermal fibres become assorted into three layers, the upper and under ones being longitudinal, the intermediate layer transverse, and the texture of the whole so compact that the traversing veins as well as the arteries preserve their open state when cut across. The fine lengthened papillæ or villi from the periphery of the derm are noticed at p. 188.

Certain Rodentia contrast with the Pachyderms in the thinness and lacerability of their derm, resembling birds in that respect. Another Lissencephalous family reproduces a crocodilian character, in the development of osseous scutes upon the peripheral part of the derm (vol. ii. p. 396, fig. 261). These scutes are small, mostly quadrilateral, and suturally united so as to form three principal groups: one protecting the trunk like an arched roof, a second forming a flatter shield or helm upon the head, the third encasing the tail, like a sheath. In most existing Armadillos the trunk-armour consists of an anterior thoracic buckler in which the ossicles form a kind of mosaic work; a middle annular' part in which they are disposed in transverse series movable upon each other; and a posterior lumbar buckler, like the thoracic

one: by this modification of the dermal plates the little animal is enabled to roll itself into a ball, and protect its snout and legs beneath the trunk-armour. In the large extinct Armadillos (Glyptodon) the annular or banded modification of the armour was not present; and the whole of the dermal scutes of the trunk were united into one massive domed roof: the marginal scutes being generally triangular, the rest more or less regularly hexagonal. The inner surface of the scutes, imbedded in the derm, is smooth; the outer surface coated with epiderm is sculptured in a definite pattern, distinct for each species and characteristic thereof. The dermal plates of the caudal sheath in certain kinds of Glyptodon formed annular series of large conical tubercles: the first ring, in all, was distinct from the rest of the caudal sheath, to facilitate the movements of the tail.

In the Pangolins (Manis) the exterior of the derm is grooved, as in Lizards, for the lodgment of the bases of the large horny scales, which protect the body and tail by their imbricated overlapping arrangement (vol. ii. fig. 158). The muscular tissue enters in greater proportion than usual into the composition of the derm of this Mammal, in connection with the thick panniculus carnosus,' and in relation to the erection of the scales, when the integument is drawn defensively about the uprolled trunk and tail.

Productions or duplications of the derm, with included muscles, &c., form the peculiar mammalian oral appendages called lips: an everted fold of skin forms the scrotum: an inverted fold the marsupium and the cheek-pouch (p. 386, fig. 300): the derm is extended between the digits to form the web' in most aquatic quadrupeds, and also beyond the digits to augment the swimming surface in the Ornithorhynchus: a duplicature of integument forms the dewlap' in certain Bovines: it forms a broad fold on each side, continued from the fore to the hind limbs to form the parachute, in the Petaurists, Pteromyds (Vol. ii. fig. 156), and Flying Dormice (Anomalurus): in the Colugos (Galeopithecus) the dermal fold expands from the nape to the fore-feet, from these to the hind-feet, and thence to the tip of the tail, forming a triangular interfemoral' flap. But the most extraordinary developments of derm are presented by the Bat tribe (vol. ii. p. 278, fig. 156): the antibrachial fold' crosses the deep interspace between the humerus and radius; the digital fold,' which mainly forms the wing, occupies the interspaces of the long and attenuated digits; the flank-folds' extend from the fifth digit to the tarsus; the interfemoral fold' passes from leg to leg, intercepting more or less of the tail.

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The wing-membrane is sometimes further developed, so as to be disposed at one part in the form of a pouch, as in the genus of Bat thence called Saccopteryx, in which the pouch is plicated, and its linear orifice is near the head of the humerus. The delicate organisation of these modifications of the derm has been noted at p. 189; and, as regards its vascular structure, at pp. 549 and 553. The conchal or auricular productions of the derm are considerable in all Bats: the two outer ears are confluent, or united by a transverse fold of skin, crossing the forehead, in Nycteris and Megaderma; in these and many other genera, e.g. Rhinopoma, Rhinolophus, Phyllostoma, the nose, also, is furnished with a crest or with foliaceous lamellæ.

486

The sudoriferous or sweat-glands, fig. 486, i, consist of a fine secerning tubule, coiled up into a ball, and situated at the under surface of the derm or in the subcutaneous tissue, h: the duct traverses the derm, at first in a wavy course, g, becoming straighter in the denser peripheral part, and spiral as it passes through the epiderm, b, to terminate at the sweat-pore. The sebaceous glands relate chiefly to the hairs, and mostly open into the hairsheath or follicle, fig. 487, h. The movements

C

d

of the derm are due either to intrinsic or extrinsic muscles: the former, ib. g, which are smooth as a rule, produce the shrinking called 'cutis anserina,' on account of the protrusion of the hair-sheaths, and the depression of the intervening part of the skin; the extrinsic muscles, which have striped fibres, move more or less of the integument, as when cattle after a shower, or a dog quitting the water, shake off the moisture, or when a fly or other irritant is sought to be dislodged.

§ 358. Epiderm.-Upon the papillose surface of the derm, in the embryo, albuminoid atoms in the solution exuding therefrom formify as cells, and between the outermost of these, condensed and dried by exposure after birth, and the derm, formifaction continues, throughout life, to produce a precipitate of cells. These, at first, are perpendicular to the derm, in one or more strata; then, as they are pushed off by newly formed cells, they assume a more rounded shape, lose their soft granular contents, afterwards their

The derm, f, so magnified, is considerably thicker than here represented.

firmer nuclei, and, finally, become pressed into dry hard scales at the periphery of the epiderm. Many of the deepest-seated and first-formed cells contain coloured particles or pigment, constituting the rete mucosum,' or 'malpighian layer,' fig. 484, d. This pigment, combined with the cells constituting the hairs or spines, gives the characteristic colour of the quadruped, and seems to affect the derm itself. It rarely manifests, in Mammals, the bright and pure colours noticed in the skin of Birds (p. 231, vol. ii.); but to the face of certain baboons it may give a red, blue, or violet tint. In quadrupeds with circumscribed patches of black hair a deposition of dark pigmentum may be traced in the corium above the sheaths whence the black hairs grow. The darker-coloured skin and hair is, as a rule in Mammals, on the upper or more exposed surface of the body, and the lighter-coloured pelt is below. But in the Ratel and Skunk the ordinary arrangement of colours is reversed, the back being light and the belly dark: the white bands of hair in the Skunk are associated with a corresponding colour of the corium, and are seen on the inner side of the dried pelt. In the human subject the amount and colour of the subcuticular pigmental cells relate, but not absolutely as regards existing continents and peoples, to the degree of solar influence to which the skin is exposed. A fair complexion and light hair do not characterise any race indigenous to tropical and warmer temperate latitudes, but are limited to cooler temperate and cold climes, which, from the present excess of dry land in that hemisphere, are northern or arctic. The continent of Europe, if the complexions of its peoples be compared from Scandinavia to the Mediterranean, exemplifies the progressive deepening of the tints of skin, hair, and eyes, as the sun exerts more power. But the Asiatic part of the Old World' shows this relation in a minor degree. The aborigines of Northern Asia to Kamtschatka are, like the Japanese, of a brownish-yellow complexion: the same prevails through all the latitudes of the vast Chinese Empire; but the southern extensions of that people into Cochin-China, Siam, and Burma, do show a deeper brown. The Hindoos retain the same almost black tint over a range of twenty-six degrees of latitude and twenty-four degrees of longitude; but these are tropical, or nearly so. The Malays of the Indian Archipelago preserve the same deep brown tint over eighteen degrees of latitude, reckoned from the equator northward, and the tint would seem still to relate to such excess of solar influence; although the sway of other causes is exemplified by the darker Mincopies, Cingalese, and Hindoos, under similar