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 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

influences. Still more strikingly is this shown by the blackness of the Melanian aborigines of New Guinea, Australia, and Tasmania, retained from the sixth to the forty-third degree of south latitude; and especially of those of the outlying islands in proximity with others inhabited by the olive-brown Polynesians, whose complexion prevails from lat. 12° S. to 46° S. (New Zealand). But the most instructive example of the closer relationship of tint to race than to climate is afforded by the aborigines of the New World, which hold nearly the same depth of copper-brown or reddish tint, latitudinally from Tierra del Fuego to Hudson's Bay, and longitudinally from the Atlantic to the Pacific. The contrast between the South American Indians and the African Negroes would seem to be decisive against the hypothesis of degrees of solar influence being the causes of degrees of darkness of complexion.

But there is an element in the problem which ought to be taken into consideration, viz. time.' If Africa be an older continent than South America, its aborigines may have been subjected to solar influences through a longer series of generations. We know not the extent of such series; some may deem that were the intertropical South American Indians subject to a vertical sun during the long ages of Africa's emersion, they would acquire a darker complexion.

Climate, however, depends on other influences than sunshine. Degrees of moisture, and whatever influences cause a contrast or gradation of seasons, &c., may have their effects upon complexion. Filthy habits, foul air, and bad food, affecting biliary and other secretions, have their share in darkening the skins or sallowing the complexions of the Esquimaux, Fins, and Laps, e.g. as compared with the cleanlier and more healthily living and better nourished Scandinavians residing some degrees further from the pole. But assuming, as the general result of the above survey of human complexions, that such complexions do, in the main, show a certain dependent relationship on solar light and heat, and postulating the effect of long periods of such subjection, we might then be led to conclude the darkest of the intertropical and warm temperate peoples to be the oldest; that the Melanians, scattered on islands to the east of the Indian Ocean, inhabit relics of a continent as old as, perhaps older than, Africa; and that the lighter-tinted races on intercalated or contiguous portions of dry land are subsequent immigrations or derivatives from lands less affected by solar influences. On this hypothesis it may be inferred that the deepest-tinted races

existing in the islands of the Malayan Archipelago are the oldest inhabitants of such-those most entitled to be termed aborigines. The Hindoos, by the same pigmental test, would be deemed older than the Parsee or Mahometan natives of Hindostan, as history, indeed, testifies. In extra-tropical latitudes, human generations may have succeeded each other for the same duration of time as in tropical ones, without further deepening or development of pigment than such diminishing influence of the sun may effect. Such peoples, migrating to tropical countries, may long maintain their inherited complexions; just as the black races migrating to extra-tropical latitudes long retain the tint inherited from forefathers in whom it has been established primarily by the requisite continuance of exposure to extreme solar heat and light.

§ 359. Callosities.-The epiderm, besides forming the firm and more or less insensible outer protection of the derm, acquires unusual thickness at certain parts in different mammals. It forms callosities over the sternum of the Camel and Dromedary, and upon the parts of the joints (carpal and rotular) on which these useful beasts of burden kneel. It defends the broad back of the penultimate phalanges of the fingers of the knuckle-walking Apes, the ischial tuberosities of most lower Catarhines, and the prehensile surface of the tail in many Platyrhines.

In the Equide there are callosities on the inner surface of the limbs, which, however, are more dermal than epidermal. In the Horse, on the inner side of the fore-leg, a little above the carpus (fore-knee' Hippotomy), and on the inner side of the hind-leg, a little below the tarsus' (hock-joint, Hippotomy), is a naked protuberance of a soft horny consistence, about the size of a chestnut, and called 'châtaigne' by the French veterinarians. Behind the metacarpo-phalangeal joint is a similar but smaller horny tubercle, called the 'ergot,' or spur. The Ass has not the châtaigne' on the hind-leg; but there is the vestige of one on the fore-leg, situated there as in the Horse; it consists of a patch of black skin devoid of hair, but not horny. There is a similar trace of the spur (ergot) behind the metacarpo- and metatarso-phalangeal joints. The Zebra resembles the Ass in these respects: the homologue of the fore-leg callosity is a patch of black naked skin about 3 inches long and 3 inches broad; the callosities behind the metacarpo-tarso-phalangeal joints are like those of the Ass.

§ 360. Hair.-The cutaneous clothing characteristic of the Mammalian class is hair.' It consists of unbranched filaments of epidermal material, usually composed of 'pith' and 'crust,'

and in which are distinguished the root,' the 'stem,' and the 'point.'

487

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The root is softer and lighter in colour than the stem,' is contained in a canal of the skin or sheath, fig. 487, e, and expands at the implanted end into the knob.' This part during the growth of the hair has a conical cavity inclosing the bulb,' ib. f, which forms the pith;' from its base there is reflected upon the knob' a capsular layer of cells which forms the crust;' this layer is continued to near the outlet of the sheath; it consists of two or more layers of cells, the outermost of which have generally lost their nuclei.' The proper tunic of the sheath is 'derm,' lined by epiderm continuous with the cuticle, which accordingly, when shed, usually brings away the hairs. In the dermic part there is a vascular and a hyaline layer; the latter ceasing with the capsular part of the hair's matrix.

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Two sebaceous glands, ib. h, usually open into the hair-sheath; and one or more delicate muscles, ib. g, of unstriped fibre, pass from the harder superficies of the derm to be inserted. into the capsule beneath the glands; these are mainly concerned in raising the hairs.

Hairs, like teeth, are of two kinds as regards growth; one temporary, the other persistent. The former are shed and succeeded by new hair, usually once a year; the latter have persistent bulbs and perennial growth. The body-hair of the Horse is an example of the first kind, the hair of the mane and tail of the second kind. In many Mammals there are two kinds of hair, according to form, length, and structure; one short, fine, more or less curled, and mostly hidden by the longer, coarser, and straighter kind, which is sometimes called the external coat, albeit the roots sink deeper into the derm than do those of the internal coat, usually called 'fur.'

These two kinds of hair-inner and outer-are most distinctly as well as abundantly shown in arctic and aquatic quadrupeds, (ermine, sable, beaver, and the seal-tribe), especially in the young state, when the heat-forming power is weak. In some species of

The contrast is striking in the hair of the Ornithorhynchus, in which the brown tint is confined to the expanded terminal part of the hair.