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COMPARATIVE

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Originally appearing in Volume V25, Page 190 of the 1911 Encyclopedia Britannica.
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COMPARATIVE ANATOMY In the larval (gastrula) stage of the Amphioxus (lancelet) cilia are present on the surface, and in the superficial epidermal cells of some fishes and amphibian larvae there is a striated layer on the free edge which is looked upon as a relic of ancestral cilia. Skin Glands.—The skin glands of the Cyclostomata (hags and lampreys) and fishes are generally unicellular and secrete slime which protects the surface of the body; the amount of slime poured cut by some of the cyclostomes is enormous. Many of these slime cells, from their shape, are spoken of as goblet cells. Some of the tele- ostean fish have poison glands at the bases of their dorsal fins and opercula. In the mud fish (Dipnoi) and amphibians multicellular spherical glands appear as involutions of the ectoderm. .4 Sometimes, as in the so-called parotids of the toad, these form large masses. Reptiles and birds are singularly wanting in skin glands, though the latter have a large uropygial gland at kj the root of the tail which secretes oil to lubricate the feathers ; it is the chief constituent of the " parson's nose " of the fowl. In mammals, except the Cetacea, the sebaceous and sudoriparous glands already described in man are found; some of the former sometimes attain a large size, as in the inter-digital gland of the sheep, Mailer's gland at the back of the 8 pig's knee and the suborbital gland of ruminants. In addi- , tion to these, special scent-producing glands are often found 'in different parts, the most remarkable of which, perhaps, are the scent glands beneath the tail of the skunk, while in male monotremes there is a special poison gland in the leg which is connected with a spur in the foot. Pigment.—Pigment cells are present both in the dermis and epidermis of fishes and amphibians, and the pigment may be either intra- or extra-cellular. In many cases it is under the control of the nervous system, so that forms like the flat-fish and the common frog can adapt their coloration to that of their 4, background. In animals permanently excluded from the light, w pigment is absent. In reptiles movable pigment cells are often found, as in the chameleon, while in birds the pigment is some-:d times of great brilliancy in the necks and wattles. In mam- mals, as in man, the pigment is confined to the cells of the o' stratum mucosum layer of the epidermis. Scales.—In yhe elasmobranch fishes scales are found composed of enamel superficially, and of dentine and bone deeply. They are developed from the epidermis and dermis, and in almost every way resemble the teeth of these animals, which are only modifications of them. The bony basal part of each scale is plate-like, hence this kind of scale is known as placoid. In the ganoid fishes, such as the sturgeon, much larger plaques called ganoid scales form a complete armature. In the teleostean fishes the scales overlap like tiles and are either cycloid, having a smooth border, or ctenoid, in which the free posterior border is serrated. Existing amphibians are usually remark-able for absence of any skin armour, though in fossil forms (Stegocephala) it was very complete. The reptilian class is specially noticeable for the production of epidermal scales, which undergo many modifications. In the Ophidia they are cast off periodically in one mass as the snake's slough, while in the Chelonia they form the different varieties of tortoise-shell. Bony structures, developed in the dermis, may underlie these epidermal horny thickenings, and are very strongly developed in the dorsal and ventral bony shields of the Chelonia (carapace and plastron), which secondarily fuse with the true endoskeleton. The armadillo is the only mammal which has a true bony exoskeleton. Feathers.—Birds are remarkable for the possession of feathers, which are highly modified scales. The embryonic or down feathers are simple, and consist of a brush of hair-like barbs springing from a basal quill or calamus. From the whole length of each barb a series of smaller barbules comes off like branches of a shrub. The adult or contour feathers are formed at the bottom of the same follicles which lodge the down feathers and, by their growth, push these out. At first they are nothing more than enlarged down feathers, but soon one of the barbs grows enormously, and forms a main shaftsor rachis to which the other barbs are attached on either side. From the sides of the barbs grow the barbules, just as in the down feathers, and these, c' I Oblique section through Papilla of hair a Pacinian corpuscle From Robert Howden, in Cunningham's Text-Book of Anatomy. and its appendages are ectodermal, and in the early embryo consist of a single layer of cells; later on this becomes double, and the superficial layer is called the epitrichium, which, after the sixth month, is cast off and mixes with the secretion of the large sebaceous glands to form the soapy vernix caseosa with which the foetus is coated at birth. In the meantime the cells of the deeper layer divide and form the various layers of the epidermis already enumerated. It is held, however, by some observers that part of the epitrichium remains as the stratum corneum. The mesodermal cells belong to the mesenchyme, and form the fibrous tissue of the true skin as well as the arrectores pilorum muscles and, in the scrotum, the dartos layer of unstriped muscle. In the sixth month fatty tissue appears in the deeper parts, and so the fat of the superficial fascia or sub-cutaneous tissue is.formed. The nails are said to appear as thickenings of the epidermis at about the ninth week, quite at the tips of the digits. Later on they shift to the dorsal side, and in doing so carry the nerves in the nail bed with them. This is toe only explanation available of the fact that the ventral nerves to the tips of the fingers encroach on the dorsal area. By about the twelfth week the nails are perfectly formed, but they do not reach the level of the finger tips until the eighth month. The hairs are developed in the third month i in the case of the large wing feathers (remiges) and tail feathers of foetal life by ingrowths of the stratum mucosum of the epidermis (rectrices), are connected by minute hooks so that the feather vane, as opposed to the shaft, has a more resistant texture than it has in the feathers of the back or breast. The bird's moult is comparable to the casting of the scales in the reptiles. Hairs.—Hairs are only found in the mammalian class, and are divided into the long tactile bristles or vibrissae and the smaller hairs which maintain the warmth of the body. In some animals the hair of the body is composed of long, stiff hairs, which are probably specialized for protective purposes, and short, soft hairs, which form the fur and keep in the warmth. Sometimes these long hairs are greatly enlarged and hardened to form protective spines as in the porcupine, hedgehog, spiny mouse and spiny ant-eater (Echidna). Horns.—Horns are of three kinds: (1) antlers, (2) hollow horns and (3) hairy horns of the rhinoceros. Antlers are growths of true bone and, except for their very vascular covering of skin (velvet), are not exoskeletal structures. They grow with great rapidity, and in the deer family are renewed each year. As soon as their growth is finished the skin covering dries up and strips off. The small horns of giraffes are also bony structures though permanent. The hollow horns of the ruminants (Bovidae) are cases of hardened epidermis which fit over a bony core and are permanent. They are found in both sexes, and in this differ from the antlers of the deer, which, except in the reindeer, are confined to the male. In the prongbuck (Antilocapral the hollow horns are shed periodically. The hairy horns of the rhinoceros are a mass of hairs cemented together by cells. The hairs grow from dermal papillae, but differ from true hairs in not being sunk into hair follicles. Claws and Hoofs.—These are modifications of nails, but whereas in nails and claws the structures are confined to the dorsal aspect of the digits, in hoofs they spread to the plantar surface as well. It has been shown in the embryological section of this article that the nail appears at the very tip of the digit, and in this position it remains in many amphibians, e.g. giant salamander, while in hoofed mammals it develops both ventrally and dorsally. In the Felidae the claws are retractile, but the real movement occurs between the middle and terminal phalanges of the digits. Spurs.—Spurs are quite distinct from nails and claws; they are very common in birds as horny epidermal sheaths covering bony outgrowths of the radial side of the carpus, metacarpus or meta-tarsus. The spur-winged goose has a carpal spur; in the screamers (Palamedea and Chauna) the spur or spurs are metacarpal, while in many gallinaceous birds (e.g. common fowls and pheasants) metatarsal spurs are found. In the mammals the male monotremes (Echidna and Ornithorhynchus) have spurs attached to an extra (? sesatpoid) bone in the hind leg, perforated for the duct of the already mentioned poison gland. Beaks.—Certain fishes belonging to the family Mormyridae have a fleshy prolongation of the lower lip, and are hence termed beaked fishes. In the Amphibia Siren and the tadpoles of most Anura (frogs and toads) have small horny beaks. In the Reptilia horny beaks are found in the Chelonia, while in birds beaks are constant and replace the teeth in modern species. In mammals a horny beak is found in Ornithorhynchus, though it coexists with true teeth in the young and with horny pads in adult specimens. In all these cases the beaks are formed from cornified epidermal scales. Baleen.—The baleen which is found in the mouths of the Balaenidae or whalebone whales is a series of flattened triangular horny plates arranged on either side of the palate. The inner edges and apices of these are frayed out into long fibres which act as strainers. In Balaena mysticetus, the Greenland whale, there are nearly four hundred of these plates, the longest of which often exceed to ft. In its development baleen resembles rhinoceros horn in that it consists of a number of epidermal hair-like fibres cemented together and growing from dermal papillae, though not from true hair follicles. For further details and literature see R. Wiedersheim, Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907) ; S. H. Reynolds, The Vertebrate Skeleton (Cambridge, 1897). (F. G. P.) ETHNOLOGY The colour of the human skin has always held an important place among physical criteria of race. Physiology explains colour as a consequence of climate and even diet. The pigment or colouring matter under the epidermis, or rather under the second or Malpighian skin, is not peculiar to the Negroid and other coloured races, but is common to all human beings. It is simply more abundant in certain peoples, and this abundance is attributed to the stimulating action of the solar heat, combined with moisture and an excess of vegetable food, yielding more carbon than can be assimilated, the character being then fixed by heredity. Theodor Waitz quotes examples proving " that hot and damp countries favour the darkening of the skin," and that the same race inclines to be darker in low marshy districts than on the hills. C. R. Lepsius asserts that the hotter the climate the darker the negro, pointing out that if you follow the line of greatest heat from Africa into Asia, it is in those regions of the latter continent that the darkest Asiatics are found. Many apparent exceptions to this general law occur, but they may be explainable as due to local causes. Thus Schweinfurth (Heart of Africa) believes that the reddish tint of the Bongos and other of the peoples inhabiting the hot, moist White Nile district is due to theferruginous nature of the laterite soil: the hue of the A-Zandeh (Niam-Niam) of the Welle valley being possibly explicable in the same way. In South America all shades of complexion intermingle. Thus in Bolivia the coppery Maropas, the dark brown Aymaras, the yellowish Moxos, and the light Mosetenos, Siriones, and Guarayos are, so to speak, neighbours. In Australasia there is the contrast between the yellow-brown Malays and the sooty-black Tasmanians. Such deviations from the colour-law may be attributed to descent (dark peoples migrating to cold, light to tropical countries), or to such varied causes as dryness, moisture, food and the vegetable peculiarities of the land, by all of which the complexion may be affected, and the influence of temperature mitigated. The colour of the human skin cannot, then, be regarded as an entirely trustworthy racial test, even blackness not being an exclusively negro characteristic. It serves, however, to divide Man into three fundamental types corresponding to the three great ethnic groups, viz. the White, the Yellow and the Black man. The first predominate in Europe, the second in Asia, while the third have their chief centres in Africa and Melanesia. Inter-breeding and, in .a lesser degree, the influence of environment have caused the occurrence within the three main groups of almost every shade and tint of complexion. Thus the colour of the skin affords a faulty basis of ethnological classification, since in the same ethnic group it varies so widely and races of one group resemble in this particular races of another. The so-called Red Indians are usually classified as a fourth group, but they are not really red-skinned. The name has come about through their custom of smearing their faces with red ochre. But among the American aborigines, side by side with the yellow, olive brown or even black (e.g. the Charruas of Uruguay), there are tribes of reddish-yellow or coppery hue. This tint is found also in certain African tribes. The palms of the hands and the soles of the feet of negroes are never black, but always yellowish, and in all coloured races the back of the body is a shade darker than the front. It is noteworthy that the skin of the coloured races is always of a lighter tint in the newly-born than in the adult; the negro baby is born a light grey colour, and the dark pigment is absent in the negro foetus. On the eighth day, sometimes as early as the third, the negro infant changes its colour to a hue nearly as dark as that of its parents. It would seem as if the blackness is associated with the general thickening of the skin and is an accompaniment of the general organic adaptation of the negro to his hot malarious climate. The effects of sunburn vary with different races. It is with the races having intermediate pigmentation, such as the dark Europeans and the yellow peoples, that the effect is most noticeable. With the former the sun burns the skin uniformly, making them of the tint of mulattoes. The colour so acquired is merely temporary. It diminishes in winter, and disappears entirely on their return to a cold temperate climate. With the Asiatics the sun causes different tints. The skins of the Indo-Chinese and the Malays become dark olive. The Fuegians and Galibis turn brick-colour or dull red. The Chinese skin turns darker in winter and paler in summer. Among certain peoples whose skins are naturally dark the parts of the body exposed to the light and air are often lighter than those covered by their clothes. This is the case with the Fuegians and the Sandwich Islanders. The fair European skin reddens under the sun, passing from pale red to brick red or to patches of deep red.
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