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COMPARATIVE See also:ANATOMY In the larval (gastrula) See also:stage of the See also:Amphioxus (lancelet) See also:cilia are See also:present on the See also:surface, and in the superficial epidermal cells of some fishes and amphibian larvae there is a striated layer on the See also:free edge which is looked upon as a relic of ancestral cilia. Skin Glands.—The skin glands of the See also:Cyclostomata (hags and lampreys) and fishes are generally unicellular and secrete slime which protects the surface of the See also: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 See also:goblet cells. Some of the tele- ostean See also:fish have See also: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 See also:toad, these See also:form large masses. See also:Reptiles and birds are singularly wanting in skin glands, though the latter have a large uropygial gland at kj the See also:root of the tail which secretes oil to lubricate the feathers ; it is the See also:chief constituent of the " See also:parson's See also:nose " of the See also:fowl. In mammals, except the See also:Cetacea, the sebaceous and sudoriparous glands already described in See also:man are found; some of the former sometimes attain a large See also:size, as in the inter-digital gland of the See also:sheep, Mailer's gland at the back of the 8 See also:pig's See also:knee and the suborbital gland of ruminants. In addi- , tion to these, See also:special See also:scent-producing glands are often found 'in different parts, the most remarkable of which, perhaps, are the scent glands beneath the tail of the See also:skunk, while in male monotremes there is a special poison gland in the See also:leg which is connected with a See also:spur in the See also:foot. Pigment.—Pigment cells are present both in the dermis and epidermis of fishes and amphibians, and the pigment may be either See also:intra- or extra-cellular. In many cases it is under the See also:control of the See also:nervous See also:system, so that forms like the See also:flat-fish and the See also:common See also:frog can adapt their coloration to that of their 4, background. In animals permanently excluded from the See also:light, w pigment is absent. In reptiles movable pigment cells are often found, as in the See also:chameleon, while in birds the pigment is some-:d times of See also: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 See also:enamel superficially, and of dentine and See also:bone deeply. They are See also:developed from the epidermis and dermis, and in almost every way resemble the See also:teeth of these animals, which are only modifications of them. The bony basal See also:part of each See also:scale is See also:plate-like, hence this See also:kind of scale is known as placoid. In the ganoid fishes, such as the See also:sturgeon, much larger plaques called ganoid scales form a See also:complete See also:armature. In the teleostean fishes the scales overlap like tiles and are either See also:cycloid, having a smooth border, or ctenoid, in which the free posterior border is serrated. Existing amphibians are usually remark-able for See also:absence of any skin See also:armour, though in fossil forms (Stegocephala) it was very complete. The reptilian class is specially noticeable for the See also:production of epidermal scales, which undergo many modifications. In the Ophidia they are See also:cast off periodically in one See also:mass as the snake's See also:slough, while in the Chelonia they form the different varieties of See also:tortoise-See also:shell. Bony structures, developed in the dermis, may underlie these epidermal horny thickenings, and are very strongly developed in the dorsal and ventral bony See also:shields of the Chelonia (See also:carapace and plastron), which secondarily fuse with the true endoskeleton. The See also:armadillo is the only mammal which has a true bony exoskeleton. Feathers.—Birds are remarkable for the See also:possession of feathers, which are highly modified scales. The embryonic or down feathers are See also:simple, and consist of a See also:brush of See also:hair-like barbs springing from a basal See also:quill or calamus. From the whole length of each barb a See also:series of smaller barbules comes off like branches of a See also:shrub. The adult or See also:contour feathers are formed at the bottom of the same follicles which See also: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 See also:main shaftsor rachis to which the other barbs are attached on either See also:side. From the sides of the barbs grow the barbules, just as in the down feathers, and these, c' I Oblique See also:section through Papilla of hair a Pacinian corpuscle From See also:Robert Howden, in See also:Cunningham's See also:Text-See also:Book of Anatomy. and its appendages are ectodermal, and in the See also:early embryo consist of a single layer of cells; later on this becomes See also:double, and the superficial layer is called the epitrichium, which, after the See also:sixth See also: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 See also:birth. In the meantime the cells of the deeper layer See also: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 See also: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 See also:fat of the superficial See also:fascia or sub-cutaneous tissue is.formed. The nails are said to appear as thickenings of the epidermis at about the ninth See also: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 See also:nail See also: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 See also:area. By about the twelfth week the nails are perfectly formed, but they do not reach the level of the See also:finger tips until the eighth month. The hairs are developed in the third month i in the See also:case of the large wing feathers (remiges) and tail feathers of foetal See also:life by ingrowths of the stratum mucosum of the epidermis (rectrices), are connected by See also:minute hooks so that the See also:feather See also:vane, as opposed to the See also:shaft, has a more resistant texture than it has in the feathers of the back or See also:breast. The See also:bird's See also: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 See also: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 See also:short, soft hairs, which form the See also:fur and keep in the warmth. Sometimes these long hairs are greatly enlarged and hardened to form protective spines as in the See also:porcupine, See also:hedgehog, spiny See also:mouse and spiny See also:ant-eater (See also:Echidna). Horns.—Horns are of three kinds: (1) antlers, (2) hollow horns and (3) hairy horns of the See also:rhinoceros. Antlers are growths of true bone and, except for their very vascular covering of skin (See also:velvet), are not exoskeletal structures. They grow with great rapidity, and in the See also:deer See also:family are renewed each See also: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 (See also:Bovidae) are cases of hardened epidermis which See also: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 See also:reindeer, are confined to the male. In the See also:prongbuck (Antilocapral the hollow horns are See also: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 See also:article that the nail appears at the very tip of the See also:digit, and in this position it remains in many amphibians, e.g. See also:giant See also:salamander, while in hoofed mammals it develops both ventrally and dorsally. In the Felidae the claws are retractile, but the real See also:movement occurs between the See also: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 See also:meta-See also:tarsus. The spur-winged See also: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 See also: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 See also:lower See also:lip, and are hence termed beaked fishes. In the See also:Amphibia See also: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 See also:constant and replace the teeth in See also:modern See also:species. In mammals a horny See also:beak is found in Ornithorhynchus, though it coexists with true teeth in the See also: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 See also:whalebone whales is a series of flattened triangular horny plates arranged on either side of the See also:palate. The inner edges and apices of these are frayed out into long See also:fibres which See also:act as strainers. In Balaena mysticetus, the See also:Greenland See also:whale, there are nearly four See also:hundred of these plates, the longest of which often exceed to ft. In its development baleen resembles rhinoceros See also: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. See also:Parker (See also:London, 1907) ; S. H. See also:Reynolds, The Vertebrate See also:Skeleton (See also:Cambridge, 1897). (F. G. P.) See also:ETHNOLOGY The See also:colour of the human skin has always held an important See also:place among See also:physical criteria of See also:race. See also:Physiology explains colour as a consequence of See also:climate and even See also:diet. The pigment or colouring See also:matter under the epidermis, or rather under the second or Malpighian skin, is not See also:peculiar to the See also: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 See also:action of the See also:solar See also:heat, combined with moisture and an excess of See also:vegetable See also:food, yielding more See also:carbon than can be assimilated, the See also:character being then fixed by See also:heredity. Theodor See also:Waitz quotes examples proving " that hot and See also:damp countries favour the darkening of the skin," and that the same race inclines to be darker in See also:low marshy districts than on the hills. C. R. See also:Lepsius asserts that the hotter the climate the darker the See also:negro, pointing out that if you follow the See also:line of greatest heat from See also:Africa into See also:Asia, it is in those regions of the latter See also:continent that the darkest Asiatics are found. Many apparent exceptions to this See also:general See also:law occur, but they may be explainable as due to See also:local causes. Thus See also:Schweinfurth (See also:Heart of Africa) believes that the reddish tint of the Bongos and other of the peoples inhabiting the hot, moist See also: The so-called Red See also:Indians are usually classified as a See also:fourth group, but they are not really red-skinned. The name has come about through their See also:custom of smearing their faces with red ochre. But among the See also:American See also:aborigines, side by side with the yellow, See also:olive brown or even black (e.g. the Charruas of See also:Uruguay), there are tribes of reddish-yellow or coppery hue. This tint is found also in certain See also: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-See also:born than in the adult; the negro baby is born a light See also:grey colour, and the dark pigment is absent in the negro foetus. On the eighth See also:day, sometimes as early as the third, the negro See also: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 See also:accompaniment of the general organic See also: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 See also:sun See also:burns the skin uniformly, making them of the tint of mulattoes. The colour so acquired is merely temporary. It diminishes in See also: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-See also:Chinese and the Malays become dark olive. The Fuegians and Galibis turn See also: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 See also:air are often lighter than those covered by their clothes. This is the case with the Fuegians and the See also:Sandwich Islanders. The See also:fair See also:European skin reddens under the sun, passing from See also:pale red to brick red or to patches of deep red. Additional information and CommentsThere are no comments yet for this article.
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