. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. n fact,composed of cells detached from the surface of the the loss is periodical, when it is termed , the mammals cast their hair, and the deer their horns,the birds their feathers, the serpents their skins, the crabstheir test, the caterpillars their outer envelop, with all thehairs growing from it. 268. The skin presents su
. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. n fact,composed of cells detached from the surface of the the loss is periodical, when it is termed , the mammals cast their hair, and the deer their horns,the birds their feathers, the serpents their skins, the crabstheir test, the caterpillars their outer envelop, with all thehairs growing from it. 268. The skin presents such a variety of structure in thedifferent groups of animals as to furnish excellent distinctivecharacters of species, genera, and even families, as willhereafter be shown. In the vertebrates we may recognizeseveral distinct layers, of unequal thickness, as may be seenin figure 94, which represents a magnified section of thehuman skin, traversed by the sudoriferous canals. Thelower and thickest layer, (a,) is the cutis^ or true skin, andis the part which is tanned into leather. Its surface presentsnumerous papillae, in which the nerves of general sensationterminate; they also contain a fine network of bloodvessels, OF THE SECRETIONS. 129. usually termed the vascular layer. Thesuperficial layer (c) is the epidermis^or cuticle. The cells of which it is com-posed are distinct at its inner portion,but become dried and flattened as theyare pushed outwards. It is supplied withneither vessels nor nerves, and, conse-quently, is insensible. Between thesetwo layers, and more especially con-nected with the cuticle, is the rele muco-sum^ (Z»,) a very thin layer of cells, someof which contain the pigment whichgives the complexion to the differentraces of men and animals. The scalesof reptiles, the nails and claws ofmammals, and the solid coverings of the Crustacea, aremerely modifications of the epidermis. On the other hand,the feathers of birds and the scales of fishes arise from thevascu
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectp, booksubjectzoology
