VERTEBRATED ANIMALS. 275 



d, d, whose ducts, e, e, pass upwards: whilst on its surface we distinguish 

 the vascular papillae, p, supplied with loops of blood-vessels from the trunk, 

 g, and a tactile papilla, t, with its nerve twig. The spaces between the 

 papillae are filled-up by the soft ' Malpighian layer/ m, of the epidermis, 

 A, in which its coloring matter is chiefly contained, whilst this is cov- 

 ered by the horny layer, h, which is traversed by the spirally twisted con- 

 tinuations of the perspiratory ducts, opening at s upon the surface, which 

 presents alternating depressions, , and elevations, b. The distribution 

 of the blood-vessels in the Skin and Mucous membranes, which is one of 

 the most interesting features in their structure, and which is intimately 

 connected with their several functions, will come under our notice here- 

 after (Figs. 479, 482, 483). In Serous membranes, on the other hand, 

 whose function is simply protective, the supply of Blood-vessels is more 

 scanty. 



671. Epidermic and Epithelial Cell-layers. The Epidermis or ' cuti- 

 cle ' covers the whole exterior of the body, as a thin semi-transparent pelli- 

 cle, which is shown by Microscopic examination to consist of a series of 

 layers of cells, that are continually wearing-off at the external surface, 

 and being renewed at the surface of the true skin; so that the 

 newest and deepest layers gradually become the oldest and most super- 

 ficial, and are at last thrown-off by slow desquamation. In their progress 

 from the internal to the external surface of the epidermis, the cells 

 undergo a series of well-marked changes. When we examine the inner- 

 most layer, we find it soft and granular; consisting of germinal corpuscles 

 in various stages of development into cells, held together by a tenacious 

 semi-fluid substance. This was formerly considered as a distinct tissue, 

 and was supposed to be the peculiar seat of the color of the skin; it 

 received the designation of Malpighian layer or rete mucosum. Passing 

 outwards, we find the cells more completely formed; at first nearly 

 spherical in shape, but becoming polygonal where they are flattened one 

 against another. As we proceed further towards the surface, we perceive 

 that the cells are gradually more and more flattened until they become 

 mere horny scales, their cavity being obliterated; their origin is indi- 

 cated, however, by the nucleus in the centre of each. This change in 

 form is accompanied by a change in the chemical composition of the 

 tissue, which seems to be due to the metamorphosis of the contents of 

 the cells into a horny substance identical with that of which hair, horn, 

 nails, hoofs, etc., are composed. Mingled with the epidermic cells, we 

 find others which secrete coloring matter instead of horn; these, which 

 are termed ' pigment-cells/ are especially to be noticed in the epidermis 

 of the Negro and other dark races, and are most distinguishable in the 

 Malpighian layer, their color appearing to fade as they pass towards the 

 surface. The most remarkable development of pigment cells in the 

 higher animals, however, is on the inner surface of the choroid coat of 

 the Eye, where they have a very regular arrangement, and form several 

 layers, known as the pigmentum nigrum. When examined separately, 

 these cells are found to have a polygonal form (Fig. 464, a), and to have 

 a distinct nucleus (b] in their interior. The black color is given by the 

 accumulation, within each cell, of a number of flat rounded or oval 

 granules, of extreme minuteness, which exhibit an active movement 

 when set free from the cell, and even whilst inclosed within it. The 

 pigment-cells are not always, however, of this simply rounded or poly- 

 gonal form; they sometimes present remarkable stellate prolongations, 

 under which form they are well seen in the skin of the Frog (Fig. 478, 



