1042 VERTEBRATES ANIMALS 



follicles. The general appearance ordinarily presented by a thin 

 vertical section of the skin of a part furnished with numerous sensory 

 papillae, is shown in fig. 775 ; where we see in the deeper layers 

 of the cutis vera little clumps of fat-cells, /, and the sweat- 

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

 we distinguish the vascular papillae, /?, 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 colouring 

 matter is chiefly contained, whilst this is covered by the horny layer, 

 h, which is traversed by the spirally twisted continuations of the 

 perspiratory ducts, opening at s upon the surface, which presents 

 alternating depressions, a, 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 in- 

 timately connected with their several functions, will come under 

 our notice hereafter. In serous membranes, on the other hand, 

 whose function is simply protective, the supply of blood-vessels is 

 more scanty. 



Epidermic and Epithelial Cell-layers. The epidermis or ' cuticle ' 

 covers the whole exterior of the body as a thin semitransparent 

 pellicle, 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 superficial, 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 

 innermost layer, we find it soft and granu- 

 lar, consisting of nucleated cells which are 

 flatter in the upper than the lower strata, 

 which make up the layer. This was for- 

 merly considered as a distinct tissue, and 

 was supposed to be the peculiar seat of the 

 colour of the skin ; it received the desig- 

 nation of Malpighian layer or rete mucosum. 

 FIG. 776. Cells from the pig- The change in form is accompanied by a 

 mentumnigrumottheeye: change in the chemical composition of the 

 S&ttSSSffi* t^ue.whichseemstobeduetothemeta.mor- 

 nucleus. phosis of the contents of the cells into a 



horny substance identical with that of which 



hair, horn, nails, hoofs, &c. are composed. Mingled with the epi- 

 dermic cells we find others which secrete colouring 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 colour ap- 

 pearing to fade as they pass towards the surface. The most remark- 

 able 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 



