ORGANS OF SENSE. 55 



epithelial cells, of which the deeper are prismatic and 

 regular. As they are nearer the surface, these cells are 

 more and more flattened, and, instead of being thick 

 and soft, as in the interior, they are thin and horny. 

 The most superficial are like little scales, and they gradu- 

 ally become detached and fall off, but are at the same 

 time continually replaced by new layers of cells pro- 

 duced from the deeper ones. This desquamation of the 

 epidermis takes place continually and imperceptibly in 

 man, but in many animals it occurs at the same time 

 over the whole surface, so that at certain intervals the 

 body seems to emerge from a thin covering. 



The coloring matter or pigment is in the deep layers of 

 the epidermis. The pigment consists of a large number 

 of microscopic granules of a brown color, which give to 

 the skin a darker or lighter shade. 



The derm, which is just under the epiderm, is com- 

 posed of strong fibres, crossing and interlacing in every 

 direction. Little conical eminences may be seen pro- 

 jecting from the surface of the derm, and in these papil- 

 lae, as they are called, are the terminations of the nerves 

 of touch. The projections of the papillae are the cause 

 of the regularly- arranged series of elevations that are 

 separated by little furrows on the surface of the skin. 

 Their number is large in all parts, but becomes enormous 

 in positions where the sense of touch is most acute. 



The sudoriferous glands are contained in the subcuta- 

 neous cellular tissue ; their excreting ducts pass through 

 the derm and open obliquely on the surface of the epiderm 

 in the furrows between the papilla. The perspiration 

 is an acid liquid, having a chemical composition that is 

 complicated as well as very variable. The sebaceous 

 glands are in the thickness of the derm ; they are 



