EXCRETION 



185 



The outer surface of the dermis is not flat, but contains 

 rooundlike projections known as papillce, which project into 

 the overlying epidermis. Some of these papillae contain nerve 

 endings of the sense of touch, while others contain capillaries, 

 which are found also in other portions of the dermis. The 

 dermis is the vascular organ of the skin, blood vessels being 

 entirely absent from the epider- 

 mis (see Figs. 86, 89). 



The epidermis consists of 

 many layers of cells, the num- 

 ber of layers being very great 

 a hundred or more on the palms 

 of the hands and the soles of 

 the feet; in other places less 

 exposed to pressure or friction 

 they may not exceed twenty. 

 The deeper cells (that is, those 

 nearer the dermis) are alive and 

 in process of active growth and 

 multiplication. The outer layers, 

 which are further from the der- 

 mis with its blood supply and 

 nearer the surface with its ex- 

 posure to drying, degenerate and 

 are gradually transformed into 

 dead, flattened horny scales 

 which, packed together, form the 

 horny layer. These scales are 

 being constantly rubbed off and their loss made good by the 

 growth and multiplication of the living cells beneath. Such a 

 covering or lining is well fitted for surfaces which are exposed 

 to friction or drying, and we accordingly find that the mouth, 

 the part of the pharynx used in swallowing, the oesophagus, 

 and the rectum are lined with the same tissue. The endings 

 of nerve fibers are found in the lower layers of the epidermis. 



FIG. 84. Hair and hair follicle 



A, horny layer of epidermis. B, 

 layer of living, growing cells ex- 

 tending (B') into the hair follicle, at 

 the bottom of which it forms the 

 mass of growing cells E over the 

 papilla (P) with its knot of capil- 

 laries; the growth, multiplication, 

 and transformation of these cells 

 into horny fibers forms the shaft of 

 the hair, D. C, capillaries in the 

 dermis. S, a sebaceous gland dis- 

 charging its oily secretion (0) into 

 the follicle to lubricate the hair and 

 the horny layer of the skin 



