308 EXCRETION 



abundant in the skin, has already been indicated in the general descrip- 

 tion of the lymphatic system. 



The Epidermis and its Appendages. The epidermis, or external 

 layer of the skin, is composed of cells. It has neither bloodvessels, 

 nerves nor lymphatics. Its external surface is marked with shallow 

 grooves, which correspond to the deep furrows between the papillae of 

 the derma. Its internal surface is applied directly to the papillary layer 

 of the true skin and follows closely all its inequalities. This portion of the 

 skin is subdivided into two layers. The internal layer is called the rete 

 mucosum, or the Malpighian layer, and the external is called the horny 

 layer. These layers present certain important distinctive characters. 



The Malpighian layer is composed of a stratum of prismoidal nu- 

 cleated cells, containing pigmentary matter, and a number of layers 

 of rounded cells without pigment. The upper layers of cells are semi- 

 transparent and nearly colorless ; and it is the pigmentary layer chiefly 

 that gives to the skin its characteristic color. All the epidermic cells are 

 somewhat colored in the dark races, but the upper layers contain no 

 pigmentary granules. The thickness of the rete mucosum is ^y 1 ^ to y 1 ^ 

 of an inch (15 to 333 /*). 



The horny layer is composed of strata of hard flattened cells, irregu- 

 larly polygonal in shape and usually without nuclei. The deeper cells 

 are thicker and more rounded than those of the superficial layers, are 

 polygonal in form and their borders present a great number of short 

 protoplasmic spines. These are called prickle-cells. There is constantly 

 more or less desquamation of the epidermis, particularly of the horny 

 layer, and the cells are regenerated from the subjacent parts. 



Physiological Anatomy of the Nails. The nails are situated on the 

 dorsal surfaces of the distal phalanges of the fingers and toes. They 

 serve to protect these parts, and in the fingers, they are quite important 

 in prehension. The general appearance of the nails is sufficiently 

 familiar. In their description anatomists have distinguished a root, a 

 body and a free border. 



The root of the nail is thin and soft, terminating in rather a jagged 

 edge, which is turned slightly upward and is received into a fold of the 

 skin extending around the nail to its free edge. The length of the root 

 varied with the size of the nail, but it usually is one-fourth to one-third 

 of the length of the body. 



The body of the nail extends from the fold of skin covering the root 

 to the free border. This portion of the nail, with the root, is closely 

 adherent by its under surface to the true skin. It is marked by fine but 

 distinct longitudinal striae and very faint transverse lines. It usually 

 presents a reddish color on account of the great vascularity of the 



