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HAND-BOOK OF PHYSIOLOGY. 



stratum Malpighii are almost all connected by processes, and thus form 

 "prickle cells" (p. 21). The pigment of the skin, the varying quantity 

 of which causes the various tints observed in different individuals and 

 different races, is contained in the deeper cells of the rete mucosum; the 

 pigmented cells as they approach the free surface gradually losing their 

 color. Epidermis maintains its thickness in spite of the constant wear 

 and tear to which it is subjected. The columnar cells of the deepest 

 layer of the "rete mucosum" elongate, and their nuclei divide into two 



FIG. 225. Vertical section of the epidermis of the prepuce, a, stratum corneum, of very few 

 layers, the stratum lucidum and stratum granulosum not being distinctly represented: 6, c, d, and e, 

 the layers of the stratum Malpighii, a certain number of the cells in layers d and e showing signs of 

 segmentation; layer c consists chiefly of prickle or ridge and furrow cells; /, basement membrane; 

 <7, cells in cutis vera. (Cadiat.) 



FIG. 226. Vertical section of skin of the negro, a, a. Cutaneous papillae, b. Undermost and 

 dark-colored layer of oblong vertical epidermis-cells, c. Stratum Malpighii. d. Superficial layers, 

 including stratum corneum, stratum lucidum, and stratum granulosum, the last two not differen- 

 tiated in figure. X 250. (Sharpey.) 



(Fig. 225, e). Lastly the upper part of the cell divides from the lower; 

 thus from a long columnar cell are produced a polyhedral and a short 

 columnar cell: the latter elongates and the process is repeated. The 

 polyhedral cells thus formed are pushed up toward the free surface by the 

 production of fresh ones beneath them, and become flattened from pres- 

 sure: they also become gradually horny by evaporation and transforma- 

 tion of their protoplasm into keratin, till at last by rubbing they are 

 detached as dry horny scales at the free surface. There is thus a con- 

 stant production of fresh cells in the deeper layers, and a constant throw- 

 ing off of old ones from the free surface. When these two processes are 

 accurately balanced, the epidermis maintains its thickness. When, by 



