THE STRUCTURE AND FUNCTIONS OF THE SKIN. 



341 



of the more superficial layers are considerably flattened. The deeper 

 surface of the rete mucosum is accurately adapted to the papillae of the 

 true skin, being, as it were, moulded on them. It is very constant in 

 thickness in all parts of the skin. The cells of the middle layers of the 

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

 " prickle cells " (Fig. 27). The pigment of the skin, the varying quan- 

 tity of which causes the various tints observed in different individuals 

 and different races, is contained in the deeper cells of 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 deep- 





FIG. 231. 



FIG. 232. 



FIG. 231. 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; ft, 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 mem- 

 brane; g, cells in cutis vera. (Cadiat.) 



FIG. 232. -Vertical section of skin of the negro, a, a. Cutaneous papillae. 6. 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 fig. X250. (Sharpey.) 



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

 two (Fig. 231, e). Lastly, the tipper 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 towards the free surface 

 by the production of fresh ones beneath them, and become flattened from 

 pressure: they also become gradually horny by evaporation and trans- 



