342 HANDBOOK OF PHYSIOLOY. 



formation 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 

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

 throwing off of old ones from the free surface. When these two pro- 

 cesses are accurately balanced, the epidermis maintains its thickness. 

 When, by intermittent pressure a more active cell-growth is stimulated, 

 the production of cells exceeds their waste and the epidermis increases 

 in thickness, as we see in the horny hands of the laborer. 



The thickness of the epidermis on different portions of the skin is 

 directly proportioned to the friction, pressure, and other sources of 

 injury to which it is exposed; for it serves as well to protect the sensi- 

 tive and vascular cutis from injury from without, as to limit the evapo- 

 ration of fluid from the blood-vessels. The adaptation of the epidermis 

 to the latter purposes may be well shown by exposing to the air two dead 

 hands or feet, of which one has its epidermis perfect, and the other is 

 deprived of it; in a day, the skin of the latter will become brown, dry, 

 and horn-like, while that of the former will almost retain its natural 

 moisture. 



B. Cutis vera. The corium or cutis vera, which rests upon a layer 

 of adipose and cellular tissue of varying thickness, is a dense and tough, 

 but yielding and highly elastic structure, composed of fasciculi of are- 

 olar tissue, interwoven in all directions, and forming by their interlace- 

 ments, numerous spaces or areolae. These areolse are large in the deeper 

 layers of the cutis, and are there usually filled with little masses of fat 

 (Fig. 234): but, in the superficial parts, they are small or entirely ob- 

 literated. Plain muscular fibres are also abundantly present. 



Papillae. The cutis vera presents numerous conical elevations, or 

 papillce, with a single or divided free extremity, which are more promi- 



FIG. 233. Compound papillae from the palm of the hand, a, basis of a papilla: 6, 6, divisions 

 or branches of the same; c, c, branches belonging to papillae, of which the bases are hidden from 

 view. X 60. (KSlliker.) 



nent and more densely set at some parts than at others (Fig. 233). This 

 is especially the case on the palmar surface of the hands and fingers, and 

 on the soles of the feet parts, therefore, in which the sense of touch is 

 most acute. On these parts they are disposed in double rows, in parallel 



