THE SKIN AND ITS FUNCTIONS. 335 



intermittent pressure, a more active cell-growth is stimulated, the produc- 

 tioii 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 sensitive and 

 vascular cutis from injury from without, as to limit the evaporation 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, t>f 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. 



Cutis vera. The cor mm or cutis, which rests upon a layer of adi- 

 pose and cellular tissue of varying thickness, is a dense and tough, but 

 yielding and highly elastic structure, composed of fasciculi of fibro- 

 cellular tissue, interwoven in all directions, and forming, by their inter- 

 lacements, numerous spaces or areolae. These areolae are large in the 

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

 of fat (Fig. 228): but, in the superficial parts, they are small or entirely 

 obliterated. Plain muscular fibre is also abundantly present. 



Papillae. The papillae are conical elevations of the cutis vera, with a 

 single or divided free extremity, more prominent and more densely set at 

 some parts than at others (Figs. 227 and 230). The parts on which they 

 are most abundant and most prominent, are the palmar surface of the 



FIG. 227. 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. 

 XGO. (Kolliker.) 



hands and fingers, and 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 curved lines, separated from each other by 

 depressions. Thus they may be seen easily on the palm, whereon each 

 raised line is composed of a double row of papillae, and is intersected by 

 short transverse lines or furrows corresponding with the interspaces 

 between the successive pairs of papillae. Over other parts of the skin 

 they are more or less thinly scattered, and are scarcely elevated above the 

 surface. Their average length is about y^- of an inch, and at their base 



