334 THE SKIN. 



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; and the more it 

 is subjected to such injury, within certain limits, the more 

 does it grow, and the thicker and more horny does it become ; 

 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 bloodvessels. 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 lat- 

 ter will become brown, dry, and horn-like, while that of the 

 former will almost retain its natural moisture. 



Cutis vera. The corium or cutis, 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 fibre-cellular tissue, interwoven in all directions, 

 and forming, by their interlacements, 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. 

 112) : but, in the more superficial parts, they are exceedingly 

 small or entirely obliterated. 



By means of its toughness, flexibility, and elasticity, the skin 

 is eminently qualified to serve as the general integument of the 

 body, for defending the internal parts from external violence, 

 and readily yielding and adapting itself to their various move- 

 ments and changes of position. But, from the abundant sup- 

 ply of sensitive nerve-fibres which it receives, it is enabled to 

 fulfil a not less important purpose in serving as the principal 

 organ of the sense of touch. The entire surface of the skin is 

 extremely sensitive, but its tactile properties are due chiefly to 

 the abundant papillae with which it is studded. These papillae 

 are conical elevations of the corium, with a single or divided 

 free extremity, more prominent and more densely set at some 

 parts than at others (Figs. 110 and 111). The parts on which 

 they are most abundant and most prominent are the palmar 

 surface of the 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 (Fig. 

 112). Thus they may be seen easily on the palm, whereon 

 each raised line is composeed 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 scat- 

 tered, and are scarcely elevated above the surface. Their 



