EXCHETIOtf. 491 



deeper layers, and a constant throwing off of old ones from the free sur- 

 face. When these two processes 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 the 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 evap- 

 oration of fluid from the blood-vessels. The adaptation of the epider- 

 mis 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. 



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 areolar 

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

 ments, 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. 298) : but, in the superficial parts, they are small or entirely oblit- 

 erated. Unstriped muscular fibres are also abundantly present. 



Papillae. The cutis vera presents numerous conical papillae, with a 

 single or divided free extremity, which are more prominent and more 

 densely set at some parts than at others. This is especially the case on 

 the palmar surface on 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 curved lines, separated 

 from each other by depressions. Thus they may be easily seen 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 -^ 

 of an inch (^ mm.), and at their base they measure about -^ of an 

 inch in diameter. Each papilla is abundantly supplied with blood, re- 

 ceiving from the vascular plexus in the cutis one or more minute arte- 

 rial twigs, which divide into capillary loops in its substance, and then 

 reunite into a minute vein, which passes out at its base. This abun- 

 dant supply of blood explains the turgescence or kind of erection which 

 they undergo when the circulation through the skin is active. The 

 majority, but not all, of the papillae contain also one or more terminal 



