1382 



HUMAN ANATOMY. 



i 



Imprint of dorsal surface of left hand near ulnar border; 

 radiating lines are produced by creases connecting points at 

 which hairs emerge. 



Fig. 1145. 



epidermis. The amount of skin-pigment not only differs permanently among races 



(white, yellow and black j) and indi- 

 FiG. 1144. viduals (blond and brunette), but 



also varies in the same person with 

 age and exposure, as contrasted by 

 the rosy tint of the infant and the 

 bronzecl tan of the weather beaten 

 mariner. 



Unless bound down to the 

 underlying tissues, as it is over the 

 scalp, external ear, palms and soles, 

 the skin is freely movable. Its 

 physical properties include con- 

 siderable extensibility and marked 

 elasticity. By virtue of the latter the 

 temporary displacement and stretch- 

 ing produced by movements of the 

 joints and muscles is overcome and 

 the smoothness of the skin, so con- 

 spicuous in early life, is maintained. 

 With advancing age the elasticity 

 becomes impaired and folds are no 

 longer effaced, resulting in the perma- 

 nent wrinkles seen in the skin of old 



people. Certain folds and furrows, however, are not only permanent and ineffaceable, 



appearing in the foetus, but are fairly constant in position and form. One group, 



produced by flexion of the joints, includes the conspicu- 

 ous creases on the flexor surface of the wrist, palm and 



fingers, and the similar markings on the soles of the feet. 



The other group, more extensive but less striking, 



includes the fine grooves that connect the points of 



emergence of the hairs and cover the trunk and extensor 



surface of the limbs with a delicate tracery (Fig. 1 144). 

 The surface modelling of the skin covering the 



palms, soles and flexor aspects of the digits is due to 



the disposition of numerous minute ridges (cristae cutis) 



and furrows (sulci cutis). The cutaneous ridges, about 



.2 mm. in width, correspond to double rows of papillae 



which they cover, the sweat glands opening along the 



summit of the crests. The patterns formed by the 



cutaneous ridges (Fig. 1145) remain throughout life 



unchanged and are so distinctive for each individual 



that they afford a reliable and practical means of identi- 

 fication. In addition to the various longitudinal, trans- 

 verse and oblique ranges of ridges that cover the greater 



part of the hand, groups of concentrically arranged 



ridges occupy the volar surface over the distal phalanges, 



the pads between the metacarpo-phalangeal joints and 



the middle of the hypothenar eminence. These highly 



characteristic areas, the so-called tactile pads (toruli 



tactiles) are most strikingly developed over the bulbs 



of the fingers, where the ridges are often disposed in 



whorls rather than in regular ovals. The markings of 



corresponding areas of the two hands are symmetrical 



and sometimes identical. 



Structure. — The two parts of which the skin is 



everywhere composed — the epidermis and the connec- 

 tive tissue stratum — are derivatives of the ectoblast and 



of the mesoblast respectively. The connective tissue portion includes two layers, 







Imprint of palmar surface of left 

 middle finger, showing arrangement 

 of cutaneous ridges ; transverse in- 

 terruptions are produced by flexion 

 creases over joints. 



