THE STRUCTURE OF SKIN 7 is lightly stroked with a blunt edge, say, of a ruler--within 10 to 15 seconds a white line develops--dermographia alba. The paleness is confined entirely (Slide 8) to the area of stimulation. This "white line" is fully developed within one minute and lasts 3 to 5 minutes. It is due to the contraction of the met- arterioles and the pre-capillary sphincters. Stroking the skin firmly with a sharp edge produces a "red line"--dermographia rubra--which appears after a latent period of 3 to 15 seconds. If the stimulus is strong enough (Slide 9) the line may last for 30 minutes or more. This is due to dilation of the smallest vessels, which have a muscular element, and this results in an increased flow of blood through all the blood vessels in the skin. This ............... .' '.:.:':.:.:.':',5":'.' ::'i!%:."'....:.:i'•"' SLIDE 6 Nail-fold capillaries in pol¾c¾ihemia vera. Dr. Gilj e.) (Courtesy of

8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS SLIDE 7 Papillary capillaries and subpapillary plexus (after Spalteholz.) From Mfiller (•.09). (Reproduced by permission of F. Enke.) SLIDE 8 A schematic representation of the structural pattern of the capillary bed. The distribution of smooth muscle is indicated in the vessel wall. From Zweifach (•.99). (l•eproduced by permission of the Josiah Macy, Jr., Foundation.)