THE STRUCTURE OF SKIN 5 of the body the dermis contains smooth muscle fibres. These may be found in the skin round the nipple, the penis and the scrotum. BLOOD SUPPLY TO THE SKIN In the deep connective tissue underlying the epidermis small arteries form a rich arterial plexus. From this plexus arterioles pass more or less perpendicularly upwards to form the sub-papillary plexus from which smaller vessels, the terminal arterioles, supply the papillary capillaries. (Slide 5) The illustrations demonstrate the arrangement of the vessels. The terminal arterioles and the metarterioles have smooth muscle in their SLIDE 4 The basement membrane of the epidermis from the scalp. The "membrane" is actually a complex argyrophilic network through the meshes of which protrude the cytoplasmic processes of the basal cells. This section was treated with Gomori's ammoniacal silver nitrate technique.

6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS walls which form pre-capillary sphincters. If these shut off, the blood _ goes round the preferential channel, but if the arterio-venous anastomosis opens, the whole of the blood is shunted away from the capillary system. It is claimed by some workers that only the pre-capillary sphincters • (Slide 6) are contractile and that the capillaries themselves are incapable of l contraction. In the resting state it has been found that there is an• intermittent opening and closing of the pre-capillary sphincters with• blood flowing sometimes through one capillary and sometimes through I another (see illustration). When the capillary sphincters are contracted,, the capillaries are ischaemic and blood flows exclusively in the preferential I channels. (Slide 7) This knowledge of the blood supply of the skin helps us to under-- stand many of its reactions. For example, if the surface of the skin• SLIDE 5 A diagram of the skin and its vessels, showing the arrangement of the arterial and venous plexuses at various levels (after Spalteholz). From Lewis (186). (Reproduced by permission of Clinical Science.)