VESSELS AND NERVES OF THE SKIN. 335 



whole wall consists of two layers of epithelial cells and a tunica 

 propria. On entering the epidermis the duct becomes spirally 

 coiled. It loses its own wall and is bounded by the cells of 

 the epidermis in which it lies. The cells of the Malpighian 

 layer of the epidermis arrange themselves concentrically around 

 the lumen. The stratum granulosum turns and follows the 

 duct downward for a short distance (Fig. 244). 



The sweat glands develop in the fifth month of embryonic 

 life as a solid outgrowth of the Malpighian layer. In the 

 course of growth they become slightly coiled, and in the 

 seventh month possess a lumen. 



Sweat glands are supplied richly with nerves. Non-medul- 

 lated fibres form a fine network on the outer surface of the 

 membrana propria, from which fine fibres pass through the 

 basal membrane. These end on the surfaces of the gland cells 

 by means of fine end bulbs. 



(e) Vessels and Nerves of the Skin. 



The number and diameter of vessels in the skin varies 

 according to the region studied. The blood supply is greatest 

 in those places which are subject to pressure (e. g., the gluteal 

 regions, the palms of the hands, and the soles of the feet). 

 The branching is greatest in parts which are most movable. 

 (Spalteholz). 



The arteries enter the corium and anastomose to form the 

 cutaneous network. From this, branches arise, which pass 

 upward toward the epidermis, and before reaching it anasto- 

 mose with one another to form the subpapillary network (Figs. 

 256 and 257). Small capillary end branches proceed from the 

 latter network into the papillae, forming capillary loops. These 

 give origin to the veins. Branches from the cutaneous network 

 form a dense plexus around the subcutaneous fat, and around 

 the bodies of the sweat glands (Fig. 256). 



The veins begin in the papillas and form four distinct net- 

 works parallel to the surface. The most superficial lies imme- 

 diately under the papillae. From this, irregular branches run 

 down to the second plexus, the meshes of which are small and 



