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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 





fig. I. Projection drawing (X 95) of 

 a group of anastomoses in the nail bed 

 of the toe. The artery crosses the center 

 of the drawing and gives rise to 1 2 

 thick-walled anastomotic branches. The 

 thin-walled venous terminations ol 

 several are shown. [From Grant & 

 Bland (99).] 



of the fingers where the maximum flow is probably 

 between 100 and 200 times the minimum (42), 

 and secondly, the fact that in normal persons the 

 blood flow to the skin is greatly in excess of its meta- 

 bolic requirements, being chiefly determined by the 

 need to maintain thermal balance (82). This lavish 

 circulation is, no doubt, valuable in the repair of 

 trauma and wounds to which the skin is especially 

 exposed. Perhaps because of methodological diffi- 

 culties, there is very little information about the 

 circulation through other areas of skin, but it is 

 almost certainly much less reactive than that through 

 the extremities. 



Arrangement of the Blood I 'esseli of the Skin 



The skin is supplied with a profuse system of capil- 

 lary loops which rise in the papillae of the corium 

 and return to enter a subpapillarv venous plexus. 

 The vessels of the latter are large and have thin walls, 

 and it is probable that when distended they contain a 



very large proportion of all the blood in the skin. 

 There are rich capillary networks around the sweat 

 glands, at the base of hair follicles, around the seba- 

 ceous glands, and in the nail bed and nail fold. 



In the skin of the extremities a special and promi- 

 nent feature is the large number of arteriovenous 

 anastomoses (50, 147). These are coiled channels 

 (fig. 1 ) with thick muscular walls and a lumen which 

 in the dilated state is between 20 and 70 n in diam- 

 eter, the average being 35 /x- They are abundantly 

 supplied with nerve endings, and a high concentra- 

 tion of cholinesterase has been reported around them 

 (28, 124, 151). They directly connect arterioles and 

 venules in the dermis at the level of, or a little super- 

 ficial to, the sweat glands. They are most numerous 

 in the nail bed, numerous at the tips of the digits, 

 less numerous on the palmar surface of the phalanges, 

 and almost absent from the dorsum of the phalanges. 

 They are fairly numerous in the palm of the hand 

 and sole of the foot, but are absent from the areas 

 of the forearm and calf which have been examined. 



