CIRCULATION THROUGH THE SKIN 



'337 



skin, causes a delayed erythema sharply confined to 

 the exposed area. It is probable that a chemical 

 agent is concerned, but that this is not histamine 



(•56). 



arterial gas embolism. After injection into the 

 brachial artery of 1 to 10 ml of gas there is usually 

 an immediate reduction in blood flow through the 

 hand lasting for a few minutes, followed by a pro- 

 longed increase, to several times the normal resting 

 rate, which does not entirely subside for many hours 

 (66). The increase in the oxygen saturation of the 

 venous blood parallels the increase in flow (fig. 8). 

 All of the several gases tested are effective, provided 

 they are given as bubbles and not in solution. The 

 response is present in both sympathectomized and 

 chronically denervated limbs, and is unaltered in the 

 presence of amounts of antihistamine substances 

 which prevent the action of histamine (67). The 

 vessels of muscle as well as of skin are affected. The 

 mechanism is not understood, but the reaction ap- 

 pears to result from some trauma to the tissues caused 

 by the bubbles, and a peripheral arterial conducting 

 mechanism may be involved (120). 



nervous control of skin blood vessels 



Vasomotor Nerves 



vasoconstrictor sympathetic nerves. Claude Ber- 

 nard (30, 31) showed that division of the cervical 

 sympathetic chain in the rabbit caused the ear on 

 the same side to become flushed and warm. Stimula- 

 tion of the trunk had the reverse effect (32). Similar 

 observations were made by Brown-Sequard (39). 

 These observations indicate that the sympathetic 

 nerves contain vasoconstrictor fibers, and that under 

 ordinary conditions the activity in these fibers keeps 

 the vessels in a partially constricted state. The warm- 

 ing of, and increased circulation through, the human 

 feet and hands following lumbar and thoracic sym- 

 pathectomy was first described by Adson & Brown 

 (5, 6) and this established that these areas were 

 similarly under sympathetic vasoconstrictor control. 

 Walker et al. (184) made quantitative measurements 

 of the effects of sympathectomy in patients with ap- 

 parently normal blood vessels in whom the operation 

 had been carried out as a treatment for excessive 

 sweating. The blood flow in milliliters per 100 ml of 

 hand per min was increased from an average value 



of 5.2 before the operation to peak values in the range 

 22.7 to 59.2 after the operation, and in the feet was 

 increased from an average value of 2.1 before the 

 operation to peak values in the range 20.8 to 28.0 

 after the operation. The averaged results on five 

 hands and six feet are shown in figure g. 



In vasoconstrictor sympathetic nerves low rates of 

 discharge have a powerful effect; in the cat's paw, 

 stimulation at the rate of 1 per sec increases the re- 

 sistance to flow about 10 times, and stimulation at 

 10 per sec increases it about 100 times (48). 



vasodilator sympathetic nerves. The evidence for 

 vasodilator nerves to the skin rests at present on 

 experiments of the type employed by Grant & 

 Holling (100), that is to say, on the simultaneous 

 observation under suitable conditions of reflex 

 stimulation of a greater blood flow in normally 

 innervated skin than in a corresponding area of skin 

 acutely deprived of its vasomotor innervation. Since 

 both areas are perfused with blood of identical com- 

 position and at the same pressure, the resistance 

 vessels may be presumed to be more widely dilated 

 in the innervated skin, and this dilatation to result 

 from nervous activity. It must be noted that chroni- 

 cally denervated skin is not a satisfactory tissue for 

 this comparison, because of the decline in blood flow 

 due to contraction of the blood vessels. 



Grant & Holling (100) found that blocking the 

 cutaneous nerves, which convey sympathetic fibers, 

 to parts of the skin of the forearm not only failed to 

 cause a flushing and rise of temperature but pre- 

 vented the vasodilatation and also the sweating 

 normally seen in the forearm during body warming. 

 Evidently, in the forearm vasomotor nerves actively 

 bring about vasodilatation. Whether they do so by a 

 direct action on the vessels or as a consequence of 

 increased sweat gland activity, or by both means 

 was, and is, uncertain. If the fibers are called "vaso- 

 dilator," it must be remembered that vasodilatation 

 may be only a consequential and not a direct effect. 



It is, of course, important for heat to be brought 

 to the skin if sweat is to be evaporated. At least 60 ml 

 of blood are required to transport from the body 

 core the heat required to evaporate 1 g of sweat when 

 the skin is 10° below the temperature of the core. At 

 least 600 ml of blood are required when the skin is 

 1 C below the temperature of the core. Sweating can 

 therefore be effective only when accompanied by 

 vasodilatation. 



