CIRCULATION THROUGH THE SKIN 



1341 



flow is seen on the day of the operation, and the 

 decline is faster than in the hand; the extent to which 

 the vessels of muscle and skin, respectively, contribute 

 to these changes has not been defined. 



The cause of the change in the vessels which leads 

 to the return of blood flow to near the normal level is 

 not known. The denervated vessels develop an in- 

 creased sensitivity to adrenaline and other vasoac- 

 tive agents (46, 74) and this develops at a rate which 

 closely parallels the decline in blood flow (22). In- 

 creased sensitivity of chronically sympathectomized 

 vessels has been demonstrated in the finger to ad- 

 renaline injected intravenously (178), and in the 

 hand to both adrenaline and noradrenaline injected 

 intra-arterially (fig. 14). Whether the return of tone 

 is due to an increased sensitivity of the vessels to 

 unknown circulating pressor substances, or to an 

 intrinsic change in the muscle of the vessel wall, or 

 to an effect of surviving accessory sympathetic fibers 

 is not decided. 



LATE EFFECTS OF TOTAL DENERVATION. "While loss 



of sympathetic supply causes the corresponding fingers 

 to be in general warmer than they otherwise would 

 be, loss of all nerve supply causes the corresponding 

 fingers to be in general colder than they otherwise 

 would be. And, since with combined loss of both 

 motor and sympathetic supply the digits remain 

 warm, it seems that sensory nerve loss must be an 

 important factor in determining the persistent cold- 

 ness in cases of mixed nerve lesions" (Lewis & 

 Pickering, 144). The extent to which the coldness of 

 denervated fingers depends on the loss of sensory as 

 opposed to sympathetic innervation has, however, 

 been questioned (62), and limbs normal except for 

 muscular paralysis are colder than normal (144). The 

 most conspicuous abnormality in the behavior of the 

 circulation in denervated digits is the great reduction, 

 under normal circumstances, in the vasodilator 

 response to cold. 



Reflex Control of Blood Vessels of the Skin 



The blood flow through the digits can be varied 

 through a very wide range by the activity of the 

 sympathetic vasoconstrictor nerves. At the upper and 

 lower extremes of the range the blood flow is normally 

 fairly steady from minute to minute. At intermediate 

 levels, such as are normally found in comfortably 

 warm subjects, the flow usually fluctuates, rising and 

 falling by 20 per cent or more several times a minute. 

 The fluctuations are abolished bv division of the 



sympathetic nerves, occur simultaneously in the 

 digits of all limbs, and are often associated with 

 simultaneous changes in heart rate (41, 42, 45). The 

 frequency of the constrictions is greater when the 

 flow is near the lower than when it is near the upper 

 end of its range, and the size and pattern of the varia- 

 tions van,- considerably in different individuals. The 

 functional significance of the fluctuations is not 

 known; their occurrence makes desirable the use of 

 repeated rather than single observations in estimates 

 of the skin circulation in the extremities. They are 

 not found in the skin of the forehead (117). 



Very little is known about the reflex responses of 

 skin other than that in the extremities, but such evi- 

 dence as is available indicates that the responses, 

 if present, are comparatively small (85, 116). 



body temperature regulation. The skin is a main 

 route for loss of heat from the body, and by far the 

 most important route capable of adjustment by the 

 temperature-regulating center. The heat lost from the 

 surface, whether by conduction, convection, radiation, 

 or the evaporation of sweat must be transported to 

 the skin, and because of the low thermal conductivity 

 of body tissue, the transport is mainly in the circu- 

 lating blood. 



Gibbon & Landis (8g) found that if one arm was 

 immersed in water at 42.5 C to 44.6 C the temperature 

 of the fingers of the opposite hand started to rise in 5 

 to 10 min, and reached 32 C in g to 16 min. If, how- 

 ever, (fig. 12) under similar conditions, the circula- 

 tion in the immersed arm was arrested (with brief 

 releases) by a pneumatic cuff for the first 35 min of 

 immersion, the rise in temperature of the fingers of 

 the opposite hand was delayed until 7 to 10 min 

 after the final release of the cuff (that is 42-45 min 

 from the start of immersion), and the fingers reached 

 32 C 11 to 16 min after the release. The response in 

 the fingers evidently depends on the return to the 

 body of hot blood from the immersed part, rather 

 than on the stimulation of peripheral receptor organs. 

 This conclusion is confirmed by the finding that 

 rapid intravenous infusions of hot saline are able to 

 provoke vasodilatation in the hand by a mechanism 

 independent of any surface heating ( 1 7g). The central 

 receptor mechanism is sensitive to the addition of as 

 little as 1 to 2 Calories of heat to the body, or to an 

 amount of heating sufficient to raise the sublingual 

 temperature by 0.15 C (88). 



The temperature of surface receptors is, however, 

 of some importance in the reflex regulation of the skin 

 circulation. Kerslake & Cooper (129) found that 



