CIRCULATION THROUGH THE SKIN 



I3 2 7 



table I. Xumber of Anastomoses per 

 Square Centimeter of Surface Area 



Hand 



Index finger 



Nail bed 501 



Tip 236 



Palm, 3rd phalanx 150 



2nd phalanx 20 



1st phalanx 93 



Palm 



Metacarpo-phalangeal joint 3rd finger 31 



Thenar eminence 113 



Hypothenar eminence 96 

 Foot 



2nd toe 



Nail bed 593 



Pad 293 



Sole, near heel 197 



These are nil for dorsal surfaces of fingers, toes, hand, and 

 foot; flexor surfaces of lower forearm and lower calf of leg; 

 lower half of ear. [From Grant & Bland (99).] 



table 2. Percentage Composition by Volume 

 of Parts of Human Limbs 



Hand 



Foot 



Forearm 



REFERENCES 



Hand Average of 3 hands (2). 



Foot Average of 2 feet (12). 



Forearm A Average of 5 forearms (56). 

 Forearm B Average of 3 forearms (2). 



Table 1, from Grant & Bland (99), summarizes the 

 distribution in the human. They have since been 

 found in the human ear (164). Some observers (151, 

 163) have reported rather smaller numbers (20-25/ 

 cm 2 ) than did Grant and Bland in the finger pad. 

 They are numerous in the external ear of the rabbit, 

 where their reactions have been carefully studied 

 (98), in the ear of the cat and dog and in the feet of 

 webfooted birds. Grant's (98) summary of the func- 

 tions of the anastomoses is still valid, and applies to 

 the human extremities as well as to the rabbit's 

 ear: "The anastomoses serve two functions (a) local, 



and (b) general, (a) It is mainly through their agency 

 that the temperature of the ears is maintained when 

 they are exposed to cold, (b) They are important 

 factors in regulating of body temperature, aiding the 

 dispersal of heat by allowing an enormous blood 

 flow through the ears." (See also Chapters 27 and 



37-) 



Measurement of the Flow of Blood Through the Skin 



The fingers and toes are composed largely of skin. 

 Of the total flow through them, the greater part 

 normally passes through skin, and total digital blood 

 flow is often used as an index of digital skin blood 

 flow (table 2). Digital flow may be directly measured 

 by venous occlusion plethysmography (40, 95), a 

 method which permits variation in the rate to be 

 followed from one heart beat to the next, and even 

 during a single beat. Flow may be estimated by 

 calorimetry (148), a method which, because of the 

 thermal capacity of the tissues, is incapable of follow- 

 ing rapid changes in flow, but which conveniently 

 integrates flow over a period of time. Calorimetry 

 finds its most successful application in the digits, 

 and the method has been rendered more versatile by 

 the use of copper-tellurium heat flow discs (47). 



In the more proximal parts of the limbs, the total 

 blood flow depends a great deal on the circulation 

 through tissues, especially muscle, deep to the skin. 

 The flow through the skin can be deduced by com- 

 paring the total flow in a pair of segments in one of 

 which the circulation through the skin has been 

 suppressed by iontophoresis of adrenaline (71). 

 More often, indirect indices of skin blood flow have 

 been employed. If venous blood can be obtained from 

 vessels exclusively draining skin, and if the oxygen 

 usage of the skin is assumed to remain constant, 

 changes in flow can be inferred from changes in the 

 oxygen content of the blood (170). Measurements 

 of the temperature of the skin have provided useful 

 qualitative information in the proximal as well as 

 in the distal parts of the limbs (100), but this tempera- 

 ture, as explained later, depends on so many other 

 factors that it is a very imperfect index of skin cir- 

 culation. It is incapable of following accurately 

 rapid fluctuations in flow; if the circulation is com- 

 pletely arrested the temperature of the skin falls 

 very slowly. A more sensitive index of blood flow is 

 the change in thermal conductivity of the skin, which 

 can be conveniently measured by a surface applicator 

 containing two small plates, one of which is elec- 

 trically warmed while the temperature difference 



