274 SCIENCE PROGRESS 



dition will be exaggerated if the arterioles simultaneously con- 

 strict, as happens under the action of histamine. 



That the capillaries are not merely passively distended or 

 depleted according to the diameter of the arterioles supplying 

 them is indicated by the common experience of two different 

 effects of external cold on the skin. The colour of the skin 

 in white races is almost entirely due to the blood in the capil- 

 laries. When the blood-supply is cut off, the skin becomes 

 white and cold. This may sometimes happen as the result of 

 the constricting effect of cold on the arterioles ; but there are 

 also two familiar states produced by cold which are associated 

 with increased depth of colour, and therefore with a greater 

 content in blood. In one of these, the skin is red and warm ; 

 in the other, on the contrary, it is blue and cold. It is clear 

 that the warmth in the former case must be due to an increase 

 in the current of warm blood flowing through the capillaries, 

 and this can only be brought about by dilatation of the arteri- 

 oles and capillaries. The capillaries are thus filled with a 

 rapidly renewed current of fresh warm blood. In this way, 

 the skin is protected from the deleterious effects of cold. A 

 compromise must be effected between the increased loss of 

 heat from the body and the protection of the skin. It will 

 be remembered that one of the means of getting rid of exces- 

 sive heat is by dilatation of the vessels in the skin. What is 

 the state of affairs when the skin is blue ? Bearing in mind 

 the fact that the colour of the blood in the veins appears to 

 be blue when looked at through the skin, we realise that this 

 is the colour assumed when oxygen has been lost. The blue 

 colour of the blood in the capillaries must be due, then, to its 

 having lost more oxygen than it does when the skin is red. 

 Although the capillaries are dilated, the current through 

 them must be decreased, and this state can only be explained 

 by an expansion of the capillaries independent of that of the 

 arterioles, which are probably constricted. The coldness of 

 the skin is readily explained on the basis of the slow circula- 

 tion through it. An exaggeration of this blue state of the 

 skin may be observed in certain pathological conditions, and 

 appears to be easily brought about by exposure to cold in 

 such cases. 



With regard to more definite experimental evidence of 

 active change in the capillaries, we may note the difficulty of 



