THE CONTROL OF THE CIRCULATION 253 



because the oxygen is so rapidly used up. In urine, where this is not the 

 case, the pressure of 2 is about the same as in the venous blood, and when 

 a neutral gas is placed in the plcural or peritoneal cavities, it ultimately 

 becomes mixed with oxygen up to 3-4 per cent. (Tobiesen cf. Krogh.) 



The independent state of contractility of the capillaries suggests that 

 their behavior towards the actions of drugs and other agencies and to 

 nervous impulses may be opposite to that of the arterioles. This is actu- 

 ally the case, at least when adrenin and histamine (page 307) are present 

 in the blood the arterioles constrict and the capillaries dilate and there 

 is convincing evidence that the same changes occur during surgical shock 

 (page 306). Capillary dilatation also occurs when mustard, chloroform, 

 etc., are applied locally to tissue, and when the salts of certain heavy metals 

 e.g., AuCl 4 Na) are injected intravenously. In these cases the capillary 

 dilatation is not to be explained by dilatation of the arterioles. The action 

 of lymphagogues and of substances causing urticaria is to be thought of 

 in this connection. The independent dilatation of capillaries also explains 

 the association of local hyperemia with cyanosis. In such cases the blood 

 is flowing so slowly in the dilated capillaries that its oxygen all becomes 

 used up. It is as yet unknown whether the capillaries are supplied with 

 vasomotor nerves. 



