THE NERVOUS CONTROL OF THE BLOOD VESSELS 1049 



have ascribed them to active contraction or change of form of the endo- 

 thelial cells, which was apparently independent of concomitant arterial 

 alteration. In a subsequent chapter we shall have occasion to study 

 in the capillary circulation the impressive changes following slight injury, 

 chemical, thermal or mechanical, which give the salient features to the 

 picture of inflammation. But it is certain that nutritive changes of less 

 degree, falling within normal physiological events, also influence consider- 

 ably the flow through the capillaries, either by increasing their lumen or by 

 altering the resistance to the passage of blood through them. Thus during 

 activity the total capacity of the capillaries of muscle may be increased from 

 O02 per cent, to 15 per cent, of the total volume of the muscle (Krogh). The 

 phenomena of dropsy show us that the capillary wall is very sensitive to the 

 continued absence of oxygen, oxygen, starvation rapidly increasing its 

 permeability ; and it seems that the presence of oxygen is an essential con- 

 dition of any reactivity to moderate nutritional changes on the part of 

 the capillaries. The dilator effect we have already studied of carbonic acid 

 and other weak acids on the arterioles seems to be shared by the capillaries. 

 In such a case it is difficult to dissociate the effects of arterial dilatation 

 from those of capillary dilatation. At least one chemical substance is known 

 however, which has diametrically opposite effects on the two sets of vessels. 

 Histamine, the amine produced by the decarboxylation of histidine, has been 

 shown by Dale to have a constrictor effect on the arterioles and a dilator 

 effect on the capillaries. It has been suggested that the production of 

 histamine or of other substances with a similar action plays an important 

 part in giving rise to the symptoms of surgical shock. In this condition, 

 which is found notably after widespread laceration, especially of the 

 muscles, and consequent destruction of the tissues, there is a continually 

 increasing depression of the blood pressure due to the ever lessening volume 

 of blood in circulation. Since this lowering of blood pressure does not 

 depend on any direct action of the heart nor is it associated with vaso-motor 

 paralysis, it has been concluded that the prime factor -at work is a general 

 dilatation of the capillaries, leading to stagnation of the blood in these vessels 

 and an increased exudation into the tissues, thus causing a constant leak of 

 blood fluid from the general circulation. 



No evidence has yet been brought forward for a direct action of the 

 central nervous system on the capillaries. Certain facts however point to 

 a connection between nerve lesions and the calibre of the capillaries supplied 

 by the nerves. Thus if in the cat the sciatic nerve be cut on the right side, 

 for the next few hours the pad of the foot on that side is flushed and warmer 

 than the left foot. The next day the flush has disappeared, in fact the pad 

 of the right foot may be paler than that of the left foot. The right foot is 

 however still a degree or two warmer than the left foot. This condition 

 may be explained on the assumption that the immediate effect of cutting 

 the sciatic nerve is to cause dilatation both of the arterioles and of the 

 capillaries. The capillary dilatation passes off, so that on the day after the 

 section, although the arterioles are still dilated and there is a more rapid 



