92 PHYSIOLOGY AND NATIONAL NEEDS 



Now all through the tissues there is a dense net- 

 work of minute vessels, the capillaries. We know 

 that these in certain conditions get dilated and that 

 blood tends to stagnate in them. This is well seen 

 in the reddening of the hands or nose in cold 

 weather, a condition which, if it goes on, passes into 

 a blueing, the result of the removal of oxygen from 

 the stagnant blood. 



A substance has been found which when injected 

 into the circulation of animals leads to a dilatation 

 of these vessels without any dilatation of the small 

 arteries, and it is a substance which might be 

 formed in lacerated tissues. 



Thus, if a considerable quantity of the blood 

 stagnates in the capillaries, the amount in effective 

 circulation will be decreased, so that the amount 

 sent to the heart will be so reduced that its action 

 will be lessened, and the head of blood required to 

 keep up the circulation will fall. As this fall occurs, 

 the force driving the blood through the capillaries 

 will be lessened and the stagnation will increase. 

 But as the blood ceases to flow freely through the 

 lungs, its supply of oxygen will also be decreased 

 and so less blood carrying less oxygen will be taken 

 to the tissues. 



This, then, being the explanation of the condi- 

 tion What is to be done ? Manifestly to get up 

 the head of fluid in the arteries so as to re-establish 

 the circulation. How is this to be done ? The 



