90 PHYSIOLOGY AND NATIONAL NEEDS 



of blood in the body is maintained by a high 

 pressure being kept up in the vessels leaving the 

 heart, and a low pressure in the vessels returning 

 blood to the heart. 



If from any cause the head or pressure of blood 

 in the arteries falls, the flow of blood must be 

 decreased. 



From 1733, when the Rev. Stephen Hales made 

 his classical experiments upon the horse, physio- 

 logists had done an enormous amount of work upon 

 animals in investigating this pressure this head 

 of blood, and they had devised methods and 

 instruments which enabled the pressure to be 

 measured in man without any operative inter- 

 ference. 



When these methods were applied hi cases of 

 wound shock it was found that there is a marked 

 decrease in the pressure, a decrease which is roughly 

 proportionate to the severity of the symptoms. 



To what is this due ? The pressure in the 

 arteries is kept up by the force of the pump the 

 heart and by the obstruction to the onflow of 

 blood offered by the smaller arteries. If either 

 of these is decreased the pressure will fall. 



The experimental work of Starling and his co- 

 workers has shown that the heart is a pump beauti- 

 fully adjusted to send on just as much blood as 

 comes to it, so that the feeble action of the heart 

 in these cases must either be due to some failure 



