84 PHYSIOLOGY AND NATIONAL NEEDS 



tint. Manifestly, then, these gassed men are not 

 getting oxygen for the blood. 



Oxygen is got by the blood from the air in the 

 lungs. Air is drawn in with every breath and 

 reaches millions of little sacs which form the lung, 

 and in the walls of these sacs run the blood-vessels 

 carrying the blood. The blood gives up the 

 carbonic acid it has brought from the tissues, and 

 takes up oxygen from the air in the sacs. The 

 examination of the lung of a patient who has died 

 from gassing at once showed that it is full, not of 

 air, but of fluid. 



Manifestly, then, if the air-sacs are full of fluid, 

 the air with oxygen cannot get to the blood, and 

 the patient is drowned as much as if he had fallen 

 into the sea. 



But how did the fluid get into the air-sacs ? 



The study of the application of irritating sub- 

 stances to the web of a frog's foot had long ago 

 shown that one result is a dilatation of all the little 

 blood-vessels, so that they become so engorged 

 with blood that it cannot freely flow away to the 

 veins. As a result of this the fluid part of the 

 blood filters out into the tissues. 



Chlorine in the lung acts just in the same way, 

 and the fluid exudes into the air-sacs. Hence 

 this drowning from within ; hence, too, this escape 

 of fluid as the patient coughs. 



But why the difiicult breathing ? Why the 



