ARTERIAL AND VENOUS BLOOD. 165 



arteries, and poured into the right side of the heart. 

 On examining the quality of the blood in these two 

 systems of vessels, it is found to have undergone a 

 great change in its passage from the one to the 

 other. The florid hue which distinguishes it in the 

 arteries has disappeared, and given place to the dark 

 colour characteristic of venous blood. Its proper- 

 ties, too, have changed, and it is now no longer 

 capable of sustaining life. 



Two conditions are essential to the reconversion 

 Of venous into arterial blood, and to the restoration 

 of its vital properties. The first is an adequate pro- 

 vision of new materials from the food, to supply the 

 place of those which have been expended in nutri- 

 tion ; and the second is the free exposure of the venous 

 blood to the atmospheric air. 



The first condition is fulfilled by the chyle or nu- 

 trient principle of the food being regularly poured 

 into the venous blood, just before it reaches the 

 right side of the heart ; and the second, by the very 

 important process of respiration, which takes place 

 in the air-cells of the lungs, and which it is our 

 present object to explain. 



The venous blood, having arrived at the right side 

 of the heart, is propelled by the contraction of that 

 organ into a large artery, leading directly, by separ- 

 ate branches, to the two lungs, and hence called the 

 pulmonary artery.* In the innumerable branches 

 of this artery expanding themselves throughout the 

 substance of the lungs, the dark blood is subjected 

 to the contact of the air inhaled in breathing ; and a 

 change in the composition both of the blood and of 



* Taking the nature of the blood for our guide, the pulmonary 

 artery ought to be named the pulmonary vein, for it contains 

 venous blood: but from its structure and office resembling 

 those of the arteries, it has been called an artery. The pul- 

 monary veins, on the other hand, contain arterial blood, although 

 named veins. To prevent confusion, it is necessary to advert to 

 this source of ambiguity. 



