REMARKABLE CHANGE IX THE BLOOD. 339 



acquires carbonic acid and other principles, and 

 changes colour from the bright scarlet to the 

 dark hue. In short, from being "arterial" it 

 becomes "venous" blood. Blood upon which 

 this change has passed may now be considered 

 as unfit to fulfil the functions previously de- 

 volving upon it. To render it again serviceable 

 for the purposes of the animal economy, it must 

 be altered, and restored to the pure and health- 

 ful state of arterial blood. It is a well-known 

 fact, that venous blood cannot circulate for any 

 length of time without producing the most 

 serious and even fatal consequences. Some che- 

 mical agency, therefore, must interfere to re- 

 store its lost wholesomeness, or the functions of 

 the animal economy would soon cease, never to 

 be recommenced. After performing its round, 

 the blood is directed into the lungs, and poured 

 through millions of fine tubes which line the 

 walls of the air-cells. Here the blood comes into 

 contact with the air inhaled into these organs. 



C 



Exposed on such a vast surface to atmospheric 

 influence, the dark fluid loses the carbonic acid 

 with which it was laden, and receives from the 

 air a fresh, and exactly equivalent, supply of 

 oxygen gas ; and now it turns from dark red to 

 scarlet, and from the deleterious character of 

 venous to the healthful composition of arterial 

 fluid. This done, it is removed from the lungs 



