Human Physiology. 



205 



in a network so fine as to task the highest powers of the micro- 

 scope to make them visible. 



From every part of the lungs, the blood, which from its con- 

 tact with air, from the absorption of oxygen, and loss of car- 

 bonic acid, etc., has, from a dark crimson hue, become a bright 

 scarlet, is sent back to the heart, when it is received into the 

 left auricle, passes to the left ventricle, and is expelled with 



Fig. 47. ARTERIES AND VEINS OF THE HEAD AND NECK. 



great force into the great aorta, or trunk of the arterial system, 

 which branches out like a tree, carrying the blood to every part 

 of the body up into the head, where the brain gets a large sup- 

 ply ; to all the organs of the body, to the upper and lower 



