THE ANIMAL KINGDOM. 325 



allowing liquids to pass through them when there is a more 

 dense liquid on the other side (as described in the ex- 

 periment at page 123, "Vegetable Kingdom,"), and, pass- 

 ing through the vessels called " lacteals," is mixed there 

 with the impure blood as it is about to be returned to the 

 heart, and from thence to the lungs, where impurities are got 

 rid of in the form of carbonic acid. The lungs (fig 91), in 



FIG. 91. LUNGS, HEART, AXD LARGE VESSELS. 



1 and 2, Right and left Lungs ; 3, Heart ; 4, Aorta; 5, Trachea. 



performing this office, expose the blood (contained in thou- 

 sands of minute blood-vessels) to the air which enters the 

 windpipe at every inspiration or breath, the windpipe 

 divides into two, and these each into two more (the bron- 

 chial tubes), and so on until they are no larger than a 

 hair ; at the end of each tube is a little " air-cell," on the 

 membraneous walls of which the vessels containing the 

 blood are distributed as a network. The blood when it 

 enters the lungs is of a dark purple or blackish colour, and 

 is loaded with carbonic acid ; this blood is received by the 

 heart from the great veins, which pour it into the right 

 auricle, this contracting, sends it into the right ventricle, 



