146 The Feeding of Animals 



THE LUNGS 



The first point where important changes occur is the 

 lungs. Here the blood loses the purplish hue which 

 it always has after being used in the body tissues 

 and takes on a bright scarlet, a phenomenon that is 

 more easily understood when we understand the lung 

 structure. 



Breathing is a matter of common experience. We 

 all know how air is drawn into the lungs at regular 

 intervals, an equivalent quantity being as regularly 

 forced out. The mechanism of respiration (breathing) 

 we will not discuss at length. It will aid us, however, 

 if we know that the passage which the air follows to and 

 from the lungs, the trachea (windpipe), divides into two 

 branches, one to each lung, and these divide and sub- 

 divide until they branch into numerous fine tubes. 

 Each of these tubes ends in an elongated dilation which 

 is made up of air cells opening into a common cavitj-. 

 These cells are so numerous in the lung tissues that only 

 a very thin wall separates adjoining ones, and in this 

 wall are carried the capillaries or fine divisions of the 

 blood-vessels leading from the heart. This arrange- 

 ment permits the blood to take up oxygen as it flows 

 along and transfer certain wastes into the lung cavities, 

 and thus be made ready to go back to the body carry- 

 ing a joint load of digested food and oxygen. Of cours6 

 the air that passes out of the lungs is less rich in 

 oxygen than when it was taken in, and there have 

 been added to it certain materials which we will notice 

 later. 



