USE OF THE DIGESTED FOOD 143 



way of the right ventricle and is returned from the 

 lungs to the left auricle to be sent to the left ventricle, 

 and from there to again start on its journey through 

 the body. (Fig. 8.) 



The nutrients, as prepared for use by digestion, enter 

 the blood on its return flow to the heart, coming into 

 the venous cavity by way of the hepatic (liver) vein and 

 the thoracic duct as previously described. When, there- 

 fore, the right side of the heart is reached, a new acces- 

 sion of food material is on its way to sustain the various 

 functions of nutrition. 



We are more interested in the object of blood circu- 

 lation than we are in its mechanism. Somehow the 

 digested food disappears into these constantly moving 

 blood currents, and the only evidence of its effect which 

 comes to us from ordinary observation is the warmth, 

 motion, and perhaps growth of the animal that is 

 nourished. 



204. The lungs. The first point where important 

 changes occur is the lungs. Here the blood loses the pur- 

 plish hue which it always has after being used in the body 

 tissues and takes on a bright scarlet, a phenomenon that 

 is more easily explained when we understand the lung 

 structure. (Fig. 9.) 



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 dis- 

 cuss 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 



