TRANSPORT OF MATERIALS 



133 



open only in such a direction as to allow blood to flow from auricle 

 to ventricle, and not in the reverse direction, the contents of the 

 ventricle are expelled into the main arterial channel, the aorta. 

 Another set of valves is necessary at the beginning of this tube, 



Right heart 



Lungs 



Left heart 



FIG. 7. Diagram of the Vascular System of the Mammal. 



in order to prevent the blood driven in, and stretching its walls, 

 from flowing back again when the ventricle relaxes in preparation 

 for another beat. The aorta gives off a large number of tubes, the 

 arteries, of which four are represented in the figure. Each of these 

 again divides into smaller vessels also with muscular walls, the 

 arterioles, and these are continued into a network of minute con- 

 nected tubes arranged around the tissue cells. These are the 



