CIRCULATION OF THE BLOOD 7 



in the possession of valves at its commencement, which 

 prevent the movement of blood except away from the 

 heart. It differs from the aorta in being much shorter 

 and breaking up much sooner into branches, though 

 these facts cannot be brought out in a diagram. It also 

 differs from the aorta in supplying branches only to the 

 lung instead of to all parts of the body. 



In the lung the branches of the pulmonary artery, like 

 the branches of the aorta, break up, as we have seen, 

 into a capillary network. The pulmonary capillary 

 network, again like the systemic capillary network, passes 

 into veins. These veins in the lung unite and become 

 larger and larger until they all end in a great trunk, the 

 pulmonary vein, which opens into the right auricle. 



We have compared the pulmonary artery to the aorta, 

 and referred also to certain points of difference. There 

 is, however, one great difference between the pulmonary 

 artery and the aorta that was a very great puzzle to the 

 ancient physiologists. The nature of that difference is, 

 however, quite clear nowadays. The pulmonary artery 

 contains dark red blood, the aorta contains light red 

 blood. This fact is easily understood if we examine 

 how the blood reaches these two great vessels. We 

 have already seen blood changed from light to dark in 

 the systemic capillaries, and passing from them into 

 the veins, thence into the vena cava, thence into the 

 right auricle, thence into the right ventricle, and thence 

 into the pulmonary artery. The pulmonary artery 

 branches and breaks up into capillaries in the substance 

 of the lung. In these pulmonary capillaries the change 

 that took place in the systemic capillaries is reversed. 

 Dark red blood enters the pulmonary capillaries and it 

 emerges from them light red. From the pulmonary 

 capillaries this light red blood passes into the pulmonary 

 vein. The pulmonary vein conveys it to the left auricle, 



