THE COURSE AND PHYSICS OF THE CIRCULATION 231 



quate for the support of life in such animals but inferior 

 to that required by the warm-blooded. 



The blood that is traversing the body in general on its 

 way from the left side of the heart to the right is said to 

 be in the greater or systemic circulation. That which is 

 passing from the right side of the heart back to the left 

 by way of the lungs is said to be in the lesser or pulmonary 

 circulation. More than three-quarters of all the blood is 

 probably in the systemic circulation at any given moment 

 But it is also true that the two ventricles pump out equal 

 quantities of blood in equal times. Students often find 

 difficulty in reconciling these two facts, but there should 

 be none. It is only necessary to reflect that no more 

 blood can be pumped out from either ventricle than the 

 other ventricle supplies to it. The service of the left 

 ventricle is much heavier than that of the right. It has 

 no more blood to drive forth into the arteries at each beat 

 but a much greater mass, upon a longer journey, has to 

 be kept in motion. 



There is a somewhat unfortunate difference between the 

 accepted significance of the nouns artery and vein and 

 that of the adjectives arterial and venous. Arteries and 

 veins, as we have indicated, are distinguished by the 

 direction of the current within them with reference to 

 the heart. But the adjective arterial, applied to blood, 

 means " fully oxygenated" while venous means " de- 

 ficient in oxygen." The systemic arteries carry arterial 

 blood but the pulmonary arteries contain blood which is 

 venous. The systemic veins convey venous blood while 

 that in the pulmonary veins, having just left the lungs, 

 is arterial. 



Arteries are elastic tubes of great strength. Their 

 walls are of considerable thickness. An important 

 feature of the smaller ones is a marked development of 

 muscle of the smooth variety. Veins are less elastic 

 than arteries, less muscular, and more capacious. An 

 artery is approximately circular in cross-section, 

 while a vein is apt to be elliptic. It follows that any 



