CHAPTER XVII 

 THE COURSE AND PHYSICS OF THE CIRCULATION 



To serve its various functions blood must be kept in 

 motion. The pump which maintains the flow is the 

 heart and the flow itself is called the circulation because 

 the blood is sent out only to return again and again. 

 In warm-blooded animals the heart is divided into right 

 and left halves and is best thought of as a pair of force 

 pumps making simultaneous strokes. On either side 

 there is a chamber above called an auricle which receives 

 incoming blood and transmits it to a second chamber 

 below called a ventricle. The ventricles are the chief 

 features of the heart from the standpoint of energy 

 evolved and applied to driving the blood. 



Vessels which conduct blood toward the heart are 

 called veins. It will be seen that they lead more or less 

 directly to the two auricles. Vessels which carry blood 

 away from the heart are called arteries. They are 

 derived from the ventricles, the smaller ones springing 

 from the larger. If we trace the course of the blood 

 along the veins we find it entering larger and larger 

 channels formed by the union of those which are more 

 slender and more numerous. If, in the same way, we 

 follow the blood in the arteries we find it introduced into 

 more and more numerous but finer branches. Both 

 systems are tree-like, but in the veins the flow is from 

 smaller to larger and in the arteries from larger to smaller 

 divisions. 



Blood which goes out from the left side of the heart 

 will return next to the right. The arteries which are 

 supplied from the left ventricle empty into veins which 

 lead to the right auricle. The connection between the 



229 



