6 CIRCULATION OF THE BLOOD 



The difficulty of understanding it is largely due to the 

 fact that the cavities which it contains are, as it were, 

 twisted on one another in an irregular fashion and in 

 three dimensions. It will be much easier to understand 

 if we imagine it straightened out on a flat surface with 

 its cavities exposed (Plate I.). 



The heart contains four cavities, two on each side. 

 The two cavities on the same side are in direct com- 

 munication with one another, so that blood passes freely 

 on each side from the upper cavity or auricle to the lower 

 cavity or ventricle of the same side. It is important to 

 remember that there is no communication between the 

 auricle and ventricle of one side of the heart and the 

 auricle and ventricle of the other side of the heart, except 

 through the blood-vessels and capillaries. The exist- 

 ence of a more direct means of communication was firmly 

 believed until the seventeenth century. This belief, as 

 we shall see, was a very great obstacle to the advance of 

 physiological knowledge. The removal of this belief 

 was one of the most important events in the history of 

 science. 



From each of the two ventricles arises a great artery 

 through which the blood is distributed. On the other 

 hand blood is brought to each ventricle from its corre- 

 sponding auricle, which in its turn receives it from a 

 corresponding great vein. The direction of the move- 

 ment of the blood is determined by a series of valves. 

 Valves are placed between auricles and ventricles on 

 both sides, and there are other valves at the entrance of 

 the great arteries that arise from each of the two ventricles. 



When the right ventricle contracts, blood is driven 

 into a great artery that resembles the aorta in certain 

 respects, and differs in others. This great artery is 

 known as the pulmonary artery. It resembles the aorta 

 in its muscular character, in size of its cross-section and 



