THE CIRCULATION. 231 



pulmonary veins, on the other hand, is responsible for the flow of 

 blood. Its direction is always in the line of least resistance. The 

 greater the difference of pressure, the greater is the velocity of the 

 blood-stream; the reduction of this difference to nil, as in death, 

 results in no movement. 



MOVEMENTS OF THE HEART. 



The heart movements consist of alternate contractions and 

 relaxations, which follow each other with a certain rhythm. Systole 

 is the name for contraction; diastole is the term for relaxation. 



The two auricles contract and relax synchronously, and these 

 movements are followed by a simultaneous contraction and relaxa- 

 tion of the ventricles. There is a systole and diastole of auricles 

 and a systole and diastole of ventricles. At last there is a very 

 short period in which the heart is in diastole. 



The auricular contraction is less sudden than the ventricular. 

 The contraction of the auricle lasts a very short time, while the 

 time of ventricular contraction is considerable, and the relaxation 

 of the ventricle is slow. 



The ventricular diastole is nearly twice as long as the ventri- 

 cular systole. The auricles have a uniform, wavelike movement; 

 the ventricles have a spasmodic action in their movement. If now 

 the venae cavae and pulmonary veins are delivering blood into the 

 two auricles, then at this time the diastole of the auricles is 

 gradually approaching completeness. The swelling of the auricles 

 is due, in part, to the pressure in the veins being greater than in 

 the cavity of the auricles and in part to the inspiratory movement 

 of the thorax sucking the blood from the veins external to the 

 thorax to the interior of the veins of the chest. During this period 

 the ventricles are filling with blood, for both the tricuspid and 

 mitral valves are open. As the cavity of the auricles is smaller 

 than that of the ventricles, the auricles are filled sooner, and con- 

 sequently contract before the ventricles, the veins offering a resist- 

 ance to the backward movement of the blood by a narrowing of 

 their opening. The systole of the auricle forces the blood chiefly 

 in the line of least resistance with the ventricle, which is not yet 

 completely filled and is undergoing diastole. Whilst the blood is 

 passing from the auricles into the ventricles the auriculo-ventri- 

 cular valves are floated gradually into a horizontal position. The 

 blood by the systole of the auricles has filled the ventricles, already 

 filled in part during the diastole of the auricle. Now the ventricles 



