CAUSES MOVING THE BLOOD. 249 



auricle itself becomes distended by the continued influx of the 

 blood pressing onwards from the pulmonary veins, and this 

 distention is the cause of its contraction ; but this contraction 

 throws a new quantity of blood into the ventricle by which it 

 is distended to such a degree as to cause its contraction ; by 

 which contraction, as above stated, the blood is thrown into 

 the aorta. The auricle and ventricle of the right side act 

 exactly in the same manner as the auricle and ventricle on the 

 left side. The two auricles contract together, and the two 

 ventricles together. 



The great cause of the movement of the blood in its circula- 

 tion is the muscular contraction of the heart, by which the 

 capacity of its cavities is suddenly diminished, and the blood 

 more or less completely expelled in a determinate direction. 

 Some other causes concur, and the investigation of these has 

 much engaged the attention of physiologists. The most strik- 

 ing of these is the effect of muscular exertion throughout the 

 body. The veins in all the muscular parts of the body con- 

 cerned in ordinary locomotion are provided with valves, which 

 permit the blood to pass in one direction only. Hence when 

 any one of these veins is momentarily compressed, the blood in 

 it is sent more swiftly towards the heart. But in active mus- 

 cular exertion such veins are subjected to repeated strokes of 

 pressure, so that the ordinary motion of the blood in them to- 

 wards the heart is very much quickened. When a man climbs 

 a high hill his pulse may rise from 70 in a minute to 150. 

 Then the first effect is probably that just referred to. By the 

 contraction of the muscles affecting the adjacent veins, the blood 

 consequently in a given time reaches the right side of the heart 

 in larger quantity ; these cavities, being more rapidly filled, con- 

 tract more frequently, so that the blood is sent through the sys- 

 tem of the pulmonary artery and that of the pulmonary veins, 

 and carried onward to the left cavities of the heart in much less 



