54 THE CIRCULATION OF THE BLOOD 



will be transmitted to it a perceptible impulse. This impulse is 

 what is called the pulse. It is caused by the force of the heart's 



FIG. 29. The left auricle and ventricle opened and part of their walls 

 removed to show their cavities. (From Yeo after Allen Thompson.} 



i, right pulmonary vein cut short; i', cavity of left auricle; 3, 3", thick wall of 

 left ventricle; 4, portion of the same with papillary muscle attached; 5, the other 

 papillary muscles; 6, 6', the segments of the mitral valve, 7, in aorta is placed over 

 the semi-lunar valves. 



action against the elastic arterial wall, and the subsequent con- 

 traction of this wall against the current it contains. 



The impulse is an index to the condition of the circulation. 



