104 THE ENGINES OF THE HUMAN BODY 



ventricle, causing the cusps to bellow out and thus 

 automatically shutting the main doorway of the chamber, 

 had only one passage open for its exit — that leading to 

 the aorta. That opening, Harvey pointed out, was 

 efficiently guarded by three semi-lunar or pocket valves, 

 which allowed free passage from the ventricle. The 

 moment the blood in the aorta sought to flow back- 

 wards, the pockets filled, their edges came together and 

 the passage became tightly closed. The more the column 

 of blood in the aorta pressed backwards, the tighter were 

 the edges of the pockets pressed together. There could, 

 therefore, be no ebb and flow of the blood to and from 

 the left ventricle ; it must pass always in one direction — 

 towards the mouth of the aorta. 



As usual the valves served Harvey as finger-posts. 

 He hastened to explain that he had not been content to 

 merely guess the meaning of the signposts ; he had 

 watched the heart at work. He described the immense 

 effort made by the left ventricle when it contracted ; it 

 became blanched and erect, its apex coming forwards. 

 At the very moment of action, when the apex of the 

 heart could be seen and felt to project, the aorta expanded 

 and became more turgid. He observed that the con- 

 traction of the left auricle immediately preceded that of 

 the ventricle ; the auricle was clearly a pump for loading 

 the ventricle. Then the left ventricle contracted and 

 emptied its load into the aorta. All these events followed 

 so fast, sometimes too quickly for the separate beats to 

 be counted, that it was difficult to be quite certain of all 

 that happened. He described how he proceeded to make 

 certain that the heart was acting as a pump. He pricked 

 the great vessels into which the ventricles pumped their 

 loads ; the spurt which issued became heightened with 

 every stroke of the ventricles. He tied the great arterial 

 channels with ligatures ; the ventricular chambers became 

 engorged, while the pulmonary artery and aorta became 

 emptier. He tied the great veins leading to the heart ; 

 immediately they became distended, while the heart became 

 empty. He tied a small vein far away from the heart, 



