140 THE HUMAN MECHANISM 



others combined. This fact is now so familiar that it is hard 

 to realize that we owe to Harvey not only the discovery of 

 the circulation but also the discovery of the meaning of the 

 heart beat. Before his time, to be sure, the living heart had 

 been seen at work, alternately shrinking in size and then 

 swelling, the shrinking being called systole and the swelling 

 diastole ; but these changes in size were regarded as the 

 results of the contraction and expansion of certain "vital 

 spirits " which the arterial blood was then supposed to contain, 

 and not as muscular contractions and relaxations. Harvey 

 showed that the heart is a powerful muscle and that its systole 

 is a muscular contraction ; that during systole it becomes 

 hard, just as the biceps muscle does when it shortens, and 

 during diastole soft and flabby; he also proved that with 

 each systole the heart drives or spouts blood into the large 

 arteries (the aorta and the pulmonary artery), and that this 

 blood is prevented from flowing back into the heart during 

 diastole by membranous valves at the very beginning of the 

 large arteries in question. 



7. The heart a muscular force pump. The beat of the 

 heart, even to its most minute detail, is one of the most 

 important as well as one of the most interesting subjects in 

 physiology; everything in the body hangs on its proper effi- 

 ciency and regulation, and it cannot be too thoroughly 

 studied. For our present purposes it will suffice to describe 

 the heart as composed essentially of a pair of muscular force 

 pumps. Dissection shows that it is divided into right and 

 left halves (see Fig. 70), completely separated from each 

 other, and that each half consists of two chambers an 

 auricle and a ventricle. The auricles, into which the great 

 veins open, have thin muscular walls and are comparatively 

 small in size ; the ventricles, on the other hand, from which 

 the great arteries arise, have thick muscular walls, especially 

 the left ventricle. The ventricles, indeed, constitute the prin- 

 cipal part of the force pump ; the auricles merely facilitate 



