644 SPECIAL PHYSIOLOGY. 



heart, have reference to the greater force needed to distribute the 

 blood through the body, than through the lungs. 



On tracing the course of the blood, through the body and the four 

 cavities of the heart, it is found that, proceeding from the left ventri- 

 cle (Fig. 108, 8), the blood passes through the aorta and arterial sys- 

 tem, into the capillaries of the whole body, and thence back by the 

 systemic veins, 1, 2, into the right auricle, 3; from the right auricle 

 it passes into the right ventricle, 4, and thence proceeds through the 

 pulmonary arteries, 5, capillaries, and veins, 6, 6, of the lungs, into 

 the left auricle, 7, from which it passes into the left ventricle, 8, and 

 thence is propelled, as before, through the aorta, 9, into the arteries 

 of the body. 



The mechanism and details of these movements, are very complex. 

 As the auricles dilate, the blood enters and distends them, that filling 

 the right auricle, coming from the two venae cavae and the nutrient 

 veins of the heart itself, that filling the left auricle, from the four pul- 

 monary veins. As the auricles are filling, some of the blood passes 

 through them into the corresponding ventricles, but the instant the 

 auricles are fully distended, they contract, and discharge nearly the 

 whole of their contents into the ventricles, which, at this period, are 

 dilating to receive the blood. The backward flow of the blood from 

 the auricles into the veins, is checked, more or less completely, by the 

 contraction of the muscular fibres surrounding the orifices of the venae 

 cavae and the pulmonary veins, and, as regards the right auricle, by 

 the Eustachian and Thebesian valves, at the mouths of the inferior 

 vena cava and cardiac sinus, and by those in the great veins at the 

 root of the neck. The blood passes from the auricles into the ventri- 

 cles, through the right and left auriculo-ventricular openings ; the 

 tricuspid and mitral valves, placed respectively at those openings, hav- 

 ing the free borders of their segments directed towards the ventricles, 

 ofter no opposition to this movement of the blood ; but the gradual 

 filling of the ventricles, and especially their complete distension, are 

 accompanied by a closure of the segments of the valves, the blood getting 

 behind them, or between them and the walls of the distended ventricles. 

 The ventricles now contract, and propel the blood into the great arte- 

 rial trunks proceeding from them, i. e., the right ventricle into the 

 pulmonary artery, and through the lungs, and the left ventricle into 

 the aorta, and through the body. The reflux of the blood towards the 

 respective auricles, is now prevented by the sudden closure of the tri- 

 cuspid and mitral valves, the segments of which are, moreover, pre- 

 vented from being forced back into the auricles, by the chordae tendi- 

 neae respectively attached to them. The blood, driven from the ven- 

 tricles into the pulmonary artery and the aorta, opens the segments 

 of the semilunar valves situated at the commencement of each of those 

 vessels, and displaces onwards the column of blood contained within 

 them, which, however, is not quite stationary. Four effects ensue : 

 First, a vibratory impulse to the blood columns in the pulmonary 

 artery and aorta, and in their branches ; secondly, an increased velo- 

 city of the blood column ; thirdly, an increased pressure within the 

 arteries ; and, fourthly, owing to the resistance offered by the blood 



