MECHANISM OF THE CIRCULATION. 57 



but the veins which leave the liver unite with the vena cava 

 inferior before that vessel enters the right auricle. 



96. Lesser Circulation. The venous blood conveyed 

 into the right auricle by the two cavae and by the coronary 

 vein, passes from this auricle into the right ventricle, and by 

 its action is driven along the pulmonary artery into the lungs 

 (Figs. 39 and 40). The latter vessel is called an artery, 

 although it carries only venous blood. In the right lung it 

 divides into three branches, and in the left into two ; thus 

 corresponding as it were with the number of the lobes of each 

 lung. 



97. The pulmonary veins, as they are called, carry back 

 from the lungs the whole of the blood which has been conveyed 

 to these organs by the pulmonary artery. In the capillary 

 system of these vessels, the blood has been in the mean time 

 aerated, revivified, and arterialized, and fitted once more to 

 perform its part in renovating the organs and maintaining 

 life. From the left auricle, in which the four pulmonary 

 veins terminate, the blood passes into the left ventricle, and 

 by its powerful action is driven through the aorta and its 

 branches into all parts of the body, to return once more by 

 the veins to the right auricle of the heart. Thus is completed 

 the double circulation in man, mammals, and birds. 



Mechanism of the Circulation. 



98. Movements of the Heart. The cavities of the 

 heart being muscular, contract and dilate : the dilatation is 

 called the diastole,* the contraction, systole.-^ As the auricles 

 of the heart dilate, they receive the blood, the right that of 

 the body, the left, that which is returning from the lungs ; 

 when full, they contract on their contents, forcing the blood 

 into the ventricles ; these in their turn suffer dilatation as 

 they fill, and, contracting suddenly, force the blood, the right 

 into the pulmonary artery, the left into the aorta. These 

 movements of the heart continue whilst life endures, and are 

 much influenced by various circumstances, such as exercise, 

 disease, &c. The number of these contractions, usually felt 

 in the radial artery at the wrist, varies with years ; they are 

 most frequent in the young, and average about seventy in 

 the adult, at noon. They are affected by every change in the 

 position of the body. 



* From Siaore'AAco, I dilate, f From crvoreAAw, I enclose. 



