232 LESSER CIRCULATION FORCES THAT MOVE THE BLOOD. 



subdivide, are distributed upon the walls of the air-cells (fig. 

 162), and the character of the blood is in them converted, by 

 exposure to the air, from the dark venous to the bright arte- 

 rial. From this capillary network the pulmonary veins arise ; 

 and the branches of these unite into trunks, of which two 

 proceed from each lung, to empty themselves into the left 

 auricle (fig. 123). This auricle delivers the blood, now arte- 

 rialized or aerated ( 253), into the left ventricle, whence the 

 aorta arises ; and by the contraction of this cavity, it is 

 delivered through that vessel to the system at large. It 

 will be observed that the vessel which proceeds from the 

 heart to the lungs is called the pulmonary artery, although it 

 carries dark or venous blood. This is because it conveys the 

 blood from the heart towards the capillaries. And, for a 

 similar reason, the vessels which return the blood from the 

 capillaries to the heart are termed pulmonary veins, although 

 they carry red or arterial blood. 



Forces that move the Blood. 



269. The mechanical action, by which the blood is caused 

 to circulate in the vessels, is easily comprehended. The cavi- 

 ties of the heart, as already explained ( 245), contract and 

 dilate alternately, by the alternate shortening and relaxation 

 of the muscular fibres that form their walls (Chap, xn.) ; and 

 the force of their contraction is sufficient to propel the blood 

 through the vessels which proceed from them. The two 

 ventricles contract at the same moment ; the auricles contract 

 during the relaxation of the ventricles, and relax whilst the 

 ventricles are contracting. The series of movements is there- 

 fore as follows : The auricles being full of the blood which 

 they have received from the venae cavse and pulmonary veins, 

 discharge it by their contraction into the ventricles, which 

 have just before emptied themselves into the aorta and pul- 

 monary artery, and which now dilate to receive it. When 

 filled by the contraction of the auricles, the ventricles contract 

 in their turn, so as to propel their blood into the great vessels 

 proceeding from them ; and whilst they are doing this, the 

 auricles again dilate to receive the blood from the venous 

 system, after which the whole process goes-on as before. It 

 is when the ventricles contract, that we feel the beat of the 

 heart, which is caused by the striking of its lower extremity 



