232 LESSER CIRCULATION—FORCES THAT MOVE THE BLOOD. 
subdivide, are distributed upon the walls of tbe air-cells (fig. 
162)* and tbe 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 cavae 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 heat of the 
heart, which is caused by the striking of its lower extremity 
