It is sent back from the lungs to the right side of the heart, from which it is sent into the great trunk of the aorta, and from thence it passes into smaller blood-vessels, until it finds its way to every part of the system.
Fig. 51.—THE ORGANS OF RESPIRATION.
A. The heart.
B B. The lungs.
C. The aorta, and on either side of the aorta the vessels which convey the venous blood to the lungs to be oxygenised, and the corresponding vessels which return it to the heart, after it has undergone that operation. (For aorta see [Fig. 50].)
![[Fig. 50]](#i-212.jpg){kind=link}
D. The trachea, or large air passage, through which the air passes into the spongy texture of the lungs, when we breathe.
E E. Arteries and veins, being the trunks of the vessels that supply the head, &c.