Then there is a large vessel, called the thoracic duct, which comes down and communicates with those little vessels (it is a sort of overseer, having a large number of workmen,) and collects the produce of their toil, and carries it upwards to the part where it passes from the organs of digestion into the vessels of circulation.
886. What becomes of the nutrition, when it has entered the vessels of the circulation?
It is sent through a large vein into the heart, entering that organ on the right side, from which the heart propels it into the lungs, mixed with venous blood; and the venous, or blue blood, is sent into the lungs, taking with it the milk, the formation of which we have traced.
887. Why are the venous blood and the chyle sent to the lungs?
Because the venous blood, in its circulation through the body, has parted with its oxygen, and taken up carbon, and it requires to get rid of the carbon, and take up more oxygen. The chyle, also, now combined with the blood, requires oxygen, and having obtained it, is converted into bright red blood, and the blue blood of the veins, having got rid of its carbon, which formed the carbonic acid of the breath, has again become bright red blood. We must therefore, in pursuing our description, cease to speak of blue, or venous blood, and of white milk, or chyle, for the two have now combined, and, with the oxygen of the air, have formed arterial blood.
"My flesh and my heart fainteth; but God is the strength of my heart, and my portion for ever."—Psalm lxxiii.