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_. [Verse: "My flesh and my heart fainteth; but God is the strength of my heart, and my portion for ever."--PSALM LXXIII.] 888. _What becomes of the arterial blood thus formed?_ 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_. [Illustration: 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.) 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. 889. _Why does the chest expand when we breathe?_ Because the lungs consist of _millions of hollow tubes_, and _cells_, which, having been emptied by throwing off _carbonic acid gas_ and _nitrogen_, become compressed, and the atmospheric air flowing into these millions of spaces, and filling the lungs, just as water fills and swells a sponge, causes them to expand, and occupy greater room. [Verse: "All the while my breath is in me, and the spirit of God is in my nostrils. My lips shall not speak wickedness, nor my tongue utter deceit."--JOB XXVII.] 890. _How does the blood communicate with the air in the lungs?_ Through the _sides of very minute vessels_, of which, perhaps, a _fine hair_ gives us the best conception. But these vessels are _twisted and wound round each other_ in such a curious manner, that they form _millions of cells_, and by being twisted and wound, a much _greater surface of air and blood_ are brought to act upon each other, than could otherwise be accomplished. 891. _Why does the blood which is thus formed, impart vitality to the parts to which it is sent?_ Because the blood is itself _vitalised_--is, in fact, _alive_, and capable of diffusing life and vitality to the organisation of which it forms a part.